Analysis of rifampin disk diffusion and stability in 7H10 agar.

Rifampin was incorporated into Middlebrook 7H10 medium either by adding an aliquot of the antibiotic into melted agar (final concentration 1.0 and 3.0 mug/ml) or by submerging a 5 or 15 mug of rifampin paper disk into 5 ml of melted agar contained in one quadrant of a Felson "X" plate. At intervals, plugs of agar were removed from the stored plates and assayed. Plates stored at 5 C for 28 days showed no loss of potency; at 37 C, the half-life of rifampin was 9 days. Stability of rifampin at these concentrations in 7H10 medium was independent of the method used for incorporation. Using the disk method, uniform rifampin concentrations of 0.75 mug/ml on day 5 for the 5-mug disk and 2.7 mug/ml on day 6 for the 15-mug disk were observed. Results indicated that the rifampin concentrations within the agar dilution and disk diffusion plates were equivalent at these times.