Effect of beta-lactam antibiotics at subinhibitory concentrations on the phagocytosis of Staphylococcus aureus by the perfused rat liver.

We evaluated whether the exposure of Staphylococcus aureus to a subinhibitory concentration of penicillin and cefamandole could modify its susceptibility to rat serum factors and to the phagocytic activity of the isolated and perfused rat liver. Control or sub-MIC treated bacteria were added to the circulating medium which contained homologous serum, and the disappearance of bacteria from the perfusate and their recovery in the liver was determined during the 10 min experimental time. Sub-MIC treated bacteria were more susceptible to the bactericidal activity of serum present in the perfusate. However, the clearance rate of bacteria by the liver was decreased for penicillin-treated organisms and unchanged for cefamandole-treated bacteria. The data suggest that beta-lactam antibiotics at sub-MIC levels may modify S. aureus susceptibility to host defense mechanisms.