Quantitative relationship between sensitivity to beta-lactam antibiotics and beta-lactamase production in gram-negative bacteria--I. Steady-state treatment.

A simple graphical method is proposed to decide whether a decreased sensitivity to beta-lactam antibiotics can be explained by the sole increase of beta-lactamase production or if other factors such as outer membrane permeability or target sensitivity must also be involved. The parameters which are necessary for the analysis can easily be measured: the relative amounts of beta-lactamase produced by the various strains; the Km of the enzyme for the various beta-lactams under study and the susceptibility of each strain to the same antibiotics, measured as MIC values.