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
reprod~cible.~’*‘Oq It has been shown, in this study and
by others,12*1t3h at 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 penicillin20,21w
ere applied. Moreover, the fact that it is
essential to obtain the desired phase of growth for
each individual strain may be overlooked.