MF19. Oral microbiology practical

Question 1

Why does TYCS distinguish between Strep. mutans and Strep. sanguinis?

Answer 1

The medium uses a high sucrose content to promote the formation of specific glucans by S. mutans thus forming distinctive colonies

Question 2

2. Why does MSB select for mutans streptococci?

Answer 2

It contains the antibiotic bacitracin to which S. mutans and S. sobrinus are resistant.

Question 3

Snyder medium is adjusted to pH___A___before use which means that only __B__ such as Lactobacillus spp are able to grow.

2) what is the selective agent of synder medium?
3) What is the indicator of pH used?

Answer 3

A) 5.0
B) acidophiles
2) pH
3) bromocresol green

Question 4

Snyder medium
At pH __A__ the bromocresol green is ___B (hint a colour)__. This turns to __c___ and then ___d___ as conditions get more acidic due to the production of organic acids such as ____e( the 3 chemicals)__from glucose.

Answer 4

3) a) 5.0
b) blue
c) green
d) yellow
e) lactic, acetic and formic

Question 5

Why is there a lot of lactobacilli in carious lessions?

Answer 5

as it is a low pH , (not a causative agent= except are part of caries progression in dentine)

Question 6

1) is blood agar selective or non-selective-why?

2) is it indicator?-why?

Answer 6

1) non-selective,yes very nutritious

2) yes haemolysis of haemoglobins by haemolysns

Question 7

microbes sampled from oral cavity and cultivated on blood agar, are they cultivated in aerobic or anaerobic conditions?- why?

Answer 7

facultative and obligate anaerobes will both grow in an anaerobic atmosphere but only facultatives will grow in the presence of oxygen. Cultivation in an anaerobic atmosphere will, therefore, more accurately represent the total number of microorganisms present

Question 8

If the loop contains 10microlitres and the diluent volume is 20 ml what is the approximate dilution achieved when the 2 liquids are mixed?

Answer 8

It is roughly a 2,000-fold dilution (note that 1,000 microlitrel = 1 ml).

Question 9

4. Which oral microorganism is most likely to grow in the Snyder Test?

Answer 9

The initial pH of the Snyder Test growth medium is 5.0. This inhibits nearly all oral bacteria except Lactobacilli.

Question 10

What is cfu?

Answer 10

colony forming unit, estimate of the number of viable cells in a sample

Question 11

you have counted 18 colonies on you agar. The sample was 10microlitesl which was added to 20ml of diluent.
100microl were plated out on Blood Agar. What is the cfu? (give you answer cfu/ml)

Answer 11

The dilution is 10 in 20,000 (ie 1 in 2,000).
182000= 3.6 10^4
units ok..
These are the numbers in 100ml so multiply by 10
3.610^4 * 10= 3.610^5 cfu/ml

Question 12

Before CHX 3500 x 10 x 2000 = 7 x 107 cfu/ml

After CHX 18 x 10 x 2000 = 3.6 x 105 cfu/ml

2) What is the percentage reduction?

Answer 12

7 x 107 - 3.6 x 105 = 6.964 x 107 per ml

6.964 x 107 x 100/ 7 x 107 = 99.5%

Question 13

1) What type of plate si a kiss plate (i.e. what is it, selective?, indicative?)
2) explain the observations seen on a kiss plate:

Answer 13

1) blood agar, nutrient-rich non-selective medium, yes indicative
2) The greenish tinge to the agar (a-haemolysis) from the inside of the mouth and tongue is caused by oral streptococci, producing H2O2 which bleaches the haemoglobin (alpha-haemolysis).
The outer regions of the mouth and nose (and chin if you have included it) produce colonies that are non-haemolytic/ gamma. These are mainly staphylococci.