Effect of combinations of antibiotics on lysis of Staphylococcus aureus by penicillin.

In the presence of penicillin, suspensions of staphylococci tend to increase greatly in density for a few hours and then undergo rapid clearing. The increase in density has been shown to be due chiefly to swelling rather than multiplication, and the rapid clearing is caused by lysis of the organisms (Chain and Duthie, 1945). This phenomenon has been observed with visual methods of comparing the turbidity of bacterial suspensions (Todd, 1945; Spicer and Blitz, 1948) and has been measured more accurately with the aid of a spectrophotometer (Chain and Duthie, 1945; Kirby, 1945). It is not known whether penicillin exerts a specific lytic action upon staphylococci or whether they are destroyed by autolytic enzymes once they become nonviable. Bronfenbrenner and Muckenfuss (1927) found some years ago that staphylococcal autolysin has a powerful lytic action upon dead, but not living, staphylococci. This same is true of pneumococci (Avery and Cullen, 1923) which also are lysed by penicillin. On the other hand, hemolytic streptococci, which do not contain an autolysin, are said to undergo lysis in the presence of penicillin (Todd, 1945; Spicer and Blitz, 1948). It has been shown recently that aureomycin, chloramphenicol, and oxytetracycline interfere with the bactericidal action of penicillin against hemolytic streptococci and enterococci (Jawetz, 1952). The present experiments were undertaken to study the effects of combinations of antibiotics upon killing and lysis of staphylococci. Striking alterations were observed in the lysis curves, and evidence was obtained which further clarifies the nature of lysis in the presence of penicillin.