Tolerance is the resistance characterised by a decrease in the bactericidal effect of 3-lactam antibiotics,as measured by MBC determinations. Both macrodilution and microdilution methods for the assessment
of the MBC are susceptible to several technical problems which make assessments of the MBC poorly reprodcible. It has been shown, in this study and by others,that technical factors are of the utmost importance. In this study the macrobroth MIC method, full 24-h incubation of the tubes, gentle shaking of MIC tubes 4 h before MBC plating and the use of 3-lactamase to prevent carry-over of penicillin 20,21 were applied. Moreover, the fact that it is essential to obtain the desired phase of growth for each individual strain may be overlooked.