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.
8. If an incubation time of 36 hr had been tested in Study 1, the average number of E.coli cells/cm2 in
10% soy broth would most likely have been:
Your Answer is
Correct Answer is H
Explanation
Looking at the line of 10% soy broth, 36 hr is between 24-48 hr, so the corresponding quantity should also be between 12000-49000, and option H is reasonable.