Binding characteristics of various penicillins in Staphylococcus aureus

Binding studies with penicillins other than penicillin G are rare in staphylococci. Therefore the binding of a series of penicillins to the isolated cytoplasmic membrane of S. aureus H was examined indirectly by inhibition of (14C)-penicillin G binding. Biphasic inhibition curves were obtained with the beta-lactamase-insensitive penicillins as well as with ampicillin, ciclacillin, and 6-aminopenicillanic acid (6-APA) indicating that there are at least two types of targets. 60--75% of them were those with a high affinity of 10(7)--10(8) l/mol, the affinities of the second type of binding sites being in the range of 10(4)--10(5) l/mol (except 6-APA: 10(5) l/mol resp.). Monophasic binding curves were produced by penicillin G, penicillin V, propicillin, mezlocillin, azlocillin, carbenicillin, ticarcillin and mecillinam. With both groups of penicillins reversible binding to some extent was found. From comparative analysis of the binding of particular penicillins it can be finally concluded that there exist four types of binding sites in staphylococcal membranes. This agrees with the current state of knowledge in the field of penicillin-binding proteins. A good correlation was found between the 50% binding value and the minimum inhibitory concentration for most of the penicillins. As half saturation also with the "biphasic" penicillins was always achieved by (irreversible) binding to the high-affinity targets, it may be suggested that the low-affinity binding sites are probably not related to the antibacterial action of penicillins in staphylococci.