The transfer of R factors in the human gastro-intestinal tract and the conditions under which it occurs was studied in healthy persons, who ingested derivatives of containing chromosomal and R-factor-mediated resistance determinants that had been prepared from strains isolated from their own faeces. The experiments were designed to obtain presumptive evidence of survival and transfer of R factors by studying resistance-marker patterns of organisms isolated from the faeces. To prove conclusively that R-factor transfer had occurred and that the observed marker patterns were not fortuitous, studies of resistance-marker patterns in one of these subjects were supplemented by further detailed investigations including serological and molecular studies, which will be described in the succeeding paper.

An incidental finding was a significant positive correlation between the mean number of antibiotic-resistance determinants in resistant organisms and the age of an individual, although these studies were limited to a small and highly selected group of subjects.

No evidence of R-factor transfer was obtained in the absence of chemotherapy in four subjects, even though the plasmids concerned could be freely transferred from the potential donors to a wide range of faecal organisms, including the genetically marked ingested potential recipients. However, treatment of three subjects with a five-day course of a relevant antibiotic after ingestion of R factor bearing organisms led to the appearance in the faeces of large numbers of resistant coliform bacilli with antibiotic-resistance patterns consistent with transfer of the R factor either to an ingested potential recipient organism (one subject) or to endogenous strains of (two subjects).

Chemotherapy greatly prolonged the persistence of ingested R factors in the faecal flora long after the antibiotics were withdrawn and caused the number of R-factor bearing organisms in the faeces to increase. No evidence was obtained in limited studies of fragmentation or loss of R factors from ingested .


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