Question

A person's mouth is sampled with a sterile, moistened swab and a plate of tryptic soy agar is inocluated with the swab. After the appropriate time of incubation is allowed, no growth appears on the plate. Which of the following is the most reasonable explanation for this result?

a. the person's mouth was sterile

b. plate should have been incubated under anaerobic conditions

c. the tryptic soy agar lacked some nutrients required the mouth microbes

d. tryptic soy agar contained a compound that inhibited the growth of the mouth microbes

Answer 1

Tryptic Soy Agar or TSA is a general-purpose media. It is suitable for the growth of several bacteria species. It is not selective or differential media. Hence with appropriate conditions provided, the bacterial cultures will thrive on the plate.

The anaerobic bacteria if present in the mouth, are growing in relatively oxygenic conditions. Hence they should be able to thrive in the aerobic conditions in the agar plate. Hence statement b is incorrect.

Tryptic soy agar is a complete medium and not minimal media, hence it is unlikely that it would lack the nutrients required by the mouth microbes. Hence the statement c is incorrect.

The tryptic soy agar is a growth media consisting of carbon source and several other nutrients and micronutrients to promote bacterial growth. It doesn't have any bacterial growth inhibitor. Hence the statement d is incorrect.

Thus, since there was no growth of bacteria, we can safely say that the swab lacked any bacteria and hence the mouth of the person was sterile. Hence the statement a is correct.