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Flashcards in Gram-Negative Bacilli Deck (121)
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What is the clinical significance of Acinetobacter?

It is an opportunistic pathogen with a high prevalence in hospitals. Because it frequently colonizes the skin and respiratory tracts of patients, its clinical significance in patient specimens can be difficult to establish.


What are some of the characteristics of Acinetobacter?

They are non-fermentative, oxidase-negative, gram-negative rods that grow on MacConkey agar. On Gram stain, they are plump coccobacilli that may be mistaken for Neisseria.


What are the characteristics of the family Enterobacteriaceae?

Gram-negative non-sporeforming rods
Falcutative anaerobes
Glucose fermenters
Most reduce nitrates to nitrites
Most are catalase-positive.


Which of the following is a characteristic of all Enterobacteriaceae: acid butt in TSI, acid slant in TSI, H2S production, or colorless colonies on MacConkey?

Acid butt in TSI. All Enterobacteriaceae ferment glucose. Not all ferment lactose or sucrose or produce H2S.


Discuss the antigens used for serological grouping of some of the Enterobacteriaceae,

O antigen - Somatic antigen - Cell wall - Lipopolysaccharide, heat stable
H antigen - Flagellar antigen - Flagella - Proteins, heat labile
K antigen - Capsular antigen - Capsule - Polysaccharide, heat labile, may mask O antigen. Removed by heating.


Which of the Enterobacteriaceae are routinely serotyped?

Salmonella, Shigella, and E. coli O157:H7.


What error may result from selecting organisms for serotyping from a sugar-containing medium such as MaConkey or TSI?

Auto-agglutination. Colonies should be selected from blood agar.


Which antigen would be expected in bacteria that grow away from the stab line in motility agar?

O and H. All bacteria have the somatic antigen (O). Bacteria that grow away from the stab line are motile by means of flagella. The H antigen is associated with flagella.


Following presumptive identification of Salmonella and Shigella by biochemical testing, what further testing is required?

Serotyping. Salmonella is typed by its O and H antigens, Shigella by O antigens only. (It is nonmotile, so does not have H antigens.)


Which antigens are associated with Salmonella typhi?

D and Vi, a capsular antigen associated with virulence.d


A suspected Salmonella is tested with polyvalent O antiserum and no agglutination is observed. What should be done and why?

Typing should be repeated on a suspension of the organism that has been heated. Heating destroys the K antigen which may mask the O antigens.


Which three enterics are most commonly isolated from clinical specimens?

E. coli, Klebsiella, and Proteus.


Which species of Proteus is most frequently isolated?

P. mirabilis.


Which Enterobacteriaceae are pathogens to the intestinal tract?

Salmonella, Shigella, diarrheagenic E. coli, and Yersinia.


Which Enterobacteriaceae are nonlactose fermenters (NLF)?

SAlmonella, Shigella, Proteus, Morganella, Providencia, Yersinia, Serratia, and Edwardsiella. Some Citrobacter and E. coli are lactose-negative also.


Describe lactose fermenters (LF) and nonlactose fermenters (NLF) on EMB, MacConkey, Hektoen Enteric, and XLD.

Lactose Fermenters:
EMB - Purple or green-black
MaConkey - Pink
Hektoen Enteric - Orange to salmon pink
XLD - Yellow

Nonlactose Fermenters:
EMB - Colorless
MacConkey - Colorless
Hektoen Enteric - Green to blue-green
XLD - Colorless or red


What color are E. coli colonies on XLD?



What color are Shigella colonies on XLD?



What color are Salmonella colonies on XLD?

Red with black center.


Which enteric is characterized by swarming growth on blood agar?



Which lactose-positive enterics are often "stringy" when touched with a loop (mucoid)?

Klebsiella and Enterobacter. This is due to their polysaccharide capsules.


Which enteric produces a characteristic green metallic sheen on EMB?

E. coli.


Which facultative gram-negative rod sometimes produces a red pigment?

Serratia marcescens.


Which member of the family Enterobacteriaceae is thought to be a potential agent of bioterrorism?

Yersinia pestis, the causative agent of plague. Although usually contracted by the bite of a flea, person-to-person transmission by droplets is possible. The key characteristic for rapid diagnosis of plague is the closed safety pin appearance of Y. pestis when stained Wayson stain. It grows best at 28C.


WHich sugars are contained in TSI tubes?

Glucose (0.1%), lactose (1%), and sucrose (1%).


Explain how to interpret TSI tubes.

Yellow butt = glucose-positive
Pink butt = glucose-negative
Yellow slant = lactose- or sucrose-positive
Pink slant = lactose- and sucrose-negative
Black precipitate = hydrogen sulfide production
Bubbles = gas production


How should the TSI reaction be recorded when the butt is totally black and the slant is yellow?

A/A, H2S. H2S is only produced in an acid environment.


Describe TSI reactions for E. coli.

A/A, Gas.


Describe TSI reactions for Proteus vulgaris.

A/A with H2S. The yellow slant is due to fermentation of sucrose, not lactose.


Describe TSI reactions for Salmonella.

K/A, Gas, H2S.