The response of Staphylococcus aureus to benzylpenicillin

The response to benzylpenicillin of 2 strains of Staphylococcus aureus was investigated in vitro using 3 techniques in parallel: continuous turbidimetric monitoring, continuous microscopic monitoring, and thin-section electron microscopy. Cultures of staphylococci were exposed to several concentrations of penicillin for various intervals of time before terminating the antibiotic activity with penicillinase. Bacterial lysis by penicillin was concentration-dependent and showed an optimal dosage effect which was very marked for one of the strains. The growth and division of cells continued for up to 1 h in the presence of penicillin. A variety of morphological responses was observed at each penicillin concentration. These changes ranged from complete bacterial lysis to apparently normal cells, and included several different types of aberrant morphological forms. On addition of penicillinase to cultures exposed to penicillin, there was a time interval before survivors began to divide. This period was increased by raising the concentration of penicillin or by increasing the period of exposure to penicillin. Bacteria resuming growth after surviving penicillin action exhibited markedly aberrant septation. Most of the survivors were found to originate from clumps of cocci rather than from individual cells or small groups.