Passage II

The number of bacteria that adhere to a surface is affected by the medium in which they are grown. Two studies examined how different media affect the number of bacteria, either Escherichia coli or Salmonella typhimurium, that adhere to a stainless steel surface.

Study 1

In each of 4 trials, Steps 1-8 were performed:

1. Five 0.36 cm2 stainless steel chips were placed into a flask.

2. A 125 mL quantity of 10% soy broth (in water) was added to the flask.

3. A 15 mL sample of an E. coli culture having a cell density of 100,000 cells/mL was added to the flask.

4. The flask was incubated at 23°C for 1 hr, 24 hr, 48 hr, or 72 hr.

5. The chips were removed from the flask and washed with Ringer's solution (an aqueous salt solution).

6. The chips were placed in a solution of euchrysine (a dye that stains DNA) .

7. The chips were washed with pure water.

8. The number of E. coli attached to each chip was counted, and the average number of cells/c㎡ of chip was determined.

Steps 1-8 were repeated; 4 trials were performed for each of 3 other media: 100% soy broth, skim milk, and 20% meat juice (in water).

The results are shown in Table 1.

Study 2

Study 1 was repeated with S. typhimurium instead of E. coli (see Table 2).

Tables adapted from Scott K. Hood and Edmund A. Zottola, “Adherence to Stainless Steel by Foodborne Microorganisms During Growth in Model Food Systems." ©1997 by Elsevier Science B. V.

9. Suppose a student wants to use a medium to grow E.coli that will minimize, as much as possible, the
adherence of the E.coli to a stainless steel surface over a 72 hr incubation period. Based on the results of Study 1, which of the 4 media tested should the student select?

Answer and Explanation

Your Answer is

Correct Answer is C

Explanation

Reducing the adhesion of E. coli to the stainless steel surface means that the average number of E. coli cells/cm² is smaller. See Table 1. The smallest value corresponds to the line of skim milk.