Question 1

The normal flora of the adult large bowel consists predominantly of&#10;A)anaerobic species.&#10;B)Staphylococcus.&#10;C)Enterococcus.&#10;D)Enterobacteriaceae.

Accepted Answer

The normal flora of the adult large bowel (colon) is established relatively early in life and consists predominantly of anaerobic species, including Bacteroides, Clostridium, Peptostreptococcus, Bifidobacterium, and Eubacterium. Aerobes, including Escherichia coli, other Enterobacteriaceae, enterococci, and streptococci, are outnumbered by anaerobes 1000:1.

Question 2

Selenite broth serves which purpose when used in the bacterial fecal culture?&#10;A)Evaluation of general flora and predominant species&#10;B)Selection of enteric pathogens such as Salmonella, Aeromonas, and others&#10;C)Inhibition of normal flora and select for enteric pathogens&#10;D)Enrichment for enteric pathogens such as Salmonella and others

Accepted Answer

Enrichment broths are sometimes used for enhanced recovery of Salmonella, Shigella, Campylobacter, and Y. enterocolitica although Shigella usually does not survive enrichment. Gram-negative broth (Hajna GN) or selenite F broth yields good recovery.

Question 3

GN Broth serves which purpose when used in the bacterial fecal culture?&#10;A)Evaluation of general flora and predominant species&#10;B)Selection of enteric pathogens such as Salmonella, Aeromonas, and others&#10;C)Inhibition of normal flora and select for enteric pathogens&#10;D)Enrichment for enteric pathogens such as Salmonella and others

Accepted Answer

Enrichment broths are sometimes used for enhanced recovery of Salmonella, Shigella, Campylobacter, and Y. enterocolitica although Shigella usually does not survive enrichment. Gram-negative broth (Hajna GN) or selenite F broth yields good recovery.

Question 4

The transport medium that best preserves the viability of most intestinal bacterial pathogens is&#10;A)buffered glycerol.&#10;B)a rectal culture swab.&#10;C)Cary-Blair.&#10;D)Campy-thioglycollate broth.

Accepted Answer

If a delay longer than 2 hours is anticipated for stool for bacterial culture, the specimen should be placed in transport medium. Cary-Blair transport medium best preserves the viability of intestinal bacterial pathogens, including Campylobacter and Vibrio spp.

Question 5

MacConkey agar serves which purpose when used in the bacterial fecal culture?&#10;A)Evaluation of general flora and predominant species&#10;B)Selection of enteric pathogens such as Salmonella, Aeromonas, and others&#10;C)Inhibition of normal flora and select for enteric pathogens&#10;D)Enrichment for enteric pathogens such as Salmonella and others

Accepted Answer

The moderately selective agar should support growth of most Enterobacteriaceae, vibrios, and other possible pathogens; MacConkey agar works well. Some laboratories use eosin-methylene blue (EMB), which is slightly more inhibitory. All lactose-negative colonies should be tested further, ensuring adequate detection of most vibrios and most pathogenic Enterobacteriaceae. Lactose-positive vibrios (V. vulnificus), pathogenic E. coli, some Aeromonas spp., and Plesiomonas spp. may not be distinctive on MacConkey agar.

Question 6

Stool received for routine culture in most clinical laboratories in the United States should be examined for the presence of at least&#10;A)Campylobacter, Salmonella, and Shigella spp.&#10;B)Salmonella and Shigella spp.&#10;C)Campylobacter and Vibrio.&#10;D)Salmonella, Shigella, and Vibrio.

Accepted Answer

Stool received for routine culture in most clinical laboratories in the United States should be examined for the presence of Campylobacter, Salmonella, and Shigella spp. under all circumstances. Detection of Aeromonas and Plesiomonas spp. should be incorporated into routine stool culture procedures. The cost of doing a stool examination on every patient for all potential enteric pathogens is prohibitive. The decision as to what other bacteria are routinely cultured should take into account the incidence of gastrointestinal tract infections caused by particular etiologic agents in the area served by the laboratory. For example, if the incidence of Yersinia enterocolitica gastroenteritis is high enough in the area served by the laboratory, then this agent should also be sought routinely. Similarly, because of the increasing prevalence of disease caused by Vibrio spp. in individuals living in high-risk areas of the United States (seacoast), laboratories in these locales may routinely look for these organisms.

Question 7

XLD agar serves which purpose when used in the bacterial fecal culture?&#10;A)Evaluation of general flora and predominant species&#10;B)Selection of enteric pathogens such as Salmonella, Aeromonas, and others&#10;C)Inhibition of normal flora and select for enteric pathogens&#10;D)Enrichment for enteric pathogens such as Salmonella and others

Accepted Answer

The specimen should be inoculated to a moderately selective agar such as Hektoen enteric (HE) or xylose-lysine desoxycholate (XLD) media. These media inhibit growth of most Enterobacteriaceae, allowing Salmonella and Shigella spp. to be detected.

Question 8

Campy-thioglycollate broth serves which purpose when used in the bacterial fecal culture?&#10;A)Evaluation of general flora and predominant species&#10;B)Selection of enteric pathogens such as Salmonella, Aeromonas, and others&#10;C)Inhibition of normal flora and select for enteric pathogens&#10;D)Enrichment for enteric pathogens such as Salmonella and others

Accepted Answer

Campy-thioglycollate enrichment broth increases the yields of positive cultures for Campylobacter spp., although it is not necessary for routine use.

Question 9

Cultures for isolation of Campylobacter jejuni and Campylobacter coli should be&#10;A)incubated in a capnophilic atmosphere at 42&#176; C and examined at 24 and 48 hours.&#10;B)incubated in a microaerophilic atmosphere at 35&#176; C and examined at 24 and 48 hours.&#10;C)incubated in regular room air at 35&#176; C and examined at 48 and 72 hours.&#10;D)incubated in a microaerophilic atmosphere at 42&#176; C and examined at 24 and 48 hours.

Accepted Answer

Cultures for isolation of Campylobacter jejuni and Campylobacter coli should be inoculated to a selective agar containing antimicrobial agents that suppress the growth of normal flora but not of Campylobacter spp. The introduction of a blood-free, charcoal-containing medium that has more selective antibiotic components has resulted in better recovery of most enteropathogenic Campylobacter spp. compared with earlier media. Commercially produced agar plates for isolation of campylobacters are available from several manufacturers. These plates are incubated in a microaerophilic atmosphere at 42º C and examined at 24 and 48 hours for suspicious colonies.

Question 10

Sheep blood agar serves which purpose when used in the bacterial fecal culture?&#10;A)Evaluation of general flora and predominant species&#10;B)Selection of enteric pathogens such as Salmonella, Aeromonas, and others&#10;C)Inhibition of normal flora and select for enteric pathogens&#10;D)Enrichment for enteric pathogens such as Salmonella and others

Accepted Answer

Blood agar (tryptic soy agar with 5% sheep blood) is an excellent general supportive medium. Blood agar medium allows growth of yeast species, staphylococci, and enterococci, in addition to gram-negative bacilli. Of importance, the absence of normal gram-negative fecal flora and/or the presence of significant quantities of organisms such as Staphylococcus aureus, yeasts, and Pseudomonas aeruginosa can be evaluated. Another benefit of blood agar is that it will allow oxidase testing of colonies. Several colonies that do not resemble Pseudomonas from the third or fourth quadrant should be routinely screened for production of cytochrome oxidase. If many are present, Aeromonas, Vibrio, or Plesiomonas spp. should be suspected.

Deck 56: Gastrointestinal Tract Infections