Question

Which of the following could be used as a negative control in Kirby-Bauer disc diffusion?

Please explain

	A. A disc containing bleach. B. A disc containing glucose C. A disc containing streptomycin D. A disc containing deionized water

Answer 1

D. A disc containing deionized water

Explanation:

If we have two possible outcomes like susceptible or resistant, then a negative control is required to compare the effects of the antibiotic.

• If both plates (positive control with antibiotic disk and a negative control with distilled water) show inhibition of bacteria that means the inhibition is not due to the antibiotic, but some other factor.
• If there is no inhibition of bacteria growth on both dishes with the antibiotic and the negative control (with distilled water), that means the drug has no effect or the bacteria is resistant.
• If there is inhibition with the antibiotic disk, but growth in negative control, that means the organism is susceptible to the antibiotic.