Factors that may prevent transfer of anti-biotic resistance between gram-negative bacteria in the gut.

Antibiotic-resistance transfer between populations of donor and recipient strains of Escherichia coli was completely inhibited in broth by dense suspensions of Bacteroides fragilis. Comparable amounts of inert bacterial matter (for-molised 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. Anaeroboisis 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 almost total inhibition 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.