Observations on the tolerance and the paradoxical effect in enterococci.
Enterococci, already know to be relatively unaffected by several antibiotics due to their inheritant characteristics, are increasingly resistant to some very important groups of drugs, by means of acquisition or exchange of new genetic traits of resistance. Resistance or moderate susceptibility towards penicillin is an interesting characteristic of enterococci, whose low degree of susceptibility to this drug is due to a low affinity for penicillin-binding proteins (PBP). Some strains of enterococci are not killed by the action of this drug but are "tolerant" (MIC/MBC > 32 mg/l). This kind of "resistance", in which the probability of surviving under selective pressure of the drug is increased, is probably linked to the deficiency of the cell's autolytic system. Only rarely does another form of resistance called the "paradoxical effect" appear, in which higher numbers of cells survive at high concentrations than at lower concentrations. In our study the degree of bactericidal activity of some beta-lactams was considered. Our results demonstrate that: i) the paradoxical effect appears more in cultures in exponential phase compared to aged cultures; ii) mutated strains show an increased number of cells that respond paradoxically (the behavior is genetically determined); iii) different beta-lactams induced different degrees of autolysis.