Question 1

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

Accepted Answer

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 2

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?

Accepted Answer

Enterobacteriaceae inhabit a wide variety of niches that include the human gastrointestinal tract,the gastrointestinal tract of other animals,and various environmental sites.Some are agents of zoonosis,causing infections in animal populations.Just as the reservoirs for these organisms vary,so do their modes of transmission to humans.

Question 3

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:

Accepted Answer

H₂S-positive is the clue that gives away Salmonella spp.in this case.Although P.vulgaris is the only other organism that is H₂S-positive,it is spot indole-positive,not negative.

Question 4

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

Accepted Answer

Sorbitol-negative E.coli can be serotyped using commercially available antisera to determine whether the somatic O antigen 157 and the flagellar H antigen 7 are present.

Question 5

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

Accepted Answer

Biochemical profiles obtained with TSI agar are often used to identify enteric pathogens presumptively.Organisms that exhibit the profiles listed require further biochemical profiling and,in the case of Salmonella spp.and Shigella spp. ,serotyping to establish a definitive identification.

Question 6

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

Accepted Answer

A glucose and lactose fermenter on LIA would produce an acid byproduct,causing the media to turn yellow,which would be A/A.

Question 7

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?

Accepted Answer

Typing is performed using a slide agglutination test.If an isolate agglutinates with the Vi antiserum and does not react with any of the O groups,then a saline suspension of the organism should be prepared and heated to 100°C for 10 minutes to inactivate the Vi antigen.

Question 8

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

Accepted Answer

The serotype O157:H7 of E.coli is associated with a serious diarrheal illness with high mortality rates among some populations.

Question 9

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

Accepted Answer

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.Shigella are lactose nonfermenters and H₂S-negative.

Question 10

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

Accepted Answer

Biochemical profiles obtained with TSI agar are often used to presumptively identify enteric pathogens.Organisms that exhibit the profiles listed require further biochemical profiling and,in the case of Salmonella spp.and Shigella spp. ,serotyping to establish a definitive identification.

Question 11

On MacConkey agar,Serratia spp.would appear:

Accepted Answer

Serratia are late lactose fermenters and would therefore appear red on MacConkey agar.

Question 12

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

Accepted Answer

Enterobacteriaceae are facultative anaerobes that ferment glucose,are oxidase-negative,and,with rare exceptions,reduce nitrates to nitrites.Further,except for Shigella dysenteriae,type 1,all commonly isolated Enterobacteriaceae are catalase-positive.

Question 13

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

Accepted Answer

Streptomycin is the drug of choice for Y.pestis infections.

Question 14

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

Accepted Answer

A red colony with a black center on XLD agar indicates the presence of a lactose nonfermenter and an H₂S-positive producer.

Question 15

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

Accepted Answer

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 16

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

Accepted Answer

In the interests of cost containment,many clinical laboratories use an abbreviated identification scheme for identification of commonly isolated enteric organisms (CLSI,35A).A spot indole test can be used to separate swarming Proteae quickly,such as P.mirabilis and P.penneri,which are negative,from a spot indole-positive P.vulgaris.

Question 17

The major etiologic agent of urinary tract infections is:

Accepted Answer

For those species that normally colonize humans,infections may result when a patient's own bacterial strains (i.e. ,endogenous strains)establish infections in a normally sterile body site.These organisms can also be passed from one patient to another.Such infections often depend on the debilitated state of a patient who is hospitalized and are nosocomially acquired.However,this is not always the case.For example,although E.coli is the most common cause of nosocomial infections,it is also the leading cause of urinary tract infections in nonhospitalized patients.

Question 18

E.coli would appear what color on HE agar?

Accepted Answer

E.coli is a lactose fermenter and is H₂S-negative;therefore,it would appear yellow on HE agar.

Question 19

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

Accepted Answer

E.coli not only causes hemorrhagic colitis,but it can also cause hemolytic-uremic syndrome,which affects the kidneys.

Question 20

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

Accepted Answer

Yersinia grows best at 25°C to 30°C and is carried by wild rats.CIN agar is a selective medium specifically used for the isolation of Y.enterocolitica from gastrointestinal specimens.

Quiz 18: Enterobacteriaceae