In the studies described in the preceding paper, circumstantial evidence was found for transfer of antibiotic resistance from an ingested strain of carrying an R factor, R-1, which determined resistance to ampicillin (A), chloramphenicol (C), kanamycin (K), streptomycin (S) and sulphonamide (Su). Resistant organisms isolated from the faeces of a single individual were examined in detail to prove conclusively that the DNA of the ingested plasmid had transferred to other organisms.

The O antigens of the resistant faecal isolates were the same as those of two organisms present before the beginning of the experiment, and these strains were resistant to all the determinants of R-1. The levels of resistance of the isolates to these antibiotics were quantitatively consistent with transfer of the plasmid R-1; and the β-lactamase conferring ampicillin resistance was of the same antigenic type.

The predominant resistant organism isolated from the faeces transferred resistance to ACKSSu to laboratory recipients, and plasmid DNA isolated from such strains had the same molecular weight as R-1 and gave 100 per cent. DNA hybridisation with DNA from R-1. Small populations of another strain were also isolated which transferred resistance to ACKSSu and to tetracycline, and this plasmid contained all of the DNA base sequence of R-1 plus additional DNA. The serotype of this strain was the same as that of a strain already resistant to tetracycline before the beginning of the experiment.

These results prove that R-factor transfer must have occurred during or after antibiotic therapy and illustrates possible dangers of chemotherapy.


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