The endotoxin-liberating effect of antibiotics on meningococci in vitro.

Three strains of Neisseria meningitis (two endotoxin-liberating and one in vitro variant non-liberating) were studied during treatment with MIC and 100 times MIC values of benzylpenicillin and chloramphenicol in a chemically-defined, protein-free medium. Treatment with the highest dose of benzylpenicillin had the most rapid effect on meningococci, although the antibacterial effect was the same for the two penicillin concentrations after 20 h. Chloramphenicol treatment showed a much slower antibacterial effect. After 2 h of antibacterial treatment, an increase of filtrable endotoxin in the medium was found for the endotoxin-ligerating strains only when the highest penicillin dose was used. During the same period there was a rapid cell death. After 20 hours of treatment, however, the endotoxin-liberating strains treated with high and low concentrations of penicillin had a markedly reduced content of filtrable endotoxin, compared to the controls and to the cultures treated with chloramphenicol. Antibacterial treatment had no, or only minimal, effect on the total content of endotoxin in the cultures, compared to the untreated controls. The endotoxin non-liberating strain had about the same total content of endotoxin as the liberating strains, but did not liberate filtrable endotoxin into the medium unless filtered with a much higher pressure through a filter with smaller pore size.