Chapter 6: Bacteriology (Aerobic Gram-Positive Bacteria)

1. A test for the hydrolysis of esculin in the presence of bile is especially useful in identifying species of the genus A. Abiotrophia B. Corynebacterium C. Enterococcus D. Staphylococcus

1. C. Enterococcus and other group D streptococci can be presumptively identified based on their ability to hydrolyze esculin in the presence of 1-4% bile salts. The medium is made selective for enterococci by the addition of either sodium azide or 4% bile salts. Organisms able to grow on this medium and hydrolyze esculin produce esculetin, which reacts with an iron salt to form a black color in the agar.

10. In the CAMP test, a single streak of a beta-hemolytic Streptococcus is placed perpendicular to a streak of beta-lysin producing Staphylococcus aureus. After incubation, a zone of increased lysis in the shape of an arrowhead is noted; this indicates the presumptive identification of A. S. agalactiae B. S. bovis C. S. equinus D. S. pyogenes

10. A. Streptococcus agalactiae isolates can be presumptively identified by the demonstration of a positive CAMP reaction. CAMP is an acronym for the scientists (Christie, Atkins, MunchPetersen) who discovered this phenomenon!. Group B streptococci elaborate the CAMP factor, which acts to enhance the zone of hemolysis produced by beta-lysin-producing strains of Staphylococcus aureus. Incubation of test plates should be earned out in ambient air, because increased CO, and anaerobic incubation increase the rate of false-positive CAMP reactions by group A streptococci.

11. Staphylococcus saprophyticus, a recognized pathogen, is a cause of A. Furuncles B. Impetigo C. Otitis media D. Urinary tract infections

11. D. The recovery rate of coagulase-negative Staphylococcus saprophyticus from urinary tract infections in young females is second only to that of Escherichia coli. The organism has a predilection for the epithelial cells of the urogenital tract and is often seen in large numbers adhering to these cells on Gram stain. Key to the identification of this coagulase-negative Staphylococcus is its resistance to novobiocin.

12. Color Plate 26 • shows the Gram stain of a blood culture on a 23-year-old pregnant woman who presented with fever and flulike symptoms in her ninth month. The isolate on blood agar produced small, translucent beta-hemolytic colonies. Which of the following is the most likely etiologic agent in this case? A. Listeria monocytogenes B. Propionibacterium acnes C. Streptococcus agalactiae D. Streptococcus pyogenes

12. A. The organism seen in Color Plate 26 is Listeria monocytogenes. Listeria is an important animal and human pathogen that is known to cause abortion, meningitis, and septicemia in humans. This gram-positive rod is actively motile at room temperature (but not at 35°C), hydrolyzes esculin, produces catalase, and is oxidase negative. When recovered on sheep blood agar plates from clinical samples, it is often initially confused with group A or group B streptococci because of its beta-hemolysis.

13. The etiologic agent most commonly associated with septicemia and meningitis of newborns is A. Streptococcus agalactiae B. Streptococcus bovis group C. Streptococcus pneumoniae D. Streptococcus pyogenes

13. A. Streptococcus agalactiae (group B Streptococcus) is a principal cause of bacterial meningitis and septicemia in neonates. The organism, which is a part of the indigenous microbial flora of the vagina, is transmitted by the mother before birth, usually as the baby passes through the birth canal. Neonatal infection with group B streptococci may occur either as an early-onset disease (at birth) or as a delayed-onset syndrome that manifests itself weeks after birth.

14. Which of the following is the most commonly isolated species of Bacillus in opportunistic infections such as bacteremia, post-traumatic infections of the eye, and endocarditis? A. B. circulans B. B. cereus C. B. licheniformis D. B. subtilis

14. B. The vegetative cells and spores of Bacillus cereus are widely distributed in the environment. The virulence mechanisms of B. cereus are an enterotoxin and a pyogenic toxin. Accidents in nature resulting in cuts or abrasions contaminated with soil or vegetation, intravenous drug abuse, ingestion of contaminated foods, and traumatic introduction into a normally sterile site through the use of contaminated medical equipment are associated with infection.

15. Loeffler's serum medium is recommended for the cultivation of A. Abiotrophia sp. B. Corynebacterium diphtheriae C. Leuconostoc sp. D. Streptococcus agalactiae

15. B. The formation of the characteristic Corynebacterium diphtheriae granules and cellular morphology seen in methylene blue stains is enhanced when the organism is grown on Loeffler's serum medium. Although this medium is primarily designed for the recovery of C. diphtheriae from clinical samples, it is not a differential medium. The agar slant, when inoculated, may demonstrate growth of corynebacteria within 8 to 24 hours.

16. On Tinsdale agar, colonies of Corynebacterium diphtheriae are characterized by the observance of A. Liquefaction of the agar surrounding the colonies on the medium B. Opalescent colonies with a white precipitate in the surrounding agar C. Black colonies on the culture medium surrounded by brown halos D. Pitting of the agar medium surrounding the colonies

16. C. Tinsdale medium, for the primary isolation of Corynebacterium diphtheriae, not only inhibits indigenous respiratory flora but differentiates colonies of C. diphtheriae. The potassium tellurite in the medium is taken up by colonies of Corynebacterium, causing them to appear black. Colonies of C. diphtheriae are presumptively identified when black colonies surrounded by a brown halo are seen on this agar medium. However, other corynebacteria and some staphylococci will produce a similar reaction.

17. Precipitates of diphtheria toxin and antitoxin formed in agar gels are an in vitro means for detecting toxigenic strains of Corynebacterium diphtheriae. The name of this test procedure is the A. D-test B. Elektest C. Hodge test D. Naglertest

17. B. The Elek immunodiffusion test is recommended for detecting toxigenic strains of Corynebacterium diphtheriae. In the test, diphtheria antitoxin is impregnated on a sterile filter paper strip, which is pressed onto the surface of an Elek agar plate. Test and control strains are then inoculated perpendicular to the strip on both sides and without touching the strip. A positive reaction by toxigenic strains produces a precipitin line at a 45-degree angle to the inoculum streak.

18. The etiologic agent of the disease erysipelas is A. Staphyloccus aureus B. Streptobacillus moniliformis C. Streptococcus agalactiae D. Streptococcus pyogenes

18. D. Erysipelas results from person-to-person transmission of group A streptococci. Symptoms occur when nasopharyngeal infection spreads to the face. The rare complication of an upper respiratory infection with Streptococcus pyogenes is characterized by sensations of burning and tightness at the site of invasion. Erythema associated with this superficial cellulitis rapidly spreads with an advancing elevated margin. Erysipelothrix rhusiopathiae causes a similar disease referred to as erysipeloid.

19. Staphylococcus aureus, when present, could most likely be recovered from a stool sample if the primary plating medium included A. Bismuth sulfite B. Phenylethyl alcohol C. Thiosulfate citrate bile salts sucrose D. Xylose-lysine-desoxycholate

19. B. Phenylethyl alcohol agar (PEA) is a selective medium for the isolation of gram-positive cocci. Blood agar medium is supplemented with 0.15% phenylethyl alcohol, which is inhibitory to most gram-negative aerobic bacilli. This medium is particularly helpful when a specimen containing gram-positive cocci is contaminated with a Proteus spp. due to the inhibition of swarming by PEA.

2. The organism associated with a disease characterized by the presence of a pseudomembrane in the throat and the production of an exotoxin that is absorbed into the bloodstream with a lethal effect is A. Arcanobacterium haemolyticum B. Staphylococcus aureus C. Streptococcus pyogenes D. Corynebacterium diphtheriae

2. D. Strains of Corynebacterium diphtheriae infected by a lysogenic bacteriophage produce an extremely potent exotoxin. Absorption of the toxin may cause a rapidly fatal hypertoxic disease characterized by myocarditis and neuritis. This disease most commonly affects children aged 1 to 10 years. Transmission is by contact with a human carrier or with contaminated fomites.

20. A common member of the normal flora of the upper respiratory tract is A. Corynebacterium jeikeium B. Lactobacillus C. Staphylococcus epidermidis D. Viridans streptococcus

20. D. Viridans streptococci are the most common normal flora in upper respiratory cultures. They are opportunistic pathogens with low virulence. Subacute endocarditis is seen in patients with previously damaged heart valves.

21. Streptococci obtain all their energy from the fermentation of sugars to A. Formic acid B. Lactic acid C. Succinic acid D. Valeric acid

21. B. Whether growing aerobically or anaerobically, streptococci obtain all their energy from the fermentation of sugars to lactic acid. Streptococci are all catalase negative and grow on coventional media such as sheep blood agar. Most are part of the normal flora of human skin, throat, and intestine but produce a wide variety of infections when introduced in tissues or blood.

22. Streptococci are unable to synthesize the enzyme A. Catalase B. Kinase C. Hyaluronidase D. Lipase

22. A. Organisms that synthesize the enzyme catalase are able to protect themselves from the killing effects of H2O2 by converting it to H2O and O2. Streptococci are unable to synthesize the heme prosthetic group for this enzyme and are catalase negative. Therefore, they grow better on blood-containing media because of the catalaselike activity of hemoglobin.

23. The beta-hemolysis produced by group A Streptococcus seen on the surface of a sheep blood agar plate is primarily the result of streptolysin A. H B. M C. O D. S

23. D. Streptolysin S is primarily responsible for the beta-hemolysis seen on the surface of a sheep blood agar plate inoculated with a group A streptococcus. Of the two hemolysins secreted by beta-hemolytic group A streptococcus, Streptolysin S is stable in the presence of atmospheric oxygen. Streptolysin O is inactivated in the presence of oxygen, and it is best demonstrated when the agar has been stabbed and subsurface hemolysis is revealed.

24. When an infection due to Streptomyces is suspected, isolates can be separated from most other bacteria by A. Heat shocking the culture B. Incubating the culture at 25°C C. Incubating the culture at greater than 37°C D. Drying the specimen before inoculating the culture media

24. B. Streptomyces are weak pathogens rarely associated with disease. The bacteria normally inhabit the soil. The most common human infection is myectoma, which is most frequently caused by S. somaliensis. Some strains of Streptomyces grow better at 25°C than at 35°C.

25. The production of H2S is one characteristic used to differentiate which of the aerobic gram-positive bacilli? A. Corynebacterium B. Erysipelothrix C. Lactobacillus D. Nocardia

25. B. Erysipelothrix is a nonmotile, catalase negative, gram-positive bacillus that often appears as long filaments. Unlike other aerobic grampositive bacilli, this organism produces H2S, which can be demonstrated in triple sugar iron agar. Erysipeloid, a skin disease of the hands usually associated with the handling of infected animals, is the human infection produced most commonly by this agent.

26. Growth in a 48-hour semisolid agar stab culture at room temperature reveals lateral filamentous growth away from the stab near the top of the medium. This observation is most characteristic of which organism? A. Rhodococcus sp. B. Corynebacterium urealyticum C. Enterococcusfaecalis D. Listeria monocytogenes

26. D. Listeria monocytogenes is motile at room temperature. When inoculated into a semisolid medium, growth away from the stab is characteristic of motility. Motility is generally enhanced just below the agar surface, giving the growth pattern an "umbrella" appearance. L. monocytogenes is nonmotile at 35°C.

27. A former species of Corynebacterium pathogenic for swine, horses, and cattle is also known to cause disease in compromised hosts. This organism when grown on culture media produces pale pink colonies that help to presumptively identify it as A. Arcanobacterium hemolyticum B. Actinomyces naeslundii C. Gardnerella vaginalis D. Rhodococcus equi

27. D. Rhodococcus equi is found in soil and commonly produces disease among livestock. These gram-positive bacilli can demonstrate primary mycelia and were formerly in the genus Nocardia. This species is characterized by its pink pigmentation on culture media and its inability to ferment carbohydrates.

28. Which one of the following is not appropriate when describing Streptococcus pneumoniae ? A. Bile-resistant B. Alpha-hemolytic C. Lancet-shaped, gram-positive diplococcus D. Virulent strains are encapsulated.

28. A. Streptococcus pneumoniae, a primary etiologic agent of lobar pneumonia, is an encapsulated, gram-positive, lanceolate diplococcus. Fastidious in its growth requirements, the organism on sheep blood agar produces characteristic alpha-hemolytic colonies, which are convex and often mucoid in appearance and bile soluble. Upon aging, colonies of S. pneumoniae undergo autolytic changes. There are approximately 80 types of pneumococci based on specific capsular antigens.

29. Nocardia can be differentiated from Actinomyces based on A. Nocardia being an obligate anaerobe B. The partial-acid fast staining reaction of Actinomyces C. The production of sulfur granules in cases of nocardiosis D. Nocardia being catalase positive

29. D. Species of the genus Nocardia are ubiquitous in the soil and thus characteristically produce exogenous forms of infection as a result of inhalation of contaminated fomites or a traumatic incident with soil contamination. A diagnostic characteristic, depending on the species, is the acid fastness of the filamentous bacilli or coccoid forms. Unlike Actinomyces spp., which are catalase-negative, gram-positive, non-sporeforming anaerobic bacilli, Nocardia spp. are catalase-positive aerobic organisms. "Sulfur granules" are characteristic of actinomycotic pus and upon examination would reveal nonacid-fast branching filaments.

3. Enterotoxin produced by Staphylococcus aureus is responsible for causing A. Carbuncles B. Enterocolitis C. Impetigo D. Scalded skin syndrome

3. B. Staphylococcal enterocolitis food poisoning cases result from the ingestion of contaminated foods containing preformed thermostable enterotoxin. This form of intoxication causes a perfuse and watery diarrhea due to the loss of electrolytes and fluids into the lumen. In many cases, the causative agent may never be recovered from patient specimens

30. Enterococcus faecium is characteristically A. Inhibited by the presence of bile in culture media B. Able to grow in the presence of high concentrations of salt C. PYR negative D. Beta-hemolytic

30. B. Enterococcus faecium is an important agent of human infection. Their differentiation from other enterococcal strains is of importance because of their resistance to most clinically useful antimicrobial agents, including vancomycin. The ability to tolerate a high concentration of salt is characteristic of the clinically significant species of Enterococcus. E. faecium is PYR positive and is usually nonhemolytic.

31. A negative PYR (L-pyrolidonyl-a- naphthylamide) test is demonstrated by A. Enterococcus faecalis B. Enterococcus faecium C. Streptococcus pyogenes D. Viridans streptococci

31. D. Viridans streptococci do not produce the enzyme pyroglutamyl aminopeptidase and, therefore, in the PYR test do not produce a positive or red color. The PYR test is used predominantly for the presumptive identification of group A streptococci and Enterococcus. Micrococcus and Lactococcus are known to produce a positive reaction as well, although the reaction may be delayed.

32. A Gram stain of a sputum specimen from a patient with a suspected case of lobar pneumonia reveals many white blood cells and many gram-positive cocci, which are primarily diplococci. Which of the following statements would be appropriate, given these findings? A. A PYR test should be performed on the culture isolate. B. An Elek test should be performed on the culture isolate. C. An optochin test should be performed on the culture isolate. D. A hippurate hydrolysis test should be performed on the culture isolate.

32. C. Streptococcus pneumoniae is a leading cause of lobar pneumonia as well as other serious bacterial infections. The Gram stain smear of clinical specimens can provide a rapid presumptive diagnosis when the characteristic morphology and Gram reaction is observed. The optochin disk test can be performed to presumptively identify this organism. Optochin lyses pneumococci, producing a zone of inhibition around the disk.

33. A child presented in August at the pediatric clinic with a superficial skin infection of the neck. The large, itchy lesions were cultured, and the diagnosis of impetigo was made. One of the etiologic agents of this clinical condition is A. Erysipelothrix rhusiopathiae B. Corynebacterium diphtheriae C. Staphylococcus saprophyticus D. Streptococcus pyogenes

33. D. Bacteriologic cultures of a typical impetigo lesion may yield either a pure culture of Streptococcus pyogenes or a mixed culture of S. pyogenes and Staphylococcus aureus. The thick crust form of impetigo, which is most commonly seen, is primarily caused by S. pyogenes. It is the bullous form of impetigo for which S. aureus is the etiologic agent. The route of infection is direct inoculation of the causative agents into abraded or otherwise compromised areas of the skin.

34. An identifying characteristic of Staphylococcus aureus is A. DNase negative B. Coagulase negative C. Mannitol fermentation postive D. Growth inhibition in presence of increased salt

34. C. Identifying characteristics of Staphylococcus aureus include the production of the extracellular enzymes coagulase and DNase and its ability to grow in the presence of high salt concentrations. Differential and selective media, such as mannitol salt agar, have been developed for the recovery of this organism. Selective media and rapid identification tests are important for this widely recognized opportunistic pathogen.

35. Which of the following organisms is able to hydrolyze sodium hippurate to benzoic acid and glycine? A. Streptococcus agalactiae B. Streptococcus pneumoniae C. Listeria monocytogenes D. Enterococcus faecalis

35. A. Group B streptococci (Streptococcus agalactiae), unlike other streptococci, can hydrolyze sodium hippurate to benzoic acid and glycine. If glycine is produced, the addition of ninhydrin to the medium will reduce the glycine to produce a purple color. The use of ninhydrin to detect glycine is a sensitive and rapid test of hippurate hydrolysis.

36. Which of the following is not characteristic of Listeria monocytogenes ? A. CAMP test positive B. Catalase negative C. Esculin hydrolysis positive D. Motile

36. B. Listeria monocy to genes and Streptococcus agalactiae produce an extracellular factor known as the CAMP factor. The test is performed by making a streak of the test isolate perpendicular to a streak of Staphylococcus aureus. A positive CAMP reaction is indicated by a zone of enhanced beta-hemolysis (arrowhead shape) at the point where the zone of hemolysis produced by S. aureus joins with that produced by the beta-hemolytic test isolate. Unlike 5. agalactiae, L. monocytogenes is catalase positive.

37. Which of the following is not associated with Staphylococcus aureus? A. Endotoxin production B. Clumping factor production C. Deoxyribonuclease production D. Hemolysin production

37. A. The production of hemolysins and the enzymes coagulase and DNase is associated with the virulence of staphylococci. The coagulaseproducing staphylococci are most commonly producers of staphylolysins, which produce betahemolysis when the isolate is grown on sheep blood agar. Many factors contribute to staphylococcal virulence by overcoming the host's natural defenses. Endotoxin is found in the cell wall of gram-negative bacteria.

38. Which of the following is a characteristic of staphylococci that would help in their isolation from clinical specimens? A. Bile resistance B. Growth at 55°C C. High salt tolerance D. Resistance to novobiocin

38. C. The physiology of staphylococci enables them to remain infectious in the environment longer than many other pathogenic bacteria. Staphylococci are somewhat heat resistant and can survive dry conditions. Their high salt tolerance enables strains to grow in salt-preserved foods and causes cases of food poisoning. Staphylococci, however, cannot resist temperatures as high as 55 °C for long periods, and they are not bile resistant. Most species are sensitive to novobiocin.

39. Which of the following species of Bacillus is nonmotile? A. B. cereus B. B. subtilis C. B. anthracis D. B. thuringiensis

39. C. Motility is a key test for the differentiation of Bacillus anthracis from other species of Bacillus. Suspect Bacillus colonies are inoculated in a broth medium and allowed to grow to a visible turbidity. A sample of this actively growing culture should be examined using the hanging-drop technique for motility. B. anthracis is nonmotile and can therefore be easily differentiated from commonly encountered motile species.

4. Abiotrophia, formerly known as nutritionally variant streptococci, will not grow on routine blood or chocolate agars because they are deficient in A. Hemin B. Pyridoxal C. Vitamin B12 D. Thiophene-2-carboxylic hydrazide

4. B. Nutritionally variant streptococci (NVS) are now termed Abiotrophia. These clinically significant microorganisms, which account for 5-6% of the cases of endocarditis, are frequently not able to be recovered because of insufficient quantities of vitamin B6 in the culture medium. The routine use of a pyridoxal disk, a streak of Staphylococcus, or vitamin B6-supplemented culture media is required for isolation.

40. Which one of the following diseases involves erythrogenic toxin? A. Cutaneous anthrax B. Diphtheria C. Impetigo D. Scarlet fever

40. D. The rash of scarlet fever is a result of the action of an erythrogenic toxin produced by group A streptococci. Because of the rapid diagnosis and treatment of group A streptococci infections, scarlet fever is rare in most developed countries. The other diseases listed do not involve an erythrogenic toxin.

41. Cultures of the posterior pharynx are most commonly submitted to the clinical laboratory for the detection of A. Corynebacterium diphtheriae B. Staphylococcus aureus C. Streptococcus pneumoniae D. Streptococcus pyogenes

41. D. Cultures of the tonsillar fossae and posterior pharynx are most commonly obtained in suspected cases of streptococcal pharyngitis. Streptococcus pyogenes is most often associated with cases of pharyngitis but is also the agent of scarlet fever and erysipelas in addition to wound infections (e.g., necrotizing fasciitis). Rapid identification of this organism and prompt antimicrobial therapy are required to prevent sequelae (i.e., rheumatic fever and acute glomerulonephritis).

42. Streptococcus sanguis, a viridans streptococcus, is most commonly associated with which of the following clinical conditions? A. Otitis media B. Pharyngitis C. Relapsing fever D. Subacute bacterial endocarditis

42. D. Subacute bacterial endocarditis is an inflammation of the lining membrane of the heart, which most often is caused by a member of the viridans group of streptococci. Streptococcus sanguis is one of several species that may lodge in an abnormal heart or on valves damaged by previous infection. Viridans streptococci are normal inhabitants of the human upper respiratory tract.

43. Rust-colored sputum in cases of lobar pneumonia is characteristic of which of the following possible etiologic agents? A. Corynebacterium jeikeium B. Staphylococcus aureus C. Streptococcus pneumoniae D. Streptococcus pyogenes

43. C. Streptococcus pneumoniae is most commonly associated with cases of lobar pneumonia. Patients characteristically produce blood-tinged, rust-colored sputum in which the characteristic gram-positive lanceolate diplococci can be found. S. pneumoniae forms alpha-hemolytic colonies when grown on sheep blood agar

44. A urine culture from a 23-year-old female grew a catalase-positive gram-positive coccus (> 100,000 cfu/mL), which would most likely be A. Staphylococcus saprophyticus B. Enterococcus faecalis C. Streptococcus bovis group D. Streptococcus viridans

44. A. Staphylococcus saprophyticus is recognized as an etiologic agent of uncomplicated cystitis cases in young females. These nonhemolytic, coagulase-negative staphylococci closely resemble S. epidermidis on sheep blood agar. Identification of 5. saprophyticus is facilitated by demonstrating its resistance to novobiocin.

45. Cystine-tellurite blood agar plates are recommended for the isolation of A. Corynebacterium diphtheriae B. Streptococcus agalaciae C. Streptococcus pyogenes D. Group D streptococci

45. A. Clinical material sent to the laboratory for the recovery of Corynebacterium diphtheriae should be inoculated on cystine-tellurite agar plates or Tinsdale medium. On tellurite-containing media, colonies of this pathogen will appear dark-brown to black, which aids in their differentiation. Suspicious colonies should be further tested for their biochemical activity and toxin production.

46. The pulmonary form of anthrax is known as A. Valley fever B. Walking pneumonia C. Farmers' lung D. Woolsorters disease

46. D. Bacillus anthracis is the causative agent of woolsorters disease or the pulmonary form of anthrax. The mode of infection is the inhalation of spores by the patient, usually during the performance of his/her occupation (sheep shearing or processing of animal hair). Prompt diagnosis and treatment of this disease is needed because it is known to progress rapidly to a fatal form of septicemia.

47. Pleomorphic gram-positive bacilli in a Gram stain best describes A. Bacillus anthracis B. Bacillus subtilis C. Listeria monocytogenes D. Corynebacterium pseudodiphtheriticum

47. D. Corynebacterium pseudodiphtheriticum is morphologically similar to all other members of the genus Corynebacterium. They are all grampositive, non-spore-forming bacilli that characteristically resemble Chinese characters or palisades. These bacteria often stain irregularly and have a pleomorphic club-shaped appearance

48. An aerobic gram-positive rod known to cause bacteremia in hospitalized immunocompromised patients is A. Bacillus anthracis B. Corynebacterium jeikeium C. Corynebacterium ulcerans D. Corynebacterium urealyticum

48. B. Corynebacterium jeikeium is a low virulence organism resistant to multiple antimicrobials. Its multiple drug resistance allows it to remain in hospital environments, and it is often cultured from the skin of hospitalized patients. In compromised patients it has been implicated in cases of septicemia, wound infections, and endocarditis in association with intravenous catheter use.

49. A bone marrow transplant patient on immunosuppressive therapy developed a pulmonary abscess with symptoms of neurologic involvement. A brain abscess was detected by MRI, and aspirated material grew an aerobic, filamentous, branching gram-positive organism, which stained weakly acid-fast. The most likely etiologic agent in this case would be A. Actinomyces israelii B. Nocardia asteroides C. Mycobacterium tuberculosis D. Propionibacterium acnes

49. B. In immunocompromised patients, Nocardia asteroides can cause invasive pulmonary infection and can often spread hematogenously throughout the body. Lesions in the brain are commonly associated with dissemination and have a poor prognosis. The organism is ubiquitous in nature, and infection is acquired by traumatic inoculation or inhalation.

5. Exfoliatin produced by Staphylococcus aureus is responsible for causing A. Enterocolitis B. Toxic shock syndrome C. Scalded skin syndrome D. Staphylococcal pneumonia

5. C. Scalded skin syndrome is a form of dermatitis produced by strains of Staphylococcus aureus that elaborate exfoliative toxin. Two types of this toxin have been identified: exfoliation A and exfoliation B. This potent toxin acts by disturbing the adhesive forces between cells of the stratum granulosum, which causes the appearance of the clear, large, flaccid bullae and the skin to peel off. Infants and children are most commonly affected with this form of dermatitis, beginning about the face and trunk and subsequently spreading to the extremities.

50. Which of the following is catalase negative? A. Bacillus B. Corynebacterium C. Leuconostoc D. Listeria

50. C. Of the genera listed, only Leuconostoc is catalase negative. Leuconostoc is vancomycin resistant and associated with infections in hospitalized patients. It has also been linked to septicemias in neonates.

51. Colonies of Listeria monocytogenes on a sheep blood agar plate most closely resemble colonies of A. Corynebacterium diphtheriae B. Streptococcus agalactiae C. Streptococcus bovis group D. Rhodococcus equi

51. B. Listeria monocytogenes is a small, grampositive bacillus that is actively motile at room temperature. When grown on sheep blood agar, this organism produces small, translucent beta-hemolytic colonies, which may be visually mistaken for beta-hemolytic streptococci. Biochemically L. monocytogenes differs from streptococci because it possesses the enzyme catalase.

52. The most common etiologic agent of infections associated with the surgical insertion of prosthetic devices such as artificial heart valves and cerebrospinal fluid shunts is A. Corynebacterium urealyticum B. Staphylococcus capitis C. Staphylococcus epidermidis D. Streptococcus mutans

52. C. Staphylococcus epidermidis is a saprophytic microorganism found on the skin and mucous membranes of humans. This coagulase-negative Staphylococcus is seen frequently as a contaminant in blood cultures when proper venipuncture technique has not been used. S epidermidis has been implicated in serious human infections associated with the surgical insertion of prosthetic devices.

53. The description of "Medusa head" colonies on solid agar is most characteristic of A. Bacillus anthracis B. Enterococcus faecalis C. Staphylococcus saprophyticus D. Streptococcus agalactiae

53. A. Bacillus anthracis is the etiologic agent of human anthrax that occurs in any of three forms: cutaneous, pulmonary, and gastrointestinal. On Gram stain this organism appears as a large, spore-forming, gram-positive bacillus that characteristically grows in long chains. Colonies on agar plates are large and opaque with fingerlike projections referred to as "Medusa head" forms

54. Which of the following is most likely to be isolated in cultures from the anterior nares of healthcare workers? A. Bacillus cereus B. Streptococcus pneumoniae C. Staphylococcus aureus D. Staphylococcus saprophyticus

54. C. Staphylococci colonize various skin and mucosal surfaces in humans. S. aureus is carried as transient flora in the anterior nares. S. saprophyticus is less likely found as normal flora and is associated with urinary tract infections. Hospital personnel may harbor resistant strains of S. aureus, and person-to-person contact is a substantial infection control concern. Cultures of the anterior nares are recommended when screening for earners in the hospital environment.

55. Ethylhydrocupreine HC1 susceptibility is a presumptive test for the identification of A. Viridans streptococci B. Streptococcus pyogenes C. Streptococcus agalactiae D. Streptococcus pneumonias

55. D. The susceptibility of alpha-hemolytic streptococcal isolates to optochin, or ethylhydrocupreine HC1, is a presumptive test for the differentiation of Streptococcus pneumoniae from viridans streptococci. Viridans streptococci are typically resistant to this agent and show no zone of inhibition or a zone of less than 10 mm with a 6-mm disk. S. pneumoniae characteristically is susceptible and produces a zone of inhibition greater than 14 mm.

56. Solubility in the presence of sodium desoxycholate is characteristic of A. Enterococcus faecalis B. Streptococcus agalactiae C. Streptococcus mutatis D. Streptococcus pneumoniae

56. D. Solubility of Streptococcus pneumoniae colonies by surface-active agents, such as sodium desoxycholate, is a widely used presumptive identification procedure. When a 10% solution of this reagent is applied to test colonies, 5. pneumoniae will be totally dissolved. Colonies of viridans streptococci typically remain intact when bile is applied.

8. Nocardia asteroides infections in humans characteristically produce A. Carbuncles B. Draining cutaneous sinuses C. Septic shock D. Serous effusions

8. B. Nocardiosis is characterized by mycetoma or chronic suppurative infection. Draining sinus tracts in the subcutaneous tissue are a common manifestation of the disease. Nocardia spp. are soil saprophytes that may produce disease in humans either by the inhalation of contaminated material or through skin abrasions. Microscopic examination of pus from suspected cases will demonstrate partially acid-fast, gram-positive, branching filamentous or coccoid organism.

57. Family members attending a picnic became ill about 2 hours after eating. The illness was characterized by rapid onset of violent vomiting. The most likely bacterial cause of such symptoms would be food poisoning caused by A. Enterococcus faecium B. Bacillus subtilis C. Listeria monocytogenes D. Staphylococcus aureus

57. D. The ingestion of food contaminated with enterotoxin produced by Staphylococcus aureus is the most likely cause of the disease in the case described. 5. aureus multiplies rapidly in improperly stored food. Within a few hours, levels of 105 organisms per gram of food can be found. Enterotoxin is elaborated when the organism reaches stationary growth phase. Ingestion of small amounts of toxin results in a rapid onset (1-6 hours) of vomiting and diarrhea as a result of a neural response.

58. The novobiocin susceptibility test is used for the identification of A. Corynebacterium diphtheriae B. Streptococcus pyogenes C. Streptococcus pneumoniae D. Staphylococcus saprophyticus

58. D. Most strains of Staphylococcus saprophyticus are resistant to novobiocin. This organism is frequently found in urine culture of young women and may be misidentified as S. epidermidis. A 5-ug disk is used in the test, and a zone of 16 mm or less determines resistance.

59. Tellurite reduction is used for the presumptive identification of A. Bacillus anthracis B. Corynebacterium diphtheriae C. Erysipelothrix rhusiopathiae D. Staphylococcus saprophyticus

59. B. On serum-cystine-sodium thiosulfate-tellurite medium (Tinsdale medium), Corynebacterium diphtheriae is differentiated from other cornybacteria and other bacteria of the respiratory tract by its ability to produce black colonies surrounded by a brown-black halo after 48 hours of incubation. Growth factors needed by C. diphtheriae are provided by the addition of the serum. Potassium tellurite is inhibitory to many gram-positive and gram-negative bacteria, but corynebacteria are resistant.

6. Streptococcus pyogenes can be presumptively identified using a(an) A. PYR disk B. ONPG disk C. SPS disk D. Optochin disk

6. A. Presumptive identification of group A streptococci can be achieved through the PYR (L-pyrrolidonyl-(3-naphthlylamide) disk test. The use of a 0.04-unit bacitracin disk is no longer recommended because groups C and G streptococci are also susceptible to this agent. A positive test result is interpreted as a bright red color change within 5 minutes.

60. The etiologic agent of the majority of adult joint infections is A. Abiotrophia sp. B. Leuconostoc sp. C. Staphylococcus aureus D. Streptococcus pneumoniae

60. C. Staphylococcus aureus is the predominant pathogen involved in joint infections of adults. Bacterial arthritis can occur following infection in other parts of the body or bacteremia. Streptococcus pyogenes and Neisseria gonorrhoeae each account for a significant number of adult infections, whereas Streptococcus pneumoniae and Haemophilus influenzas predominate in childhood infections.

6.1. Which of the following is associated with infections in humans often linked to deli meats and improperly pasteurized dairy products? A. Bacillus subtilis B. Listeria monocytogenes C. Leuconostoc D. Streptococcus agalactiae

61. B. Listeria monocytogenes is a cause of human and bovine abortion. In humans, the mother's symptoms are usually mild, resembling the flu and causing a low-grade fever. The organism can be isolated from aborted fetuses as well as from the maternal placenta. When infection with this etiologic agent is detected early, appropriate therapy can be initiated, which may prevent the death of the fetus.

62. Bacillus cereus has been implicated as the etiologic agent in cases of A. Food poisoning B. Impetigo C. Pelvic inflammatory disease D. Toxic shock syndrome

62. A. Bacillus spp. are gram-positive, spore-forming bacilli widely found in the environment. Bacillus cereus is of particular interest as an etiologic agent of human cases of food poisoning. This enterotoxin-producing microorganism is most commonly associated with cases of food poisoning following ingestion of reheated rice served at Asian restaurants.

63. The causative agent of "malignant pustule" is A. Bacillus anthracis B. Corynebacterium ulcerans C. Erysipelothrix rhusiopathiae D. Listeria monocytogenes

63. A. Bacillus anthracis infects humans by three routes: respiratory, gastrointestinal, and cutaneous. Malignant pustule is the name given to lesions seen in cutaneous anthrax in humans. The lesion is, however, neither malignant nor a pustule. The disease produces a localized abscess on the skin, which forms a characteristic black eschar surrounded by a red raised ring.

64. An infant was hospitalized with a severe, tender erythema. The child's epidermis was loose, and large areas of skin could be peeled off. The condition described is most consistent with a clinical syndrome associated with A. Streptococcus pyogenes B. Staphylococcus aureus C. Bacillus anthracis D. Erysipelothrix rhusiopathiae

64. B. Scalded skin syndrome is the dermatitis associated with the effects of the exfoliative toxin produced by strains of Staphylococcus aureus. Exfoliatin acts in humans to disrupt the adhesive forces between cells of the stratum granulosum, creating large flaccid bullae. This syndrome occurs primarily in infants and children; the primary infection is usually unrelated to the areas where lesions appear.

65. A catalase-negative gram-positive coccus is isolated from a urine sample of a hospitalized patient. The bacterium produced a black pigment on bile-esculin agar and formed acid from glucose in the presence of 6.5% NaCl. What is the most likely identification of this bacterium? A. Abiotrophia sp. B. Enterococcus faecalis C. Group B streptococci D. Group D streptococci

65. B. Enterococcus faecalis and E. faecium grow in the presence of bile, hydrolyze esculin, and produce acid from glucose in the presence of high salt concentration. These bacteria also express streptococcal group D antigen. The ability to tolerate high salt concentrations differentiates the enterococci from the group D streptococci like the S. bovis group.

7. A gram-positive coccus that is catalase positive, nonmotile, lysostaphin resistant, and modified oxidase positive is best identified as a member of the genus A. Micrococcus B. Lactococcus C. Pediococcus D. Staphylococcus

7. A. Staphylococci and micrococci are both catalase positive gram-positive cocci. Staphylococci are more clinically significant, so it is important to differentiate Micrococcus from Staphylococcus. Micrococci are modified oxidase positive, whereas Staphylococci are negative.

9. Erysipelothrix infections in humans characteristically produce A. Pathology at the point of entrance of the organism B. Central nervous system pathology C. Pathology in the lower respiratory tract D. The formation of abscesses in visceral organs

9. A. Infection caused by Erysipelothrix rhusiopathiae in humans is primarily erysipeloid. Erysipeloid is usually the result of contact with an infected animal or contaminated animal product. The characteristic presentation is cutaneous spreading lesions of the fingers or hand that are raised and erythematous. Although generally confined to the skin, E. rhusiopathiae has been implicated in rare cases of endocarditis.