Mechanisms of resistance to growth inhibition and killing by beta-lactam antibiotics in enterococci.

Enterococci are characterized by an intrinsic resistance to growth inhibition by beta-lactam antibiotics. The low susceptibility of enterococci to beta-lactam antibiotics is associated with the synthesis of a particular penicillin-binding protein (PBP) that has a low affinity for beta-lactam agents. This protein appears to be capable of taking over the function of the other PBPs when they are saturated with beta-lactam molecules or inactivated by mutations. A quantitative correlation can be established between the binding of several beta-lactam molecules to the low-affinity PBP and the minimal inhibitory concentration for enterococcal strains. In contrast, the mechanism of enterococcal resistance to the bactericidal activity of beta-lactam agents that inhibit growth at relatively low concentrations appears to be associated with an alteration in the pattern of autolytic enzyme activity. In particular, lack of or poor activity of an autolytic enzyme appears to be responsible for the paradoxical bactericidal response often exhibited by clinical isolates of Enterococcus faecalis in the presence of penicillin.