Effect of subinhibitory concentrations of mecillinam on the serum susceptibility of Escherichia coli strains

For serum-resistant clinical isolates of Escherichia coli were grown in the presence of various subinhibitory concentrations of mecillinam or pivmecillinam and then exposed to the bactericidal action of human serum. All strains became more serum susceptible as a result of pregrowth in medium containing mecillinam, but the concentration of antibiotic needed to produce the effect varied according to the strain being used. Production of ovoid or round cells was a prerequisite for sensitization to serum. Growth in the presence of mecillinam did not alter the response to serum of a serum-susceptible E. coli strain.