Exam 5
What are the identifying characteristics of Acinetobacter spp.?
Acinetobacter spp. are gram-negative coccobacillary organisms rather than true bacilli. They are oxidase-negative, which is uncharacteristic of most other nonfermenters. In addition, they have a bluish-purple appearance on MacConkey agar and are often susceptible to the antimicrobial combination of ampicillin and sulbactam, which is unusual for many other nonfermenters.
An oxidase-positive, indole-positive, β-hemolytic, gram-negative bacillus resistant to O/129, cannot grow in 6% NaCl broth, and is Voges-Proskauer positive is isolated from an adult stool culture. You should suspect:
Aeromonas hydrophila
An organism that can live in reduced concentrations of oxygen but prefers an anaerobic environment is known as a(n):
Aerotolerant anaerobe
When attempting to recover enteric Campylobacter spp., which specimen, media, and incubation conditions should be used?
Appropriate specimens for the isolation of enteric campylobacters are stool samples and rectal swabs; stool samples are preferred. Two categories of media are available for isolation: blood-based and charcoal-based media. A commonly used blood-based medium is CAMPY-BAP. This is a Brucella agar-based medium, with 10% sheep red blood cells and a combination of antimicrobials. Charcoal cefoperazone desoxycholate agar is an alternative. The addition of antimicrobial agents and incubation of the plates at 42° C inhibits normal fecal biota. Because the campylobacters require oxygen at a concentration less than room air, they must be incubated in a microaerophilic atmosphere.
Which statement about assays for the antigenic detection of both Clostridioides difficile toxins A and B is TRUE?
The antigenic assays give rapid results.
Which of the following organisms is Gram-negative?
Bacillus Vibrio Staphylococcus Nocardia
Darting motility is a characteristic of:
Campylobacter
A small, microaerophilic, curved, gram-negative rod which was isolated from a diarrhea stool specimen gave the following results: Grew best at 42 ºC Oxidase positive Resistant to cephalosporin Susceptible to nalidixic acid Hippurate hydrolysis positive
Campylobacter jejuni
An organism grew at 37° C and 42° C from a stool culture that was oxidase, catalase, and hippurate positive. This organism will be identified as:
Campylobacter jejuni
The recovery of this bacterial species from diarrhea stool specimens requires the use of selective culture media such as Butzler selective media or Skirrow blood agar incubated at 42° C in an atmosphere of 5% oxygen, 10% CO2, and 85% nitrogen. Growth on Skirrow blood agar as illustrated in the upper image are confluent smooth non-hemolytic colonies growing along the streak line with lateral extensions (arrows). Slender, long branching, S-shaped Gram negative rods are observed on Gram stain, as observed in the lower image. Positive reactions for oxidase, indoxyl, and hippurate provide for a more definitive identification. With these observations, select the name of this isolate.
Campylobacter jejuni
Which of the following is a selective media recommended for the isolation of Clostridioides difficile?
Cefoxitin-cycloserine-fructose agar (CCFA)
A patient presents at the ER with severe diarrhea and dehydration, marked with rice-water stool. The patient has a travel history to India. A stool culture was obtained and grew comma-shaped, gram-negative bacilli. What is the most likely diagnosis for this patient?
Cholera
Rice water stools that are most commonly due to ingestion of contaminated water and have been a major public health concern for centuries, exhibit colonies on TCBS agar consistent with those in the image to the right. These colonies are caused by a condition called:
Cholera
The irregular-sized, gray-white colonies, with irregular margins, as shown in the image to the left in the upper composite photograph, were recovered from a stool specimen in a case of antibiotic-associated colitis. The colonies are flat, gray-white, and spreading with irregular margins. Some patients with this infection develop pseudomembranous colitis that can be life-threatening. Growth is enhanced when Cycloserine-cefoxitin-fructose agar (CCFA) is used, as seen in the upper right photograph. The observation of elongated, gram-positive bacilli with distinctive subterminal and central spores in Gram stain mounts provides for presumptive identification. Additional key identifying reactions are the absence of lipase and lecithinase and a chartreuse pigment under fluorescence light. With these observations, select the presumptive identification of this isolate.
Clostridioides difficile
What organism is responsible for a potentially lethal type of food poisoning caused by improperly canned food?
Clostridium botulinum
A gram-positive bacillus was isolated from a wound specimen and had the following characteristics: double zone of β-hemolysis, lecithinase positive, lipase negative, spot indole negative. What is the most likely identification of this organism?
Clostridium perfringens
A sample from a wound infection, suspected case of gas gangrene, was cultured and showed anaerobic colonies that are small, smooth, light gray-yellow, and surrounded by a distinctive double zone of beta hemolysis (arrows). Gram stain, as illustrated in the lower photomicrograph, are large, non-spore forming, "box-car" shaped gram-positive bacilli. What is this organism that is also related to outbreaks of food-borne diarrhea following ingestion of contaminated meat products?
Clostridium perfringens
Which of the following organisms is responsible for myonecrosis with gas gangrene, food poisoning as well as necrotizing enteritis, a life threatening that causes ischemic necrosis of the jejunum?
Clostridium perfringens
Which of the following organisms produces a double zone of hemolysis on blood agar?
Clostridium perfringens
Which of the following is the best media for recovery and presumptive identification of Clostridioides (formerly Clostridium) difficile in suspected cases of pseudomembranous colitis?
Cycloserine-cefoxitin fructose agar (CCFA)
Campylobacter jejuni is most noted for causing:
Gastroenteritis
Which of the following statements is correct regarding the glutamate dehydrogenase (GDH) assay for Clostridioides difficile?
Detection of the GDH gene and 16srRNA genes for C. difficile without the presence of a toxin gene indicates a carrier or nonpathogenic state.
The following statements regarding the cell cytotoxicity neutralization assay (CCNA) for Clostridiodes (formerly Clostridium) difficile are correct EXCEPT?
Detects toxin B in stool samples Detects cytopathic effect on human cells Provides rapid results Procedure is complex
Endogenous anaerobes least likely to be involved in cases of bacteremia are:
Eubacterium
Clostridium spp. are especially easy to identify in Gram-stained smears of clinical specimens because they always appear as gram-positive rods with terminal or subterminal spores.
FALSE
Exogenous anaerobes more commonly cause infectious diseases than endogenous anaerobes.
FALSE
Based on carbohydrate utilization, what is the difference between nonfermentative and fermentative organisms?
Fermentative gram-negative bacilli are able to metabolize carbohydrates to derive energy under anaerobic conditions. Phenotypically, these bacteria can produce an acid "butt" in specific media (e.g., triple sugar iron [TSI] agar or Kligler iron agar [KIA]). Nonfermenters cannot ferment sugars and are not able to acidify the butt of TSI or KIA.
Which of the following is a risk factor for acquiring V. alginolyticus infection?
Fishing or swimming in ocean water
A microbiologist was surprised when he could not recover Helicobacter pylori from gastric biopsies in which organisms were seen in the tissue sections. He was advised to switch from the Campy BAP selective medium to Skirrow's blood agar because the latter is:
Free of cephalothin
An anaerobic, pleomorphic, gram-negative bacillus was recovered from a liver abscess. The special potency antimicrobial disks demonstrated that the organism was vancomycin resistant and colistin and kanamycin sensitive. Other results were as follows: chartreuse fluorescence, spot indole positive, and lipase positive.
Fusobacterium necrophorum
Which of the following genera is typically microaerophilic?
Helicobacter
A culture was performed on a stool sample. After growth was obtained on the blood agar plate, the gram stain showed curved gram-negative rods. The oxidase, catalase, and urease were positive. What is the most likely bacterial identification?
Helicobacter pylori
The bacterial species presented here serves as the causative agent of active chronic gastritis of the lower pyloric antrum of the stomach with the complications of peptic ulcer disease and less commonly carcinoma. Small, curved, plump bacilli are seen on stains of tissue sections obtained from infected intestinal mucosa as illustrated in these photomicrographs. This ulcer and carcinoma-producing bacterium can be presumptively identified by the use of the urea breath test in which strong urease produced by this bacterial species is detected by capturing radioactive labeled CO2 in the exhaled breath from an infected patient. From these observations, select the causative bacterial species.
Helicobacter pylori
The organism seen in the image is found in the stomach mucosa and presumptive identification is often obtained from a positive urease test. Select the organism below.
Helicobacter pylori
Which of the following species of bacteria is positive for rapid urease production, as demonstrated by a color change in urea agar from yellow to pink within two hours?
Helicobacter pylori
Which of the following tests is most helpful in differentiating C. jejuni from the other Campylobacter spp.?
Hippurate hydrolysis
Each of the following practices will better ensure the successful recovery of Campylobacter jejuni from stool specimens except:
Incubation at 42°C Use of Campy BAP medium Use of selenite enrichment broth Incubation in an atmosphere of 5% O2, 10% CO2, and 85% N2
Which one of the following statements about Campylobacter jejuni is false?
It can occur in domestic animals, raw milk, and undercooked poultry. It does not require special incubation conditions to grow in culture. It is associated with neutrophils in the stool. It is a curved gram-negative rod.
What are the typical susceptibility patterns of the most commonly isolated nonfermenters?
Many of the nonfermenters, especially P. aeruginosa, A. baumannii complex, and S. maltophilia, can be resistant to agents used to treat infections caused by fermentative gramnegative bacilli. They are resistant to penicillin, ampicillin, most third-generation cephalosporins (except ceftazidime), macrolides, lincosamides, and agents active against grampositive bacteria. There is variability of their in vitro and in vivo responses to aminoglycosides; quinolones; aminopenicillins, such as piperacillin and ticarcillin; and SXT. Specific susceptibility tests need to be performed if the nonfermenter is considered clinically relevant.
What is the typical natural habitat of most nonfermenters?
Most nonfermenters exist in the environment, often a moist or aquatic environment. They are not usually part of the normal human biota. Nonfermenters may also be found in soil and on plants, as well as in hospital environments on countertops, on equipment, and occasionally in contaminated liquids used for dispensing medications and disinfectants
What initial clues indicate that an isolate is a nonfermenter?
Most nonfermenters will grow on media selective for gram negative bacilli, such as MacConkey agar, but will remain lactose-negative. In addition, many of the nonfermenters will be oxidase-positive. The Gram stain of many nonfermenters is that of thin bacilli, often a little longer than the rods of the fermenters, such as E. coli and other Enterobacteriaceae. When a nonfermenter is placed on media to determine carbohydrate utilization, such as TSI or KIA, there will be no acidification of the butt. This result will increase the likelihood of a nonfermenter
Match the following infectious diseases with their associated causative organism: Myonecrosis Tetanus Botulism Pseudomembranous colitis Actinomycosis
Myonecrosis - Clostridium perfringens Tetanus - Clostridium tetani Botulism - Clostridium botulinum Pseudomembranous colitis - Clostridioides difficile Actinomycosis - Actinomyces spp.
What types of infections do nonfermenters cause?
Nonfermenters rarely cause infections outside of the hospital environment, except when traumatically implanted (e.g., into the skin from soil, vegetation, or water sources) or in the case of immunocompromised patients. In the hospital, nonfermenters may be the cause of nosocomial urinary tract infections, post surgical wound infections, pneumonia (particularly ventilator associated pneumonia), and/or bacteremia. The incidence of infection is greater in the immunocompromised patient.
Various methods have been employed for the detection of Clostridioides difficile disease, which method is the new gold standard for detection?
Nucleic acid amplification tests (NAATs)
Which one of the following results is typical of Campylobacter jejuni?
Optimal growth at 42 ºC
What are the four most common nonfermentative, gram-negative bacilli isolated in the clinical laboratory?
Pseudomonas aeruginosa is the most common nonfermenter associated with clinical infections, especially nosocomial infections. Acinetobacter baumannii complex, Burkholderia spp., and Stenotrophomonas maltophilia are often isolated from hospitalized patients, especially from respiratory specimens, but they are more often colonizers and are not always clinically significant. The isolation of a nonfermenter from a single blood culture, or as part of a polymicrobial infection, often indicates that the organism is acting as a colonizer or a laboratory contaminant rather than a relevant pathogen. However, if one of the nonfermenters is seen on a Gram stain from a sterile site, is the only organism isolated, and is present in high numbers, its clinical significance needs to be considered
Which nonfermenters are most commonly isolated from patients with cystic fibrosis
Pseudomonas aeruginosa is the species most often isolated; occasionally Burkholderia cenocepacia and Burkholderia multivorans, both members of the B. cepacia complex, are recovered from lower respiratory tract specimens from patients with cystic fibrosis.
Illustrated in this photomicrograph of a gastric biopsy specimen are a few thin, curved and S-shaped bacteria (arrows). This picture is suspicious for Helicobacter pylori. An additional test helpful in confirming this identification is:
Rapid urease
A 62-year-old female undergoing chemotherapy for breast cancer developed fever, abdominal pain, and foul-smelling diarrhea after receiving clindamycin for 5 days. The photomicrograph is an H & E-stained section of colon biopsy revealing focal ulceration and pseudomembranous inflammation. Each of the following laboratory findings will confirm the presumptive diagnosis of Clostridioides difficile-associated colitis EXCEPT:
Recovery of C. difficile in the stool Detection of cytotoxin B of C. difficile Positive latex agglutination of glutamate dehydrogenase of C. difficile Positive EIA for C. difficile enterotoxin
What risk factors are associated with infections caused by nonfermentative, gram-negative bacilli?
Risk factors for infection by nonfermenters include immunocompromised states from cancer or cancer chemotherapy, transplantation, and steroid use. In the immunocompetent individual, infections are associated with burns, catheters, prior use of broad-spectrum antimicrobial agents, metabolic disorders (e.g., diabetes mellitus), and foreign body implantation, traumatically or via transplanted organs.
Which statement about bacterial culture for Clostridioides (Clostridium) difficile is TRUE?
Routine bacteriological media will provide adequate recovery. The culture is specific for toxigenic strains. Culture on appropriate media provides an effective means of recovering the organism. Culture for C. difficile provides desirable turnaround times.
Some anaerobes are particularly susceptible to oxygen because they lack the enzyme:
Superoxide dismutase
A pleomorphic gram-positive bacillus that is spot indole and catalase positive can be presumptively identified as Cutibacterium acnes.
TRUE
Failure to isolate fusiform gram-negative organisms that were observed on a Gram-stained smear of a clinical specimen could be an indication that a problem exists with the primary medium used for the isolation of anaerobes or the system being used for anaerobic incubation of primary plates.
TRUE
Large, dark colonies (>1 mm) growing on a BBE agar plate at 24 hours can presumptively be called a member of the Bacteroides fragilis group.
TRUE
How would you differentiate P. aeruginosa from other fluorescent group pseudomonads?
The ability to grow at 42° C and the presence of a blue-green pigment diffusing through the medium are characteristics of P. aeruginosa that can be used to differentiate this organism from other members of the pseudomonad fluorescent group. In addition, the distinct grapelike odor of P. aeruginosa may be a clue.
What nonculture methods are used to diagnose Helicobacter pylori infections?
The most commonly used nonculture method for the diagnosis of H. pylori is the noninvasive 13C- or 14C-labeled urea breath test. The patient receives an oral dose of labeled urea. Urease activity by H. pylori results in the formation of radioactivelabeled CO2, which is absorbed into the bloodstream and then exhaled. Other nonculture methods include microscopic examination of stained gastric tissue, direct fecal antigen detection, polymerase chain reaction assay, and determining urease activity of gastric biopsy material.
Which of the following specimens would be unacceptable for anaerobic culture?
Urethral swab
Which of the following Vibrio spp. would you expect to be most likely isolated from a blood culture?
V. vulnificus
Growing on the surface of the anaerobic blood agar plate illustrated in the upper image are indistinct small, gray, convex, translucent colonies free of beta hemolysis. Gram-negative cocci lying singly and in clusters are seen on Gram stain, as illustrated in the lower photomicrograph. Nitrates are reduced, indole is negative, and carbohydrates are not utilized (asaccharolytic). This isolate is part of the microbiota of the mouth, upper respiratory tract, and intestine. What is this organism?
Veillonella parvula
A gram-negative bacillus isolated from a stool specimen produces clear colonies on MacConkey agar and yellow colonies on thiosulfate citrate bile salt sucrose medium. The isolate is subcultured to a sheep blood agar plate with an O/129 disk. The isolate is sensitive to O/129 and is oxidase-positive. You should suspect
Vibrio cholerae
The fastidious Gram-negative bacillus featured here has been recovered from individuals in pandemic outbreaks of diarrhea after ingestion of contaminated stream, lake, or well water. In many cases, life-threatening profuse diarrhea resulting in dehydration and electrolyte loss is experienced. Smooth, entire, convex light gray-yellow colonies are observed on blood agar (left upper image). Note the yellow pigmentation of the colonies growing on TCBS agar (Thiosulfate Citrate Bile Sucrose) on the right of the composite image, indicating strong acid production from sucrose. Cytochrome oxidase is also positive. Small, curved, and S-shaped, gram-negative bacilli are seen on the Gram stain. From these observations, select the correct genus/species identification of this isolate.
Vibrio cholerae
Illustrated in this photograph is the surface of a thiosulfate citrate bile sucrose (TCBS) agar plate which are growing colonies of an unknown bacterial species from a diarrheal stool specimen. The most likely identification is: [yellow colonies]
Vibrio cholerae is correct because Vibrio cholerae ferments sucrose, resulting in the yellow, acid pH colonies. Bacterial species that can grow on TCBS agar, but do not ferment sucrose are semitransparent and will appear green.
