MFD 26 Oral MIcrobiology (practical)

Question 1

1. Can you assume that each type of colony represents a different species?

Answer 1

Generally, different types of colony are likely to be derived from different species. However, sometimes different strains of a species will actually produce quite different colonies (for example, not all strains of Staph. aureus produce the golden pigment). Therefore, two different colonies may be formed by the same species. Conversely, very similar colonies may be produced by different species. Colony morphology is just one of many traits that can help to identify bacteria, but it is dangerous to use any one trait on its own.

Question 2

Is the relative number of colonies on non-selective FAA plates, is it safe to assume the colonies presents the total bacterial population:

Answer 2

No! Although FAA is non-selective, it will not culture everything. For a start, obligate aerobes will not be present since the plates were incubated anaerobically. There are also many species in the mouth that are difficult or impossible to culture in the lab.

Question 3

How can you explain the appearance of the different colony types on TYCS:

2) give description of colony:
a) strep mutans Type A
b) Type B
c) s.sanguis

Answer 3

This is due to the different glucans being produced by the different species present.

2) a) yellow, b) grey white
c) white

Question 4

calculate CFU/ml, if there are 50 colonies MSB plate, 50-fold dilution of the sample/saliva and 100microlites was spread on the plate:

Answer 4

50 colonies in 100 microlites
=500colonies in one ml
. due to dilution:
500*50=25,000CFU/ml

Question 5

Which bacteria in saliva would you expect to be responsible for the acid produced in the Snyder test?

Answer 5

Lactobacilli

Question 6

What carbon source are the bacteria fermenting?

Answer 6

Glucose (see the list of ingredients for the Snyder medium).

Question 7

What acid(s) is being formed in the synder medium when bacteria are fermenting?

Answer 7

The by-products of particular species are, therefore, either entirely lactate or a mixture of lactate and, mainly acetate. (some lactobacilli can switch between homo and heterofermentation -some can’t

Question 8

Why do the number of lactobaccilli bacteria increase in caries?

Answer 8

Caries is initiated and then progresses because the pH of the immediate environment is reduced to below the Critical pH (approx 5.2-5.5). This acidic environment favours lactobacilli but not most other oral bacteria. The numbers of lactobacilli therefore increase because they still multiply whereas other species are inhibited.