Antibiotic-resistance transfer between populations of donor and recipient strains of Escherichia coli was compIeteIy inhibited in broth by dense suspensions of Bacteroides fragilis. Comparable amounts of inert bacterial matter (formolised suspensions of E. coli or B. fragilis), or smaller numbers of viable B. fragilis, Streptococcus faecalis, Staphylococcus albus, Neisseria catarrhalis, or solutions of sodium taurocholate or glycocholate were only moderately inhibitory. Anaerobiosis had no effect upon plasmid transfer. Population densities of enteric organisms in these studies were similar to those found in faeces. The presence of dense cultures of B. fragilis provide a satisfactory explanation for the almost total inhbition of conjugation in the human gut. Other factors inhibiting conjugation to a lesser degree may reinforce the effect of B. fragilis in vivo.

As well as selecting for resistant organisms, antibiotics may also indirectly increase populations of R-factor-bearing organisms in the gut by interfering with the anaerobic flora and so permit an increase in the frequency of conjugation.

I am very grateful to Professor W. A. Gillespie for his interest and to Miss M. A. C. Jones for technical help. M. A. C. J. was supported by a Programme Grant to Professor M. H. Richmond from the Medical Research Council.


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