8. The supervisor wants you to predict what growth will occur and on what plates

Question

8. The supervisor wants you to predict what growth will occur and on what plates, based on what you see microscopically in the gram stain from a fecal sample.

http://homepage.usask (9.19.09)

a. What organism do you think it is?

b. What media will it grow on and how do you incubate it?

9. One of the enterotoxins produced by enterotoxigenic Eschericha coli in traveler’s diarrhea is similar to a toxin produced by __________________________.

a. How does this toxin work?

10. You have narrowed the id of an organism from a fecal sample down to either Salmonella or Citrobacter.

a. What media would you use to differentiate the two organisms

b. Describe the growth on the media for each organism.

Explanation / Answer

8. a. The link is not working properly. Please provide with proper link. As I cannot able to see the picture of gram stain I am assuming the most common fecal pathogen which is gram negative , S or comma shaped Campylobacter, common species is jejuni.

b. The organism is microaerophilic. The media can be used is Campy CVA agar which is incubated at 420 C in microaerobic condition for 72 hour.

9. ETEC ( Enterotoxigenic Escherichia coli) produce enterotoxins which may be either heat labile (LT) or heat stable ( ST) or both. E.coli LT resembles the cholera toxin in its structure, antigenic properties and mode of action. Cholera toxin is produced by Vibrio cholerae.

a) The toxin molecule of approx. 84000 MW consists of 1A and 5 B subunits. The B binding units attach to the GM1 ganglioside receptors on the surface of jejunal epithelial cells. The A active subunit , on being transported into the enterocyte, dissociates into 2 fragments : A1 and A2 . The A2 fragment only links biologically active A1 to the B subunit. The A1 fragment causes prolonged activation of cellular adenylate cyclase and accumulation of cAMP , leading to outpouring into the small intestinal lumen , of large quantities of water and electrolytes and the consequent watery diarrhea. The toxin also inhibits intestinal absorption of sodium and chloride.

10. a) Xylose Lysine Desoxycholate media or XLD media can be used to distinguish the two organisms.

b) Salmonella helps thiosulfate to metabolise resulting in production of hydrogen sulfide which in turn leads to the formation of pink or reddish colonies with black center whereas Citrobacter produces yellow to orange colonies due to acidifation of the medium as a result of lactose fermentation.

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