clinical microbiologist makes serial dilutions of several antimicrobial agents in broth, and then incubates Irug dilution series with a standard amount of a patient’s isolated pathogen. What is this microbiologist up?
by-Bauer
B. antibiogram
C. E-test
D. MIC
E. therapeutic index (TI)
The Correct Answer and Explanation is:
The correct answer is D. MIC (Minimum Inhibitory Concentration).
Explanation:
The clinical microbiologist is performing a serial dilution of antimicrobial agents in broth, and then incubating these dilutions with a standard amount of a patient’s isolated pathogen. This technique is used to find the lowest concentration of an antimicrobial agent that will inhibit visible growth of the microorganism. This lowest concentration is called the Minimum Inhibitory Concentration (MIC).
What is MIC?
- MIC is the smallest concentration of an antibiotic that prevents visible bacterial growth after overnight incubation.
- It is a quantitative measure, providing specific numerical data on the sensitivity of the pathogen to the antimicrobial drug.
- This allows clinicians to determine the effective dosage of the drug needed to inhibit the pathogen.
Why serial dilution?
- Serial dilutions involve preparing a series of tubes or wells with decreasing concentrations of the antimicrobial agent.
- Each tube is inoculated with the same amount of bacteria.
- After incubation, the tubes are checked for bacterial growth.
- The MIC is identified as the lowest drug concentration in which no visible growth occurs.
How is MIC used clinically?
- It helps tailor antibiotic therapy to the patient’s infection by selecting a drug and dose that will achieve concentrations at or above the MIC in the body.
- Knowing the MIC can prevent under-dosing (which may fail to clear infection) or over-dosing (which may cause toxicity).
- It is especially important for serious infections or when resistant organisms are suspected.
Other options explained:
- A. Kirby-Bauer (Disk Diffusion) Test: This involves placing antibiotic-impregnated disks on an agar plate inoculated with bacteria. The zone of inhibition is measured, but this is a qualitative test, not serial dilution.
- B. Antibiogram: A profile summarizing the susceptibility of various pathogens to different antibiotics, often compiled from multiple MIC or disk diffusion tests. It is not the procedure described here.
- C. E-test: A plastic strip with a gradient of antibiotic concentrations is placed on agar inoculated with bacteria; the MIC is read where bacterial growth intersects the strip. This is similar to MIC but uses an agar method, not broth serial dilution.
- E. Therapeutic Index (TI): This is the ratio of the toxic dose to the therapeutic dose of a drug; it is a pharmacological concept, not related to microbiological dilution testing.
In summary, the microbiologist is performing a MIC test by serially diluting antibiotics and incubating with the bacteria to find the minimum concentration that inhibits bacterial growth. This is a critical lab method for guiding effective antimicrobial therapy.