Comparison of tube dilution and microtitre methods for the detection of antibiotic tolerance in strains of Staphylococcus aureus.

Comparison of the MICs and MBCs of oxacillin for ten isolates of Staphylococcus aureus determined by tube dilution with those by microtitre dilution tests demonstrated that, whereas the MICs did not differ, the mean MBC by tube dilution was significantly higher (P less than 0.05) than that by microtitre after both 24 and 48 h of incubation. In addition, tolerance (MBC/MIC ratio greater than 32) was seen in four of ten strains with the tube dilution method but in only one strain with the microtitre technique after 24 h of incubation. These results and the poor correlation (r = 0.33) between MBCs by these methods indicate the importance of technical factors in the quantitation of MBCs and resulting identification of tolerance in Staph. aureus, and the need for a reference procedure in determination of MBCs.