Antimicrobial sensitivity testing of the organism can be performed when there are choices to be made regarding antimicrobial therapy, particularly when resistant organisms are more likely to be present.
The Kirby-Bauer plate shown below illustrates the diffusion disk method in which antibiotic-impregnated disks are placed on a culture plate with the organism, and the zone of inhibition of growth is measured, compared to controls. In general, less than 2 mm of inhibition defines resistance, while a large zone of inhibition suggests sensitivity. Smaller zones imply intermediate sensitivity.
In the example below, the organism is sensitive to Amikacin and Penicillin; intermediate sensitivity to Gentamicin and Cefotaxime; and resistance to Methicillin, Vancomycin, and Nafcillin.
The series of test tubes shown below demonstrate the minimum inhibitory concentration (MIC) method. 1 ml of broth containing the organism is added to each tube; the tubes contain serial dilutions of an antibiotic, ranging from 0.5 microgram/mL to 64 microgram/mL. A control tube contains no antibiotic. The MIC is the tube with the lowest level (highest dilution) of antibiotic in which no growth occurs.
In the example below, the MIC is 2 microgram/mL for ampicillin.
The MIC is an in vitro assay. One may want to know what effectiveness an antibiotic has in the patient's serum. A minimum inhibitory dilution (MID) may be performed by taking a patient serum sample, making serial dilutions, and placing those patient serum dilutions into broth growing the organism isolated from the patient. In the example below, the MID is performed to correlate with doses of the antibiotic given and measured in the patient's serum as "peak" and "trough" levels just after and just before administration of the dose, respectively. The MID is at a 1:4 dilution for the trough level of amikacin, and at 1:32 for the peak level. In general, an MID of 1:32 suggests good antibiotic effectiveness.