Evidence for extensive resistance gene transfer among Bacteroides spp. andamong Bacteroides and other genera in the human colon

Transfer of antibiotic resistance genes by conjugation is thought to play a n important role in the spread of resistance. Yet virtually no information is available about the extent to which such horizontal transfers occur in n atural settings. In this paper, we show that conjugal gene transfer has mad e a major contribution to increased antibiotic resistance in Bacteroides sp ecies, a numerically predominant group of human colonic bacteria. Over the past 3 decades, carriage of the tetracycline resistance gene, tetQ, has inc reased from about 30% to more than 80% of strains. Alleles of tetQ in diffe rent Bacteroides species, with one exception, were 96 to 100% identical at the DNA sequence level, as expected if horizontal gene transfer was respons ible for their spread. Southern blot analyses showed further that transfer of tetQ was mediated by a conjugative transposon (CTn) of the CTnDOT type. Carriage of two erythromycin resistance genes, ermF and ermG, rose from <2 to 23% and accounted for about 70% of the total erythromycin resistances ob served. Carriage of tetQ and the emt genes was the same in isolates taken f rom healthy people with no recent history of antibiotic use as in isolates obtained from patients with Bacteroides infections, This finding indicates that resistance transfer is occurring in the community and not just in clin ical environments. The high percentage of strains that are carrying these r esistance genes in people who are not taking antibiotics is consistent with the hypothesis that once acquired, these resistance genes are stably maint ained in the absence of antibiotic selection. Six recently isolated strains carried ermB genes. Two were identical to erm(B)-P from Clostridium perfri ngens, and the other four had only one to three mismatches. The nine strain s with ermG genes had DNA sequences that were more than 99% identical to th e ermG of Bacillus sphaericus. Evidently, there is a genetic conduit open b etween gram-positive bacteria, including bacteria that only pass through th e human colon, and the gram-negative Bacteroides species. Our results suppo rt the hypothesis that extensive gene transfer occurs among bacteria in the human colon, both within the genus Bacteroides and among Bacteroides speci es and gram-positive bacteria.