Deck 19: Enterobacteriaceae

Question 1

A green colony with a black center is isolated on HE agar from a stool specimen.It is most likely which organism?

A) Escherichia coli
B) Salmonella spp.
C) Shigella spp.
D) Klebsiella pneumoniae

Answer 1

B
The black center indicates H₂S production; therefore Salmonella spp.is the only H₂S producer in the group.A green colony represents a lactose nonfermenter.E.coli and K.pneumonia are both lactose-positive and would appear yellow on HE.Although Shigella are lactose nonfermenters and H₂S-negative.

Question 2

Enterotoxigenic and enteroinvasive are terms often linked to which of the following bacterial species?

A) Escherichia coli
B) Salmonella enteritidis
C) Shigella sonnei
D) Yersinia pestis

Answer 2

A
Although E.coli is a normal bowel inhabitant, its pathogenic classification is somewhere between that of overt pathogens and opportunistic organisms.Strains of this species, such as enterotoxigenic E.coli (ETEC), enteroinvasive E.coli (EIEC), and enteroaggregative E.coli (EAEC), express potent toxins and cause serious gastrointestinal infections.In addition, in the case of enterohemorrhagic E.coli (EHEC), life-threatening systemic disease can result from infection.

Question 3

Most strains of Escherichia coli can be presumptively identified on the basis of which characteristic?

A) Spot indole-positive
B) Citrate-positive
C) Methyl red-negative
D) Urease-positive

Answer 3

A
In the interests of cost containment, many clinical laboratories use an abbreviated identification scheme for identifying commonly isolated enteric organisms (Clinical Laboratory Standards Institute [CLSI], 35A).E.coli, the most commonly isolated enteric organism, can be identified using a positive spot indole test.

Question 4

Escherichia coli can produce a toxin similar to Shigella dysenteriae and can cause hemorrhagic colitis.What other organ does it affect?

A) Heart
B) Lungs
C) Liver
D) Kidney

Question 5

Of the following bacterial species, which is associated with a serious diarrheal illness with high mortality rates among some populations?

A) Escherichia coli
B) Shigella spp.
C) Salmonella spp.
D) Vibrio spp.

Question 6

This organism, belonging to the family Enterobacteriaceae, grows best at 25º to 30º C.Domestic and wild rats and other animals can carry this organism.What is the medium of choice?

A) MacConkey agar
B) Xylose-lysine-deoxycholate (XLD) agar
C) Cefsulodin-irgasan-novobiocin (CIN) agar
D) Hektoen enteric (HE) agar

Question 7

For the most part, all Enterobacteriaceae are positive for which biochemical reactions?

A) Glucose, oxidase, and catalase
B) Glucose, indole, and oxidase
C) Glucose, reduce nitrates to nitrites, and oxidase
D) Glucose, catalase, and reduce nitrates to nitrites

Question 8

The major etiologic agent of urinary tract infections is:

A) Escherichia coli.
B) Shigella spp.
C) Salmonella spp.
D) Vibrio spp.

Question 9

A patient bitten by a flea developed a painful bubo at the site of the bite.The drug of choice for this organism is:

A) ampicillin.
B) gentamycin.
C) streptomycin.
D) ciprofloxacin.

Question 10

In addition to being a group of gram-negative bacteria considered normal flora of the gastrointestinal tract of humans and animals, which one of the following also contains agents of important diseases such as gastroenteritis and enteric fevers, urinary tract infections, and bacterial pneumonia?

A) Neisseriaceae
B) Micrococcaceae
C) Enterococcus
D) Enterobacteriaceae

Question 11

Which is correct for the description "alkaline slant, acid butt, no H₂S"?

A) Salmonella spp.
B) Escherichia coli
C) Proteus spp.
D) Pseudomonas spp.

Question 12

Escherichia coli would appear what color on HE agar?

A) Green
B) Green with a black center
C) Yellow
D) Yellow with a black center

Question 13

Which of correct for the description "acid slant, acid butt, no H₂S"?

A) Salmonella spp.
B) Escherichia coli
C) Proteus spp.
D) Pseudomonas spp.

Question 14

A green colony with a black center was isolated and presumptively identified as a Salmonella spp.The organism was serotyped with polyvalent antisera; however, the antisera tested negative for all antigens.What is the next course of action?

A) Regrow the organism on sheep blood agar, and serotype again the next day.
B) Make a suspension, and heat to 100º C for 10 minutes.
C) Make a suspension, and place on ice for 30 minutes.
D) Report the organism out as a nontypable Salmonella spp.

Question 15

The swarming gram-negative bacillus that can be presumptively identified on the basis of a positive indole test is:

A) Klebsiella pneumoniae.
B) Proteus vulgaris.
C) Enterobacter aerogenes.
D) Shigella sonnei.

Question 16

If an organism were a glucose and lactose fermenter, the LIA reaction would be:

A) K/A.
B) A/K.
C) A/A.
D) K/K.

Question 17

On MacConkey agar, Serratia spp.would appear:

A) red.
B) colorless.
C) green.
D) yellow.

Question 18

A gram-negative bacillus that is hydrogen sulfide (H₂S)-positive, citrate-positive, spot indole-negative, and associated with gastroenteritis in the United States is most likely:

A) Klebsiella pneumoniae.
B) Proteus vulgaris.
C) Salmonella enteritidis.
D) Shigella sonnei.

Question 19

MacConkey-sorbitol agar is useful for detecting pathogenic strains of:

A) Escherichia coli.
B) Shigella spp.
C) Salmonella spp.
D) Vibrio spp.

Question 20

An organism that appears red with a black center on XLD agar is lactose-_____ and H₂S-_____.

A) positive; positive
B) positive; negative
C) negative; positive
D) negative; negative