Beta-lactamases as the main resistance factor to penicillin-related antibiotics.

The interplay between the three factors involved in the resistance of bacteria to beta-lactam antibiotics (sensitivity of target, synthesis of beta-lactamase, permeability barrier) is analysed and discussed on the basis of a simple kinetic model. The three factors do not act independently. In Gram-negative bacteria, the permeability barrier is only significant when the bacterial cell also produces a beta-lactamase. Special attention is devoted to cases where large periplasmic beta-lactamase concentrations prevail, a situation which has been observed in some clinical isolates.