Kirby-Hinton Incar Lab Report

The Kirby-Bauer test is standardized in order to allow others to be able to reproduce the test since there are many factors that could affect the zone of inhibition. One of the standardizations is the medium used for the test, a Mueller-Hinton agar, which is different from the general mediums used in laboratories. The Mueller-Hinton agar plate is larger in size than standard agar plates so that multiple types of antibiotics are able to fit without interfering with each other (Slonczewski & Foster, 2015). The medium is thicker in depth (5 mm) and contains loose agar, which allows the antibiotics to diffuse out of the disk and into the media because if this test was conducted on a thinly poured agar plate, then the zone of inhibition would appear larger than it really is (Slonczewski & Foster, 2015). Another important factor about the Mueller-Hinton agar is that it doesn’t contain para-aminobenzoic (PABA) acid, which is usually present in most mediums. But because sulfonamides, a type of sulfa drug, has a similar structure to PABA and both compete for the same enzyme (PABA needs this same enzyme to produce folic acid), PABA is taken out the medium (Slonczewski & Foster, 2015). The Kirby Bauer susceptibility test used antibiotics that targeted the cell wall synthesis such as penicillin, ampicillin, vancomycin, oxacillin,

Sulfa drugs prevent the synthesis of tetrahydrofolic acid (THF), which is needed in order to synthesize nucleic acid. Bacteria synthesize their own folic acid, which is a necessary precursor of THF, and the sulfa drugs interfere with the synthesis of folic acid. This prevents the production of THF, which inhibits the synthesis of DNA (Slonczewski & Foster, 2015). Trimethoprin/sulfamethoxazole was used in the