Effect of serum and blood on Enterobacteriaceae grown in the presence of subminimal inhibitory concentrations of ampicillin and mecillinam.

Bacteria from the strains of five species of Enterobacteriaceae grown in the presence of subminimal inhibitory concentrations of ampicillin or mecillinam formed into filamentous or round cells. These filamentous and round cells as well as normal control bacteria were incubated with either fresh human serum or blood, the bactericidal effects of which were then determined. In most cases, the bactericidal effect of either serum or blood on filamentous or round cells was less than the effect on control cells. In some cases, the effect on these drug-exposed cells was similar to that on control cells, but in no instance was the effect greater for the drug-exposed cells than for the control cells. However, in all cases in which the bactericidal effect of either serum or blood on the control cells was greater than or equal to 99%, the bactericidal effect on the drug-exposed cells was close to 90%. Although drug-exposed cells were not much more resistant to the bactericidal effect of serum or blood than were normal bacteria, they clearly were not more susceptible to these effects.