Mechanism of intrinsic penicillin-resistance in Staphylococcus aureus. Binding of penicillin G to the cytoplasmic membrane of resistant staphylococci.

In staphylococci (and other gram-positive bacteria) the phenotypical expression of intrinsic penicillin resistance as well as the frequency of this resistance are unknown. A permeability barrier in the cell wall, as it was established in gram-negative bacteria, is unlikely in staphylococci and was excluded by experiments with whole cells. Assuming that in staphylococci intrinsic resistance is also located in the cell envelope, the cytoplasmic membrane should be the site of responsibility. Therefore the binding of (14C)-penicillin G to the isolated membrane fraction of four staphylococcal strains resistant to penicillin G was studied. Two strains showed an additional resistance to methicillin, one of them was penicillinase-negative. Compared with two sensitive strains the penicillin binding was greatly reduced in the membranes of the resistant ones. This was not due to a loss of specific receptors. The maximum specific binding capacity of about 20 nmol of (14C)-penicillin G/g membrane protein was found in the resistant strains as well as in the sensitive controls. Furthermore penicillin G irreversibly bound also in the resistant bacteria. Differences, however, were found regarding the affinity of the binding sites for penicillin. In all resistant strains it was reduced by a factor of about 500. Our findings confirm the assumption that in staphylococci the mechanism of intrinsic penicillin resistance is located in the cytoplasmic membrane. They can best be explained when considering that not the penicillin binding components themselves but the membrane structure in their environment is altered thus preventing the access of the antibiotic to its site of action in the membrane. In addition, the homogeneity of our results indicates that intrinsic resistance to penicillin G is, contrary to current opinion, a general phenomenon in resistant staphylococci.