You are working with a new molecule that you think may be an effective antibioti

Question

You are working with a new molecule that you think may be an effective antibiotic. Below is some experimental data you have collected. You inoculated a series of liquid cultures with different concentrations of drug to a starting OD of 0.050. The next day, you read the optical density of those cultures. Then, you take cells from that culture and plate them on solid media without drug. 6. OD of a liquid culture grown with drug 0.845 0.793 0.826 0.067 0.053 0.064 0.066 Number of colonies on a plate without drug 6.4 x 108 6.2 x 108 6.3 x 108 3.5x 107 3.6 x 107 3.4 x 107 5 Drug concentration No drug 1 ug/mL 2 ug/mL 4 ug/mL 8 ug/mL 16 ug/mL 32 ug/mL What is the MIC of your molecule? a. b. What is the MBC of your molecule? c. Is your molecule bactericidal or bacteriostatic? Why?

Explanation / Answer

a) the minimum inhibitory concentration (MIC) is defined as the lowest concentration of a chemical at which there is no visible growth of a bacterium. and is usually measure by looking at OD of liquid cultures.

So in the given example at 4 microgram/ ml, no growth in liquid culture is observed.

Hence MIC is 4

b) The minimum bactericidal concentration is defined as the lowest concentration of an antibacterial agent that is required to kill a particular bacterium. The MBC is ususally determined by measuring the cfu.

Therefore in the given example, 32 microgram/ml is the concetration at which no growth in bacteria is observed.

Hence MBC is 32.

c) The molecule is bacteriocidal. This is because, cfu is measured by plating bacteria on plates that do not contain any of the antibacterial agent.

If the given molecule was baceriostatic (chemicals that does not kill but arrests the growth of bacteria), as long as the agent was present, the baceria dont divide. But as soon as the agent is removed, like on the plate the bacteria would divide and form a lot of colonies.

Instead, if the molecule is bacteriocidal (chemicals that kill bacteria), the cells die in its presence and hence even when plalted without the chemical there are no live cells to form colonies.

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