Comparison of effects of penicillin minimal inhibitory and sub-inhibitory concentration on Staphylococcus aureus and Streptococcus faecium does not support the view that antibiotic sub-inhibitory concentrations can specifically interfere with bacterial virulence.
The effect of minimal (MIC) and sub-minimal (sub-MIC) inhibitory concentrations of penicillin on Staphylococcus aureus and Streptococcus faecium were compared. It was found that similar alterations in both cell shape and ultrastructure were found in the presence of penicillin MIC and sub-MICs, the only difference being that while in the presence of penicillin MIC all individual cells were altered, in the presence of sub-MICs the damaged portion was smaller the lower the penicillin concentration and the longer the incubation time. By testing the effect of inoculum size on the penicillin MIC, it was found that penicillin concentrations, which were sub-MICs for rather dense population, turned out to be the MICs for lower density populations. These findings do not support the view that sub-MICs of antibiotics can cause specific damage to bacterial cells which, although not leading to growth inhibition, lowers their virulence. On the contrary, it is suggested that penicillin sub-MICs have no specific effect on Staphylococcus aureus and Streptococcus faecium cells, but simply differ from MIC in that they do not inhibit all cells.
D008826:Microbial Sensitivity Tests; D008854:Microscopy, Electron; D010400:Penicillin G / Q000494:pharmacology*; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000254:growth & development / Q000472:pathogenicity / Q000648:ultrastructure; D013291:Streptococcus / Q000187:drug effects* / Q000254:growth & development / Q000472:pathogenicity / Q000648:ultrastructure; D014774:Virulence
D010400:Penicillin G