The Bactericidal Action Of -Lactam Antibiotics on an Autolysin-deficient Strain of Bacillus subtilis

Abstract

An autolysin-deficient mutant of B. subtilis was completely tolerant to 5 h incubation with 50-100 .mu.g cycloserine ml-1; the wild-type was rapidly lysed and killed by 12 .mu.g ml-1. Lysis did not occur when low concentrations of .beta.-lactams were added to exponentially growing cultures of the mutant, but > 90% of the bacteria were killed within 90-120 min. Protein, lipid and peptidoglycan synthesis and growth were inhibited after .apprx. 60 min. At this time, but not earlier, small amounts of these 3 cell components appeared in culture supernatants. Earlier, at .apprx. 20-30 min, the intracellular pools of amino acids started to decline rapidly; there was a temporary apparent increase in the rate of lipid synthesis. Neither of the latter phenomena occurred with cycloserine, with which protein and lipid synthesis declined only slowly and the rate of peptidoglycan synthesis was 80% inhibited within 30 min. Only occasional cells with damaged walls were seen 30-90 min after addition of .beta.-lactams or cycloserine to the culture. Thus, wall hydrolysis or penetration by residual autolysins in the mutant are probably not responsible for mass cell death caused by the .beta.-lactams.