R-selection of subbacteriostatic tetracyclin-concentrations.

We investigated the smallest R-factor selecting tetracyclin concentration in a chemostat. All experiments were performed with a miscellaneous culture of three clones of the E. coli K12 921 strain. The first clone was plasmid-free, the second one harboured the R270 (Tc, Su) Inc F II rep and the third one the F'lac-plasmid Inc F I derep. The influence of different tetracyclin concentrations on the development of the viable count of these three clones was investigated in batch and in steady state culture phase. The steady state was kept for about three hours. It served to enforce the selection effect of the antibiotic. A selection effect was already stated with 0.25 microgram Tc/ml. That is to say with 1/10 of the MIC (2.5 micrograms Tc/ml) of the clones used. The importance of these results will be discussed.