Using a loop, make a single small streak of each culture onto the appropriate se

Question

Using a loop, make a single small streak of each culture onto the appropriate section of the plate. Incubate the plate at 37 degree C for 24-48 h. A mixed culture containing E. coil, salmonella choleraesuis, and Enterococcus faecalis is steaked onto Hektoen enteric agar. E. coli is a Gram-negative lactose fermenter that produces hydrogen sulfide, S. choleraesuis is a Gram-negative nonfermenter that produces hydrogen sulfide, and E. faecalis is a Grampositive, lactose fermenter. Explain the appearance of each species after incubation of the plate. How would the appearance of each of the three organisms in the previous question change if eosin methylene blue agar was used instead of Hektoen enteric agar? What if trypticase soy agar was used?

Explanation / Answer

E. coli and E. faecalis will have dark coloured colonies when stained with eosin methylene blue, while S. cholerasuis will have a colourless colony. This is because fermenting strains produce acids, which lower the pH and encourage absorption of the dye. Trypticase sog agar is a non-selective media, and all the three stains should grow. It used in combination with the above test, S. cholerasuis will not grow as eosin methylene blue is toxic to non-fermenting bacteria.

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