Question

If a bacterial species is not susceptible to an antibacterial drug at the concentration present in a particular disk, does that necessarily mean the species is completely resistant to the drug? Explain your answer

Answer 1

Antibacterial sensitivity test are performed by adding a specific concentration of an antibiotic on a disc and placing them on the agar plate containing the test organism. If there is a zone of inhibition around the disc (no growth) after incubation, then it indicates that the organism is sensitive to the antibiotic at that concentration. However, if no zone of inhibition is obtained, then the test organism is resistant to the drug at that concentration only. It is quite possible that the test organism may be inhibited at a higher concentration of the drug that what was applied on the disc.

In order to find the effective concentration, a minimum inhibitory concentration test should also be performed. MIC test can be performed in test tubes containing fixed number of test organism and varying the concentration of the antibiotic. If there is no growth at higher concentration than what was applied to disc, then only it can be said that the test organism is resistant to the drug. This susceptibility testing can also be performed using disc diffusion method as well where disc impregnated with varying concentration of the drug are applied on same plate with test organism.

Some antibiotics also have poor diffusion capacity in agar due to their higher molecular weight. These include Vancomycin, colistin, and clarithromycin etc. Hence, the organism shows growth, not because it is resistant, but because of exposure to less concentration of antibiotic. Disc diffusion is used when antibiotic sensitivity of the test organism to multiple antibiotics needs to be studied simultaneously.