Extracellular and membrane-bound beta lactamase of Staphylococcus aureus: their importance for the expression of penicillin resistance.

The synthesis and excretion of beta lactamase by several strains of Staphylococcus aureus from different clinical sources and the ability of both the extracellular and membrane-bound enzyme to mediate penicillin resistance was studied. When beta-lactamase production was maximally induced with penicillin G or ampicillin, about 50% of the beta lactamase was excreted from the cells, the amount of extracellular enzyme correlating well with the degree of resistance established by an in-vitro test model. From penicillin-binding experiments it became apparent, however, that the membrane-bound beta lactamase can also constitute a barrier, strong enough on its own to prevent penicillins from reaching their target. This could be of clinical relevance if, under certain conditions in vivo, the extracellular beta lactamase is insufficient for full protection of the staphylococcal cells.