BOC Microbiology
The ONPG test allows organisms to be classified as a lactose fermenter by testing for which of the following? a. permease b. beta-galactosidase c. beta-lactamase d. phosphatase
*b. beta-galactosidase* Through the action of the enzyme beta-galactosidase, ONPG cleaves into galactose and o-nitrophenol (a yellow compound).
A penicillin-resistant Neisseria Gonorrhoeae produces: a. alpha-hemolysin b. beta-lactamase c. enterotoxin d. coagulase
*b. beta-lactamase* Antimicrobial resistance in Neisseria Gonorrhoeae is widespread. The production of beta-lactamase (penicillinase) breaks open the beta lactam ring of penicillin, destroying its activity. Thus, N Gonorrhoeae that produce beta lactamase are resistant to penicillin.
A reliable test for distinguishing Staphylococcus aureus from other staphylococci is: a. oxidase b. coagulase c. catalase d. optochin susceptibility
*b. coagulase* Coagulase is the biochemical test used to distinguish S aureus (positive) from coagulase-negative staphylococci (negative).
The optimal wound specimen for culture of anaerobic organisms should be: a. a swab of lesion obtained before administration of antibiotics b. a swab of lesion obtained after administration of antibiotics c. a syringe filled with pus, obtained before administration of antibiotics d. a syringe filled with pus, obtained after administration of antibiotics
* a syringe filled with pus, obtained before administration of antibiotics* The use of swabs for collection of specimens for anaerobic culture is discouraged. Aspiration with a needle and syringe is recommended. Whenever possible cultures should be obtained before the administration of antibiotics to optimize organism recovery.
Characteristically, enterococci are: a. unable to grow in 6.5% NaCl b. relatively resistant to penicillin c. sodium hippurate positive d. bile esculin negative
* b. relativity resistant to penicillin* Enterococci are bile esculin-positive, hippurate-negative and have the ability to grow in 6.5% NaCl. Enterococci are relatively resistant to penicillin and require combination therapy to treat serious infections.
The expected colony count in a suprapubic urine from a healthy individual is: a. 0 CFU/mL b. 100 CFU/mL c. 1,000 CFU/mL d. 100,000 CFU/mL
*a. 0 CFU/mL* Urine in the bladder is normally sterile. Suprapubic aspiration removes urine directly from the bladder and yields a specimen free of urethral contamination.
"Nutritionally deficient" streptococci are: a. enterococci b. group D nonenterococci c. cell wall-deficient streptococci d. in the genera Granulicatella and Abiotrophia
*d. in the genera Granulicatella and Abiotrophia* Organisms that used to be categorized as nutritionally variant or deficient streptococci have been reclassified into the genera Abiotrophia and Granulicatella.
A diabetic foot swab from a 82-year-old woman with recurrent infections is submitted for culture. The Gram stain reveals: many neutrophils, no squamous epithelial cells many gram-negative bacilli many gram-positive cocci in chains The physician requests that all pathogens be worked up. In addition to the sheep blood and MacConkey agar plates routinely used for wound cultures, the technologist might also process a(n): a. CNA agar plate b. chocolate agar plate c. XLD agar plate d. chopped meat glucose
*a. CNA agar plate* CNA agar is a selective medium commonly used in the isolation of gram-positive aerobic and anaerobic organisms. Since the Gram stain indicates a mixture of gram-positive and gram-negative organisms, use of CNA will aid in the recovery of the gram-positive cocci in culture.
Which of the following must be incubated in a microaerophilic environment for optimal recovery of the organism? a. Campylobacter jejuni b. Escherichia coli c. Pseudomonas aeruginosa d. Proteus mirabilis
*a. Campylobacter jejuni* Most Campylobacter species grow best under lower oxygen tension in an atmosphere of 5% oxygen, 10% carbon dioxide and 85% nitrogen. E coli and Proteus mirabilis are facultative anaerobes and Pseudomonas aeruginosa is an aerobe.
Which selective medium is used for the isolation of gram-positive microorganism? a. Columbia CNA with 5% sheep blood b. trypticase soy agar with 5% sheep blood c. eosin methylene blue d. modified Thayer-Martin
*a. Columbia CNA with 5% sheep blood* Columbia CNA agar contains colistin and nalidixic acid, which inhibit most gram-negative organisms. Eosin methylene blue is selective and inhibits gram-positive organisms and modified Thayer Martin is selective and inhibits gram-positive organisms, gram-negative bacilli and yeast.
The best medium for culture of Bordetella pertussis is: a. Regan-Lowe agar b. cystine blood agar c. Martin Lewis agar d. Ashdown agar
*a. Regan-Lowe agar* Regan-Lowe agar is an enriched and selective medium for the isolation of B pertussis. Cephalexin is added to inhibit nasopharyngeal flora. It provides better isolation of B pertussis than Bordet-Gengou medium.
Which of the following media can be used to culture Campylobacter jejuni? a. Skirrow medium b. CIN agar c. anaerobic CNA agar d. bismuth sulfite
*a. Skirrow medium* Skirrow medium is an enriched selective blood agar medium used for the isolation of Campylobacter from specimens with mixed flora. CIN and bismuth sulfite agars are selective and differential for Yersinia enterocolitica and Salmonella, respectively. CNA agar is selective for gram-positive organisms.
Gram-positive cocci in chains are seen on a Gram stain from a blood culture. The organisms grows as a beta-hemolytic colony. Further tests that could be performed include: a. bile esculin, PYR, bacitracin, and hippurate b. catalase and coagulase c. oxidase and deoxyribonuclease d. Voges-Proskauer and methyl red
*a. bile esculin, PYR, bacitracin, and hippurate* Bile esculin, PYR, bacitracin and hippurate are biochemicals/tests used in the presumptive or definitive identification of beta-hemolytic streptococci such as S pyogenes, S agalactiae and Enterococcus.
Gram stain examination from a blood culture bottle shows dark blue, spherical organisms in clusters. Growth on sheep blood agar shows small, round, pale yellow colonies. Further tests should include: a. catalase production and coagulase test b. bacitracin susceptibility and serological typing c. oxidase and deoxyribonuclease reations d. Voges-Proskauer and methyl red reactions
*a. catalase production and coagulase test* The Gram stain and culture growth describe a Staphylococcus species. Catalase production confirms that the organism belonged to the genus Staphylococcus and coagulase is used to differentiate S aureus from coagulase-negative staphylococci.
Which type of microscope would be *most* useful in examining viruses and the structure of microbial cells? a. electron b. phase-contrast c. dark-field d. bright-field
*a. electron* Due to the small size of viruses, they are not visible using light microscopy. Electron microscopy is used to visualize viruses and the internal structure of microorganisms.
Which of the following is the *most* appropriate specimen source and primary media battery? a. endocervical-chocolate, Martin Lewis b. sputum-sheep blood, Thayer-Martin, KV-laked blood c. CSF-Columbia CNA, MacConkey d. urine-sheep blood, chocolate, Columbia CNA
*a. endocervical-chocolate, Martin Lewis* Chocolate agar and chocolate agar-based selective media (Martin Lewis) are routinely used for the recovery of Neisseria gonorrhoeae from genital specimens. Sputum and urine specimens are routinely processed using a general purpose media (blood agar) and a selective agar (EMB or MacConkey). In addition chocolate agar is routinely included to enhance recovery of fastidious organisms such as H influenzae. CSF is routinely processed using blood and chocolate agars.
A beta-hemolytic streptococcus that has been isolated from an ear culture grows up to the edge of a 0.04 unit bacitracin disk. Which of the following tests would help to determine if the organism is Enterococcus? a. growth in 6.5% NaCl broth b. growth in the presence of penicillin c. optochin susceptibility d. fermentation of 10% lactose
*a. growth in 6.5% NaCl broth* Of the biochemicals listed, only growth in 6.5% NaCl will aid in the identification of Enterococcus, which has the ability to grow in the presence of high salt concentrations.
In order to isolate Campylobacter coli/jejuni, the fecal specimen should be: a. inoculated onto selective plating media and incubated in reduced oxygen with added CO₂ at 42°C b. stored in tryptic soy broth before plating to ensure growth of the organism c. inoculated onto selective plating media and incubated at both 35°C and at room temperature d. incubated at 35°C for two hours and cary-blair media before inoculating onto selective plating media
*a. inoculated onto selective plating media and incubated in reduced oxygen with added CO₂ at 42°C* Campylobacter coli/jejuni require a microaerophilic atmosphere for optimal recovery. The use of selective media is recommended for recovery from fecal specimens. Selective media for campylobacter contains antibiotics to inhibit the growth of enteric gram-negative flora. Unlike other enteric pathogens, C coli/jejuni grow well at 42°C.
Group B, beta-hemolytic streptococci may be distinguished from other hemolytic streptococci by which of the following procedures? a. latex antigen typing b. growth in 6.5% NaCl broth c. growth on bile esculin medium d. bacitracin susceptibility
*a. latex antigen typing* Growth in 6.5% NaCl, growth in bile esculin medium and susceptibility are not used in the routine identification of Streptococcus agalactiae. Polysaccharide typing for group B antigen is routinely used for identification of S agalactiae.
The lowest concentration of antibiotic that inhibits growth of a test organism is the: a. minimum inhibitory concentration b. serum inhibitory concentration c. minimum bacterial titer d. maximum inhibitory titer
*a. minimum inhibitory concentration* The MIC is a basic laboratory measurement of the activity of an antibiotic against an organism. It is the lowest concentration of antibiotic that inhibits visible growth of the organism. It does not represent the concentration of antibiotic that is lethal to the organism.
After 24 hours a blood culture from a newborn grows catalase-negative, gram-positive cocci. The bacterial colonies are small, translucent and beta-hemolytic on a blood agar plate. Biochemical test results of a pure culture are: bacitracin: resistant CAMP reaction: positive bile esculin: not hydrolyzed 6.5% NaCl broth: no growth Assuming that all controls react properly and reactions are verified, the next step would be to: a. perform a Streptococcus group typing b. report the organism as Streptococcus pneumoniae c. report the organism as Staphylococcus aureus d. report the organism as Staphylococcus epidermidis
*a. perform a Streptococcus group typing* The colony description and biochemical results presented describe Streptococcus agaloactiae. The identification of this organism is confirmed by streptococcus antigen typing.
An Enterococcus isolated from multiple blood cultures in a patient endocarditis should be: a. screened for high level aminoglycoside resistance b. checked for tolerance c. assayed for serum antimicrobial activity d. tested for beta-lactamase production
*a. screened for high level aminoglycoside resistance* Systemic enterococcal infections, such as endocarditis, are commonly treated with a cell-wall-active agent and an aminoglycoside. These agents act synergistically to kill the organism. If the organism is resistant to one or both, there is no synergy, and the combination will fail. It is important to detect aminoglycoside and beta-lactam resistance in these cases. Enterococci have intrinsic moderate level resistance to aminoglycosides. Acquired resistance corresponds to very high MICs (greater than 500 ug/ mL) for gentamicin and is termed high level resistance .
When processing throat swabs for a group A Streptococcus culture, the medium of choice is: a. sheep blood agar b. rabbit blood agar c. human blood agar d. horse blood agar
*a. sheep blood agar* Sheep blood agar is preferred because clear-cut patterns of hemolysis are obtained.
The susceptibility results below are reported on an Enterococcus faecalis isolated from peritoneal fluid. ampicillin: susceptible vancomycin: resistant clindamycin: susceptible levofloxacin: resistant linezolid: susceptible The physician calls questioning the results. Which of the following should have been done before the report was released? a. the clindamycin result should have been removed from the report since it is inactive against Enterococcus b. the ampicillin result should have been changed to resistant since the isolate is vancomycin resistant c. the linezolid result should have been removed from the report since it is inactive against Enterococcus d. ciprofloxacin should have been added to the report since levofloxacin was resistant
*a. the clindamycin result should have been removed from the report since it is inactive against Enterococcus* Clindamycin, cephalosporins and trimethoprim/sulfamthoxazole may appear active in vitro against Enterococcus species but are not effective clinically and should not be reported as susceptible.
Upon review of a sputum Gram stain, the technician notes that the nuclei of all of the neutrophils present in the smear are staining dark blue. The best explanation for this finding is: a. the slide was inadequately decolorized with acetone/alcohol b. the sputum smear was prepared too thin c. the cellular components have stained as expected d. the iodine was omitted from the staining procedure
*a. the slide was inadequately decolorized with acetone/alcohol* Problems with analysis Gram staining generally result from errors including interpretation of the slide (smear prepared too thick), excessive heat fixing, and improper decolorization. Inadequate decolorization with acetone/alcohol results in a smear in which host cells (neutrophils and squamous cells), as well as bacteria, all appear blue.
The steam autoclave method of sterilization: a. uses 15 lbs of pressure for 15 minutes b. utilizes dry heat for 20 minutes c. produces a maximum temperature of 100°C d. requires a source of ethylene oxide
*a. uses 15 lbs of pressure of sterilization* The traditional gravity displacement steam sterilization cycle is 121°C for 15 minutes at 15 pounds per square inch. Ethylene oxide is an alternative sterilization method.
Which of the following clean catch urine culture colony counts indicates the patient likely has a urinary tract infection? a. 10¹ CFU/mL b. 10³ CFU/mL c. 10⁵ CFU/mL d. no growth
*b. 10³ CFU/mL* Patients with infection often have at least 100,000 bacteria/mL of urine in the bladder. However one third of young women with symptomatic cystitis have less than 100,000 bacteria/mL of urine. The infectious Disease Society of America consensus definition of cystitis is greater than equal to 1,000 CFU/mL of a uropathogen.
A vaginal/rectal swab is collected from a pregnant patient to screen for group B streptococcus colonization. What is the best medium to use for specimen inoculation? a. blood agar b. LIM broth c. CNA agar d. thioglycollate broth
*b. LIM broth* Detection of group B Streptococcus (GBS) in the genital and gastrointestinal tracts of pregnant women can identify infants at risk for GBS infection. The CDC currently recommends the collection of vaginal and rectal swabs or a single swab inserted first into the vagina and then the rectum at 35-37 weeks gestation. The swab(s) should be inoculated into a selective broth medium such as LIM broth (Todd-Hewitt broth with colistin and nalidixic acid). The use of vaginal/rectal swabs and selective broth medium greatly increases the recovery of GBS.
Production of beta-lactamase is inducible in which of the following: a. Haemophilus influenzae b. Staphylococcus aureus c. Corynebacterium diphtheriae d. Streptococcus pyogenes
*b. Staphylococcus aureus* Some bacteria such as Enterococcus sp, H influenzae and Neisseria gonorrhoeae continually produce only after exposure to an inducing agent (such as penicillin). C. diphtheriae and S pyogenes do not produce beta-lactamase.
Children who have infections with beta-hemolytic streptococci can develop: a. acute pyelonephritis b. acute glomerulonephritis c. chronic glomerulonephritis d. nephrosis
*b. acute glomerulonephritis* Noninfectious sequelae associated with infection with Streptococcus pyogenes are glomerulonephritis and rheumatic fever.
An antibiotic used to suppress or kill contaminating fungi in media is: a. penicillin b. cycloheximide c. streptomycin d. amphotericin B
*b. cycloheximide* Cycloheximide, which inhibits protein synthesis, is the common agent used in Mycosel or mycobiotic agar to inhibit faster-growing saprophytic fungi. Penicillin and streptomycin do not inhibit fungi. Amphotericin B is not routinely used as an additive in fungal media.
The best medium for culture of Francisella tularensis is: a. Bordet-Gengou medium b. cystine glucose blood agar c. Loeffler medium d. charcoal selective medium
*b. cystine glucose blood agar* Francisella tularensis is fastidious and not readily recovered in culture. Cysteine blood glucose agar is an enriched medium with beef heart infusion, peptones, glucose and rabbit blood. It also includes cystine, which is required by F tularensis for growth.
In disk diffusion susceptibility testing, as an antimicrobial agent diffuses away from the disk, the concentration of the antibiotic is: a. increased b. decreased c. unchanged d. inoculum dependent
*b. decreased* The amount of antibiotic used in disk diffusion susceptibility testing is standardized in constant. Once the disk is placed on the inoculated plate and makes contact with the agar, the antibiotic in the disk begins to diffuse out. As it diffuses into the media, the concentration of antibiotic gets lower the further it diffuses from the disk.
After satisfactory performance a daily disk diffusion susceptibility quality control is documented, the frequency of quality control can be reduced to: a. twice a week b. every week c. every other week d. every month
*b. every week* Daily disk diffusion quality control can be converted to weekly testing when thirty days of consecutive testing demonstrates no more than three antibiotic/organism combinations outside of the acceptable limits.
A liquid fecal specimen from a three-month-old infant is submitted for microbiological examination. In addition to culture on routine media for Salmonella and Shigella, this specimen should be routinely: a. examined for the presence of Entamoeba hartmanni b. examined for the presence of Campylobacter sp c. screened for the detection of enterotoxigenic Escherichia coli d. placed in the thioglycollate broth to detect Clostridium botulinum
*b. examined for the presence of Campylobacter sp* Campylobacter continues to be the most common enteric pathogen isolated from patients with diarrhea. Rountinly fecal specimens should be cultured for Salmonella, Shigella and Campylobacter. Fecal specimens are not routinely cultured for enterotoxigenic E coli or C botulinum. E hartmanni is a nonpathogenic parasite and does not cause diarrhea.
Which 2 diseases are usually preceded by infection with beta-hemolytic streptococci? a. rheumatic fever, undulant fever b. glomerulonephritis, rheumatic fever c. rheumatic fever, tularemia d. glomerulonephritis, undulant fever
*b. glomerulonephritis, rheumatic fever* Noninfectious sequelae associated with infection with Streptococcus pyogenes are glomerulonephritis and rheumatic fever.
A beta-hemolytic Streptococcus that is bacitracin-sensitive and CAMP-negative is: a. group B b. group A c. beta-hemolytic, not group A,B, or D d. beta-hemolytic, group D
*b. group A* Group A streptococci
A urine Gram stain shows gram-positive cocci in clusters. The organism tested catalase positive. To *speciate* this organism from culture, the technician should perform a coagulase test and a/an: a. polymyxin B susceptibility b. novobiocin susceptibility c. oxidase d. beta-lactamase
*b. novobiocin susceptibility* The organism in this urine culture is a Staphylococcus species. Coagulase will differentiate S aureus from coagulase-negative staphylococcis (CNS) and novobiocin susceptibility will differentiate S saprophyticus from other CNS. S saprophyticus is a common cause of urinary tract infections in young females.
A 73-year old man diagnosed as having pneumococcal meningitis is not responding to his penicillin therapy. Which of the following tests should be performed on the isolate to best determine this organism's susceptibility to penicillin? a. beta lactamase b. oxacillin disk diffusion c. penicillin disk diffusion d. Schlichter test
*b. oxacillin disk diffusion* With the exception of the oxacillin disk screening test, disk diffusion is not recommended for testing S. pneumoniae against beta-lactam agents. S. pneumoniae does not produce beta-lactamase, so beta-lactamase testing would not be useful. The Schlichter test is not a method for determining an organism's susceptibility to a given agent.
An antibiotic that inhibits cell wall synthesis is: a. chloramphenicol b. penicillin c. sulfamethoxazole d. colistin
*b. penicillin* Penicillin inhibits penicillin binding proteins that are essential to peptidoglycan (cell wall) synthesis. Chloramphenicol inhibits protein synthesis, colistin increases cell membrane permeability, and sulfamethoxazole inhibits folate metabolism.
Three sets of blood cultures were obtained from an adult patient with fever and suspected endocarditis. The aerobic bottle of one set had growth of Staphylococcus epidermidis at 5 days of incubation. This indicates that: a. there was low-grade bacteremia b. the organism is *most* likely a contaminant c. the patient has a line infection d. blood culture bottles are defective
*b. the organism is most likely a Contaminant* Coagulase-negative staphylococci are commonly associated with contaminated blood cultures; however, they are also increasing as a cause of true bacteremia. Significant bacteremia in a patient with endocarditis is usually continuous and low grade. In most cases, all blood cultures drawn will yield positive results. The facts that only 1 bottle of 1 set was positive, and that the bottle did not become positive until day 5 of incubation, indicate that this isolate is most likely a contaminant.
Which of the following is the *most* important variable in the recovery of organisms in patients with bacteremia? a. subculture of all bottles at day 5 of incubation b. the volume of blood cultured c. use of chlorhexadine for skin antisepsis d. collection of multiple blood culture sets from a single venipuncture
*b. the volume of blood cultured* The volume of blood collected is the single most important variable in the recovery of organisms in patients with blood stream infections. Since many cases of adult bacteremia are low magnitude,there is a direct relationship between the yield of blood culture (positivity) and volume of blood collected. The collection of multiple blood culture sets from a single venipuncture is an unacceptable practice due to the potential for contamination. The practice of terminal subculture of bottles at 5 days is no longer recommended. The use of chlorhexadine for skin antisepsis does not affect organism recovery, but aids in decreasing blood culture contamination.
Which of the following factors would make an organism appear to be more resistant on a disk diffusion susceptibility test? a. too little agar in the plate b. too many organisms in the inoculum c. the presence of 0.5% NaCl in the medium d. a medium with a pH of 7.4
*b. too many organisms in the inoculum* To ensure the reproducibility of disk diffusion testing, the inoculum must be standardized. If the inoculum is too dense (too many organisms), zone sizes would be smaller than expected and appear falsely resistant.
When performing a Kovac indole test, the substrate must contain: a. indole b. tryptophan c. ornithine d. paradimethylaminobenzaldehyde
*b. tryptophan* The indole test is used for determining an organism's ability to produce indole from deamination of tryptophan by tryptophanase.
When performing a stool culture, a colony type typical of an enteric pathogen is subcultured on a blood agar plate. The resulting pure culture is screened with several tests to obtain the following results: *TSI:* Acid Butt, Alkaline slant, no gas, no H₂S *phenylalanine deaminase:* negative *motility:* positive *serological typing:* Shigella flexneri (Shigella subgroup B) The serological typing is verified with new kit and controls. The best course of action would be to: a. report the organism as Shigella flexneri without further testing b. verify reactivity of motility medium with positive and negative controls c. verify reactivity of the TSI slants with positive and negative controls for H₂S production d. verify reactivity of phenylalanine deaminase with positive and negative controls
*b. verify reactivity of motility medium with positive and negative controls* All of the biochemical and serological reactions listed are consistent with an identification of Shigella flexneri, with the exception of motility. All Shigella are nonmotile.
The Gram stain from a blood culture shows gram-positive cocci in chains. No growth occurs on blood agar plates incubated both aerobically and anaerobically. Additional testing should be done to detect the presence of: a. Staphylococcus saprophyticus b. Aerococcus urinae c. Abiotrophia defectiva d. Streptococcus pneumoniae
*c. Abiotrophia defectiva* Nutritionally deficient streptococci such as Abiotrophia do not grow on sheep blood agar without the addition of cysteine or proximity to S aureus colonies.
A catheterized urine is inoculated onto blood and MacConkey agar using a 0.01 mL loop. After 48 hours, 68 colonies of a small translucent nonhemolytic organism grew on blood agar but not on MacConkey. Testing reveals small gram-positive, catalase-negative cocci. The preliminary report and follow-up testing would be: a. growth of 680 colonies/mL of gram-positive cocci, optochin and bacitracin susceptibility tests to follow b. growth of 6,800 colonies/mL of a Staphylococcus species, coagulase test to follow c. growth of 6,800 colonies/mL of a Streptococcus species, esculin hydrolysis and NaCl growth test to follow d. growth of 6,800 colonies/mL of a Streptococcus species, no further testing
*c. Growth of 6,800 colonies/mL of a Streptococcus species, esculin hydrolysis and NaCl growth test to follow* The number of colonies isolated is multiplied by 100 when a 0.01 mL loop is used for inoculation. Gram positive, catalase negative cocci are indicative of streptococci.
The optochin (ethylhydrocupreine hydrochloride) disk is used for the identification of: a. Haemophilus influenzae b. group A beta-hemolytic streptococci c. Streptococcus pneumoniae d. alpha-hemolytic streptococci
*c. Streptococcus pneumoniae* Optochin susceptibility is used to differentiate S pneumoniae, which are susceptible, from other alpha-hemolytic streptococci, which are resistant.
Which one of the following organisms does not require susceptibility testing to the antimicrobial indicated when isolated from a clinically significant source? a. Staphylococcus aureus-clindamycin b. Proteus mirabilis-gentamicin c. Streptococcus pyogenes-penicillin d. Escherichia coli-levofloxacin
*c. Streptococcus pyogenes-penicillin* Susceptibility testing should be performed when the susceptibility of the organism cannot reliably be predicted and resistance is known or suspected. Susceptibility testing of penicillins for treatment of S pyogenes does not need to be performed routinely since resistance has not been documented.
In a quality control procedure on a new batch of Mueller-Hinton plates using a stock culture of Staphylococcus aureus (ATCC 25923), allthe disk zone sizes are too small. The *most* likely reason for this is that the: a. Mueller-Hinton plates were poured too thin b. potency of the antibiotic disks is too high c. bacterial suspension was not diluted to the proper concentration d. disks should have been set up on mannitol salt
*c. bacterial suspension was not diluted to the proper concentration* Quality control zone sizes that are too small could indicate that the organism inoculum is too high, plates were poured too thick, or that the potency of the antibiotic disks is too low.
In the Kirby-Bauer disc diffusion susceptibility test, which variable is critical when testing Pseudomonas species for antibiotic susceptibility to aminoglycosides? a. incubation temperature b. duration of incubation c. cation content of media d. depth of agar
*c. cation content of media* Variations in the concentrations of divalent cations primarily calcium and magnesium affect the results of aminoglycoside, tetracycline and colistin tests with P. Aeruginosa isolates. A cation concentration that is too high results in smaller zone sizes, and a concentration that is too low increases zone sizes.
The enterotoxin produced by certain strains of hemolytic, coagulase positive Staphylococcus aureus: a. is destroyed by boiling for 15-30 minutes b. is identical to the dermonecrotic toxin c. causes one type of bacterial food poisoning d. is highly antigenic
*c. causes one type of bacterial food poisoning* Staphylococcus aureus produces an enterotoxin that is asociated with short-incubation food poisoning.
Which of the following combinations is useful for confirming the presence of extended spectrum beta- lactamases in E coli? a. ampicillin + cefepime b. cefoxitin + penicillin c. ceftazidime + clavulanic acid d. cefpodoxime + cefotaxime
*c. ceftazidime + clavulanic acid* Extended spectrum beta lactamases (ESBL) are inhibited by clavulanic acid. Confirmatory tests of the presence of ESBL are based on the enhanced activity of a beta-lactam antibiotic, usually cefotaxime or ceftazidime, when it is tested with clavulanic acid compared to the activity of the beta lactam tested alone.
First-generation cephalosporins can be adequately represented by: a. cefotetan b. ceftriaxone c. cephalothin d. cefoxitin
*c. cephalothin* Cephalothin is a first-generation cephalosporin, cefotetan and cefoxitin are second-generation cephalosporins, and ceftriaxone is a third generation cephalosporin.
Media used to support growth of Legionella pneumophilia should contain which of the following additives? a. X and V factors b. hemin and Vitamin K c. charcoal and yeast extract d. dextrose and laked blood
*c. charcoal and yeast extract* Buffered charcoal yeast extract medium is a specialized enrichment medium for the isolation of Legionella. The nutritive base includes yeast extract. Charcoal is added to the medium as a detoxifying agent.
Cerebrospinal fluid from a febrile 25-year-old man with possible meningitis is rushed to the laboratory for a stat Gram stain and culture. While performing the Gram stain, the technologist accidentally spills *most* of the specimen. The smear shows many neutrophils and no microorganisms. Since there is only enough CSF to inoculate one plate, the technologist should use a: a. blood agar plate b. chopped meat glucose c. chocolate agar plate d. Thayer-Martin plate
*c. chocolate agar plate* Enriched media such as chocolate agar has no inhibitory effects on bacterial growth and contains additional nutrients that support the growth of fastidious organisms such as H influenzae and Neisseria.
A medium that aids in the presumptive identification of organisms based on their appearance on the medium is called: a. enriched b. selective c. differential d. specialized
*c. differential* Differential media contain compounds, often carbohydrates, that provide a presumptive identification based on colony color or a precipitate around the colony. Examples include MacConkey, Hektoen and xylose lysine desoxycholate agar.
The organism *most* commonly associated with neonatal purulent meningitis is: a. Neisseria meningitidis b. Streptococcus pneumoniae c. group B streptococci d. Haemophilus influenzae
*c. group B streptococci* All of the organisms listed are potential causes of meningitis. Group B Streptococcus is associated with neonatal meningitis and meningitis in the elderly.
A beta-hemolytic Streptococcus that is bacitracin-sensitive and CAMP-positive is: a. group A or B b. group A c. group B d. beta-hemolytic, group D
*c. group B* Group B streptococci
A gamma-hemplytic Streptococcus that blackens bile esculin agar but does not grow in 6.5% NaCl broth is *most* likely: a. group B Streptococcus b. Enterococcus c. group D Streptococcus d. Streptococcus pneumoniae
*c. group D Streptococcus* Group D streptococci and Enterococcus produce a positive bile esculin test; however, of these 2, only Enterococcus grows in the presence of 6.5% NaCl.
Which of the following groups of specimens would be acceptable for anaerobic culture? a. vaginal, eye b. ear, leg tissue c. pleural fluid, brain abscess d. urine, sputum
*c. pleural fluid, brain abscess* Materials collected from sites not harboring indigenous flora (sterile body fluids, abscess exudate and tissue) should be cultured for anaerobic bacteria. However, since anaerobes normally inhabit the skin and mucous membranes as part of the indigenous flora, specimens such as urine, sputum, and vaginal, eye and ear swabs are not acceptable for culture.
In the optochin (ethylhydrocupreine hydrochloride) susceptibility test, if there is a zone of inhibition of 19-30 mm surrounding the disk following overnight incubation at 37°C, the colony *most* likely consists of: a. staphylococci b. streptococci c. pneumococci d. intestinal bacilli
*c. pneumococci* Optochin susceptibility is used to differentiate S pneumoniae, which are susceptible, from other alpha-hemolytic streptococci, which are resistant.
Test for beta-lactamase production in Haemophilus influenzae: a. are not commercially available b. include tests that measure a change to an alkaline pH c. should be performed on all blood and CSF isolates d. are not valid for any other bacterial species
*c. should be performed on all blood and CSF isolates* As many as 20%-40% of H influenzae produce beta-lactamases. Detection of these enzymes should be performed on any isolate considered to be a pathogen using the chromogenic cephalosporin (nitrocefin) test.
Susceptibility testing performed on quality control organisms using a new media lot number yielded zone sizes that were too large for all antibiotics tested. The testing was repeated using media from a previously used lot number, and all zone sizes were acceptable. Which of the following best explains the unacceptable zone sizes? a. the antibiotic disks were not stored with the proper desiccant b. the depth of the media was too thick c. the depth of the media was too thin d. the antibiotic disks were not properly applied to the media
*c. the depth of the media was too thin*
The ability to detect oxacillin-resistant Staphylococcus aureus may be enhanced by: a. shortening incubation of standard susceptibility plates b. incubating susceptibility plates at 39°-41°C c. using Muellar-Hinton broth with 2% NaCl d. adjusting inoculum to 0.1 before inoculating susceptibility plates
*c. using Mueller-Hinton broth with 2% NaCl* For optimum detection of oxacillin-resistant S aureus, a suspension with a turbidity equivalent to a 0.5 McFarland standard be inoculated into a cation-adjusted Mueller-Hinton broth with 2% NaCl. Plates should be incubated at 35+/- 2°C for 24 hours. Temperatures above 35°C may not detect oxacillin resistance.
In a disk diffusion susceptibility test, which of the following can result if disks are placed on the inoculated media and left at room temperature for an hour before incubated? a. the antibiotic would not diffuse into the medium, resulting in no zone b. zones of smaller diameter would result c. zones of larger diameter would result d. there would be no effect on the final zone diameter
*c. zones of larger diameter would result* A delay of more than 15 minutes between placing the disks on an inoculated plate and incubation permits excess prediffusion of the antimicrobial agent from the disk. This would result in a larger than expected zone diameter.
A sheep blood agar plate inoculated with 0.001 mL of urine grows 70 colonies of Staphylococcus Aureus. How many colony forming units per mL of urine should be reported? a. 70 b. 700 c. 7000 d. 70,000
*d. 70,000* When 0.001 mL of urine is plated, the growth of one colony is equivalent to 1000 CFU/mL. Thus 70 colonies is 70,000 CFU/mL.
A Gram stain from a swab of a hand wound reveals: moderate neutrophils no squamous epithelial cells moderate gram-positive cocci in chains moderate large gram-negative bacilli Select the appropriate media that will selectively isolate each organism. a. KV-laked agar, Thayer-Martin b. sheep blood, MacConkey c. Columbia CNA, chocolate d. Columbia CNA, MacConkey
*d. Columbia CNA, MacConkey* Columbia CNA agar is a selective medium used for the isolation of gram-positive organisms. The medium contains colistin and nalidixic acid, which inhibits gram-negative organisms. The medium contains bile and crystal violet, which inhibits gram-positive organisms.
Which of the following organisms may be mistaken for Neisseria gonorrhoeae in Gram stained smears of uterine cervix exudates? a. Lactobacillus species b. Streptococcus agalactiae c. Pseudomonas aeruginosa d. Moraxella osloensis
*d. Moraxella osloensis* Moraxella osloensis is a Gram-negative coccobacillus that is often plump and occurs in pairs and demonstrates a morphology similar to Neisseria. The presence of this organism in endocervical specimens contaminated with vaginal secretions can lead to over interpretation of smears for N gonorrhoeae.
Of the following bacteria, the *most* frequent cause of prosthetic heart valve infection occurring within 2-3 months after surgery is: a. Streptococcus pneumoniae b. Streptococcus pyogenes c. Staphylococcus aureus d. Staphylococcus epidermidis
*d. Staphylococcus epidermidis* Staphylococcus epidermidis is the most common cause of prosthetic valve endocarditis.
A gram-positive coccis isolated from a blood culture has the following characteristics: optochin susceptibility: negative bacitracin (0.04 U) susceptibility: negative bile esculin hydrolysis: negative hippurate hydrolysis: positive catalase: negative This organism is *most* likely: a. Staphylococcus aureus b. Streptococcus pneumoniae c. Streptococcus pyogenes d. Streptococcus agalactiae
*d. Streptococcus agalactiae* Streptococcus agalactiae is catalase and bile esculin hydrolysis negative and bacitracin and optochin resistant. Streptococcus agalactiae hydrolyzes hippurate.
An important cause of acute exudative pharyngitis is: a. Staphylococcus aureus (beta-hemolytic) b. Streptococcus pneumoniae c. Streptococcus agalactiae d. Streptococcus pyogenes
*d. Streptococcus pyogenes* Streptococcus pyogenes is the cause of exudative pharyngitis, commonly called strep throat.
Which of the following is the *most* appropriate organism and media combination? a. Legionella species - Regan Lowe b. Clostridium difficile - phenylethyl alcohol (PEA) c. Campylobacter species - charcoal yeast extract d. Yersinia enterocolitica - cefsulodin-irgasan-novobiocin (CIN)
*d. Yersinia enterocolitica - cefsulodin-irgasan-novobiocin (CIN)* CIN agar is a selective and differential medium for the isolation and differentiation of Y enterocolitica. This medium contains sodium desoxycholate, crystal violet, cefsulodin, irgason (triclosan), and novobiocin as selective agents, and mannitol as the carbohydrate.
Streptococcus pneumoniae can be differentiated best from the viridans group of streptococci by: a. Gram strain b. the type of hemolysis c. colonial morphology d. bile solubility
*d. bile solubility* Bile solubility testing of alpha-hemolytic streptococci differentiate S. pneumoniae (soluble) from other alpha-hemolytic streptococci such as viridans streptococci (insoluble).
Viridans streptococci can be differentiated from Streptococcus pneumoniae by: a. alpha hemolysis b. morphology c. catalase reaction d. bile solubility
*d. bile solubility* Bile solubility testing of alpha-hemolytic streptococci differentiates S pneumoniae (soluble) from other alpha-hemolytic streptococci, such as viridans streptococci (insoluble).
The *most* common cause for failure of a GasPak anaerobic jar to establish adequate environment for anaerobic incubation is: a. the failure of the oxidation-reduction potential indicator system due to deterioration of methylene blue b. the failure of the packet to generate adequate H₂ and/or CO₂ c. condensation of water on the inner surface of the jar d. catalysts that have become inactivated after repeated use
*d. catalysts that have become inactivated after repeated use* The two most common causes of failure of the GasPak system is a defective gasket in the jar lid that allows escape of gas from the inside of the jar and inactivated catalyst pellets.
Proper media for culture of a urethral discharge from a man include: a. sheep blood and phenylethyl alcohol agars b. eosin-methylene blue and sheep blood agars c. thioglycollate broth and chocolate agar d. chocolate and modified Thayer-Martin agars
*d. chocolate and modified Thayer-Martin agars* Chocolate agar and chocolate agar-based selective media should be used for recovery of Neisseria Gonorrhoeae from urethral discharge. Chocolate agar provides the nutrients required by N Gonorrhoeae and selective media contains antimicrobial agents that inhibits other organisms and permits recovery of pathogenic Neisseria.
Which of the following methods is inadequate for the detection of vancomycin-intermediate S aureus? a. broth macrodilution b. agar dilution c. gradient diffusion d. disk diffusion
*d. disk diffusion* The disk diffusion procedure will not differentiate S aureus strains with reduced susceptibility to vancomycin (MICs 4-8 micro grams per mL) from susceptible stains even when incubated for 24 hours.
A beta-hemolytic, catalase-positive, gram-positive coccus is coagulase negative by the slide coagulase test. Which of the following is the *most* appropriate action in identification of this organism? a. report a coagulase-negative Staphylococcus b. report a coagulase-negative Staphylococcus aureus c. reconfirm the hemolytic reaction on a fresh 24-hour culture d. do a tube coagulase test to confirm the slide test
*d. do a tube coagulase test to confirm the slide test* Staphylococcus aureus are usually beta-hemolytic. Some strains may not produce bound coagulase detected by the slide coagulase test. A tube coagulase is performed to detect free coagulase and should be performed on colonies with typical S aureus morphology that are slide coagulase-negative.
It is important to differentiate between Enterococcus and group D streptococci because: a. viridans streptococci are often confused with enterococci b. several enterococci cause severe puerperal sepsis c. group D streptococci are avirulent d. enterococci often show more antibiotic resistance than group D streptococci
*d. enterococci often show more antibiotic resistance than group D streptococci* Enterococcus species are more resistant to antimicrobial therapy than group D streptococci such as S bovis.
An aspirate of a deep wound was plated on blood agar plates and incubated aerobically and anaerobically. At 24 hours there was growth on both plates. This indicates that the organism is a(n): a. nonfermenter b. obligate anaerobe c. aerobe d. facultative anaerobe
*d. facultative anaerobe* Facultative anaerobes are organisms that can grow under both aerobic and anaerobic conditions.
Which of the following antibiotics would routinely be tested and reported for isolates of Pseudomonas aeruginosa? a. penicillin b. erythromycin c. clindamycin d. gentamicin
*d. gentamicin* Aminoglycoside antibiotics such as gentamicin are active against Pseudomonas and routinely tested and reported on these isolates. Penicillin, erythromycin, and clindamycin are not active against Pseudomonas.
To quality control the autoclave, a vial of Bacillus stearothermophilus is autoclaved and should then be: a. inoculated to blood agar b. incubated at 37°C c. inoculated to chocolate agar d. incubated at 56 °C
*d. incubated at 56 °C* Bacillus stearothermophilus is commonly used as an indicator organism for the appropriate functioning of autoclaves. Unlike most Bacillus species, B stearothermophilus grows at 56°C.
Which one of the following combinations of organisms would be appropriate as controls to test the functions listed? a. beta-hemolysis-negative Escherichia coli and Streptococcus pyogenes b. catalase-negative Staphylococcus aureus and Staphylococcus epidermidis c. H₂S production-negative Proteus mirabilis and Staphylococcus epidermidis d. indole-negative Escherichia coli and Proteus mirabilis
*d. indole-negative Escherichia coli and Proteus mirabilis* Of the combinatins listed, the use of E coli and Proteus mirabilis will produce a positive and negative result for indole, respectively. The remainder of the organisms are all positive for the test described.
The *most* sensitive substance for the detection of beta-lactamases is: a. penicillin b. ampicillin c. cefoxitin d. nitrocefin
*d. nitrocefin* The chromogenic cephalosporin test using nitrocefin is the most sensitive and specific test for detection of beta lactamase. Acidimetric tests employing penicillin are less expensive, but not as sensitive, as the nitrocefin assay.
Which of the following antimicrobials would be inappropriate to report on an E coli isolated from a wound culture? a. gentamicin b. ampicillin c. cefazolin d. nitrofurantoin
*d. nitrofurantoin* Certain antimicrobials, such as nitrofurantoin and norfloxacin, are used only or primarily to treat urinary tract infections. These agents should not be reported for pathogens recovered from other sites of infection.
An expectorated sputum is sent to the laboratory for culture from a patient with respiratory distress. The gram stain of the specimen shows many squamous epithelial cells (> 25/lpf) and rare neutrophils. The microscopic appearance of the organisms present include: a. moderate gram-positive cocci in chains and diplococci b. moderate gram-negative diplococci c. moderate palisading gram-positive bacilli all in moderate amounts This gram stain is *most* indicative of: a. a pneumococcal pneumonia b. an anerobic infection c. an Haemophilus pneumonia d. oropharyngeal flora
*d. oropharyngeal flora* Sputum specimen quality is assessed to determine if a specimen is representative of the site of infection. The presence of white blood cells is an indicator of infection, and presence of squamous epithelial cells is an indicator of oropharyngeal contamination. In this specimen, > 25 epithelial cells per lpf is an indicator of poor specimen quality, and the bacteria present are representative of oropharyngeal flora.
Susceptibility testing is performed on a Staphylococcus aureus isolate from a blood culture with the following results: oxacillin: resistant cefazolin: susceptible clindamycin: susceptible erythromycin: susceptible trimethoprim/sulfamethoxazole: susceptible vancomycin: susceptible What should the technologist do next? a. ceftriaxone should be reported instead of cefazolin b. clindamycin should be tested for inducible resistance prior to reporting c. the trimethoprim/sulfamethoxazole result should be removed since all S aureus are resistant d. the cefazolin result should be changed to resistant since the oxacillin result is resistant
*d. the cefazolin result should be changed to resistant since the oxacillin result is resistant* Oxacillin resistant staphylococci are resistant to all beta-lactam agents, beta-lactam/beta-lactamase inhibitor combinations and carbapenems. Results for these antibiotics should be reported as resistant or should not be reported.
Vibrio parahaemolyticus can be isolated best from feces on: a. eosin methylene blue (EMB) agar b. Hektoen enteric (HE) agar c. Salmonella Shigella (SS) agar d. thiosulfate citrate bile salts (TCBS) agar
*d. thiosulfate citrate bile salts* TCBS is a highly selective and differential medium for the recovery of most Vibrio species including V parahaemolyticus. Hektoen and Salmonella-Shigella agars are selective and differential for the isolation and differentiation of enteric pathogens such as Salmonella and Shigella. EMB is a selective and differential medium for gram-negative enteric bacilli.
Which of the following specimen requests is acceptable? a. feces submitted for anaerobic culture b. Foley catheter tip submitted for aerobic culture c. rectal swap submitted for direct smear for gonococci d. urine for culture of acid-fast bacilli
*d. urine for culture of acid-fast bacilli* Urine is an appropriate specimen for the detection of renal tuberculosis. Since feces contain anaerobic organisms as part of the indigenous flora, it is an unacceptable specimen for anaerobic culture. Foley catheter tips are also not acceptable for culture, because they are contaminated with colonizing organisms. Gram stain smears of rectal swabs for N gonorrhoeae should also not be performed, since the presence of organisms with similar morphologies may lead to overinterpretation of smears.
The procedure that assures the *most* accurate detection of mec-A-mediated oxacillin resistance in routine broth microdilution susceptibility testing against S. Aureus is: a. addition of 4% NaCl b. incubation at 30°C c. incubation for 48 d. use of cefoxitin for testing
*d. use of cefoxitin* Cefoxitin is used as a surrogate for mecA-mediated oxacillin resistance in S. Aureus. S. Aures with cefoxitin MICs >4 ug/mL are considered oxacillin resistant. The Clinical Laboratory Standards Institute (CLSI) recommends addition of 2% NaCl, incubation at 35 +/- 2⁰C, and incubation for 24 hours when performing susceptibility testing of S. aureus against oxacillin.
When using a control strain of Staphylococcus aureus, the technologist notices that the zone around the oxacillin disk is too small. Which of the following is the *most* likely explanation? a. inoculation of the plates 10 minutes after preparing the inoculum b. incubation of the Muller-Hinton plates at 35°C c. use of a 0.25 McFarland standard to prepare inoculum d. use of outdated oxacillin disks
*d. use of outdated oxacillin disks* Deterioration of the antimicrobial agent in the disk will cause the zone sizes to be too small (falsely resistant). Standardization of the inoculum turbidity to less than a 0.5 McFarland standard would result in an inoculum that is too light and resulting zone sizes that are too large. Incubation of the plates at 35°C and inoculating plates within 10 minutes of preparation would not have an adverse effect on zone sizes.
A 21-year-old patient presents with pharyngitis. A throat swab is collected and submitted for anaerobic culture. This specimen should be: a. set up immediately b. rejected as unacceptable c. inoculated into thioglycollate broth d. sent to a reference laboratory
*rejected as unacceptable* Many anaerobic bacteria are commensal flora in the oropharynx. Anaerobic bacteria do not cause pharyngitis. The most common cause of pharyngitis is Streptococcus pyogenes. Other causes include Arcanobacterium haemolyticum, Corynebacterium diphtheriae, Neisseria gonorrhoeae and viruses.
A tech is reading a Gram stain from CSF and observes many neutrophils and lancet- shaped gram-positive diplococci. Which set of Chemistry and hematology CSF results would *most* likely be seen in someone with this type of infection? CSF Results: WBC Glucose Protein A increased increased increased B decreased decreased decreased C increased decreased increased D decreased increased decreased a. result A b. result B c. result C d. result D
*result C* The classic CSF alternations associated with bacterial meningitis are a high WBC count with a neutrophil predominance as well as a low CSF glucose and a high CSF protein.