Antibiotic Combinations and Resistance: Response of E. coli to Antibiotics, Singly and in Pairs

The data presented confirm and extend the observations previously made with staphylococci and some streptococci using other antibiotic combinations. For E. coli, and within the limitations of the methods employed, it was shown that following repeated subcultures in pairs of antibiotics, resistance generally developed more slowly and to a smaller degree than when the same organisms were exposed to the same antibiotics individually. There were wide variations, however, depending mainly on the antibiotics used, but perhaps also with the strain of organism. Cross-resistance to neomycin frequently accompanied increases in resistance to streptomycin resulting from exposure to the latter, alone or in combinations. Likewise, cross-resistance to oxytetracycline and chlortetracycline from exposures to chloramphenicol and to chloramphenicol and chlortetracycline following subcultures on oxytetracycline accompanied the development of resistance to the homologous antibiotic, whether present alone or paired with another agent.