Question

1. The size of the zone of inhibition often varies for two different species of bacteria when challenged with the same antibiotic. What is a possible explanation for these differences?

2. Antibiotics can be bactericidal (they kill the microbe) or bacteriostatic (they inhibit microbial growth, but do not kill). When observing a zone of inhibition around a disk, how would you to determine whether the antibiotic causing that zone was bactericidal or bacteriostatic?

3. When performing antibiotic sensitivity testing, it is important that the test conditions are kept very standardized (e.g. same kind of media, agar depth, age of culture). Why?

4. Are there any other substances besides antibiotics we could evaluate using this method?

Answer 1

1. Varying size of zone of inhibition in diffrent bacteria for the same antibiotic is due to the resistivity. Some bacteria has resistance for some antibiotics. This means that the zone of inhibition is reduced when there is a resistivity.

2. Size of the zone of inhibition defines the bactericidal or bacteriostatic activity of an antibiotic. If the zone is larger then, it has bactericidal and if the zone is medium then, it has bacteriostatic activity. This should be compared with the standard chart.

3. Yes, all the plates should be maintained in a standardised condition. This is to make sure that the growth is affected due to antibiotic not because of lack of condition.

4. Yes, we can use chemicals such as disinfectant. This would help to know the antibacterial activity of that agent.