[In vitro investigations of rifampicin. Correlation between minimal inhibitory concentration and minimal inhibitory zone].

Regression analyses demonstrated a good correlation between the minimum inhibitory concentrations of rifampicin determined by the agar dilution method in Mueller-Hinton or Iso-Sensitest agar and the diameters of the inhibition zones produced by the antibiotic in the agar diffusion test on the same two media. The inhibition zone diameters produced in cultures of gram-positive bacteria most of which are highly sensitive to rifampicin (MIC less than 0.125 microgram/ml), are greater than 32 mm. Clearly discernible zones of inhibition ranging from 9 to 24 mm in diameter are, however, also found with gram-negative bacteria, which are less sensitive to the drug (MIC 4--32 microgram/ml. The simple agar diffusion method consequently provides a reliable indication of the sensitivity of bacteria to rifampicin.