COMPARISON OF TUBE DILUTION AND MICROTITRE METHODS FOR THE DETECTION OF ANTIBIOTIC TOLERANCE IN STRAINS OF STAPHYLOCOCCUS-AUREUS

Comparison of the MIC [minimum inhibitory concentration] and MBC [minimum bactericidal concentration] of oxacillin for 10 isolates of S. aureus determined by tube dilution with those by microtiter dilution tests demonstrated that, whereas the MIC did not differ, the mean MBC by tube dilution was significantly higher (P < 0.05) than by microtiter after 24 and 48 h of incubation. Tolerance (MBC/MIC ratio > 32) was seen in 4 of 10 strains with the tube dilution method but in only 1 strain with the microtiter technique after 24 h of incubation. These results and the poor correlation (r = 0.33) between MBC by these methods indicate the importance of technical factors in the quantitation of MBC and resulting identification of tolerance in S. aureus, and the need for a reference procedure in determination of MBC.