[Selection of R plasmid-carrying E. coli pathogens in a mixed population by subinhibitory antibiotic concentrations]

The effect of subinhibitory concentrations of tetracyclin, chloramphenicol, gentamicin and ampicillin in a mixed culture of E. coli was investigated in a chemostat in order to determine minimal selection concentration of these drugs. The mixed culture consisted of three clones of E. coli K12 921. One of these clones was plasmid free, one harboured the F'lac plasmid and the third one an R-factor. The antibiotics tetracyclin, chloramphenicol and gentamicin selected the antibiotic resistant R-factor bearing strain at a concentration of 1/10 of the minimal inhibition concentration. Subinhibitory concentrations of ampicillin failed to select the R-factor bearing strain because this strain produced a secreted betalactamase.