Modification of bacterial serum susceptibility by rifampin

A subinhibitory concentration of rifampin converted a strain of Escherichia coli from serum resistant to serum susceptible. When continually cultured in nutrient broth containing 1.5 microgram of rifampin per ml, this strain of E. coli became susceptible to killing by both normal human serum and normal rabbit serum. Copared to the original strain, the rifampin-treated E. coli displayed no detectable change in adherence capability, but appeared less virulent in the rabbit model of endocarditis. A rifampin-resistant mutant of the E. coli strain was not found to undergo conversion to serum susceptibility upon culture in rifampin.