Inhibition of the autolytic system by vancomycin causes mimicry of vancomycin-intermediate Staphylococcus aureus-type resistance, cell concentration dependence of the MIC, and antibiotic tolerance in vancomycin-susceptible S-aureus

Treatment of the fully vancomycin-susceptible Staphylococcus aureus strain COL with subinhibitory concentrations of vancomycin allowed its continued growth but generated a phenotype reminiscent of some S, aureus isolates with vancomycin-intermediate S. aureus (VISA)-type resistance: the bacteria grew in multicellular clusters; electron microscopy showed inhibition of cell separation and accumulation of amorphous cell wall-like material at the bacterial surface. Titration of free vancomycin showed a gradual disappearance of the drug from the medium, which-eventually-coincided with an increase in the growth rate, burst in viable titer, and dispersal of cellular clusters. Addition of inhibitory concentrations of vancomycin to the same strain at a higher cell concentration caused a very different-antibiotic-tolerant-response: an immediate halt in growth, followed by a prolonged lag, during which there was neither a loss of viable titer or optical density nor a change in cell morphology but a gradual removal of vancomycin from the medium to the cell wall of the bacterium, from which the antibiotic could be recovered in a biologically active form. Eventually, the drug-treated culture resumed normal growth. The transient appearance of both the VISA phenotype and vancomycin tolerance could be traced to the inhibition of the autolytic system of the bacterium by vancomycin molecules attached to the cell wall, blocking the access of a staphylococcal murein hydrolase(s) to its cell wall substrate.