This set of three Petri dish culture plates contained a Mueller-Hinton agar growth medium that had been inoculated with a culture composed of Staphylococcus sp. bacteria. Atop the growth medium, antibiotic-impregnated disks had been laid, which gave rise to this example of an antibiotic sensitivity test. The three plates were kept, clockwise, at a respective pH 8.0, pH 6.5, and pH 7.2. As you can see, the pH of the agar had a measurable impact on the growth of these bacteria.
The size of the inhibition zones (IZ) surrounding the antibiotic-impregnated paper disks depends on the sensitivity of the bacteria present in the agar, to the antibiotic, the antibiotic's ability to diffuse through the culture medium, and the pH of the medium. In this case, the results were as follows:
- Tetracycline Disks (TE) produced an IZ of 18mm/pH 8.0; 27mm/pH 6.5; 22mm/pH 7.2
- Lincomycin Disks (L) produced an IZ of 23mm/pH 8.0; 12mm/pH 6.5; 18mm/pH7.2
- Streptomycin Disks (S) produced an IZ of 23mm/pH 8.0; 16mm/pH 6.5; 20mm/pH 7.2
- Erythromycin Disks (E) produced an IZ of 32mm/pH 8.0; 23mm/pH 6.5; 27mm/pH 7.2.