Evidence for Extensive Resistance Gene Transfer among Bacteroides spp. and among Bacteroides and Other Genera in the Human Colon

Abstract

Transfer of antibiotic resistance genes by conjugation is thought to play an important role in the spread of resistance. Yet virtually no information is available about the extent to which such horizontal transfers occur in natural settings. In this paper, we show that conjugal gene transfer has made a major contribution to increased antibiotic resistance in Bacteroides species, a numerically predominant group of human colonic bacteria. Over the past 3 decades, carriage of the tetracycline resistance gene, tetQ, has increased from about 30% to more than 80% of strains. Alleles of tetQ in different Bacteroides species, with one exception, were 96 to 100% identical at the DNA sequence level, as expected if horizontal gene transfer was responsible 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 observed. Carriage of tetQ and the erm genes was the same in isolates taken from 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 clinical environments. The high percentage of strains that are carrying these resistance genes in people who are not taking antibiotics is consistent with the hypothesis that once acquired, these resistance genes are stably maintained in the absence of antibiotic selection. Six recently isolated strains carried ermB genes. Two were identical to erm(B)-P from Clostridium perfringens, and the other four had only one to three mismatches. The nine strains with ermG genes had DNA sequences that were more than 99% identical to the ermG of Bacillus sphaericus. Evidently, there is a genetic conduit open between gram-positive bacteria, including bacteria that only pass through the human colon, and the gram-negative Bacteroides species. Our results support the hypothesis that extensive gene transfer occurs among bacteria in the human colon, both within the genus Bacteroides and among Bacteroides species and gram-positive bacteria.

Bacteroides: the Good, the Bad, and the Nitty-Gritty

“…Conjugative transposons have been largely responsible for the spread of tetracycline and erythromycin resistance in clinical isolates of Bacteroides (250). Fifty years ago, tetracycline resistance was almost nonexistent in this genus (103).…”

“…Conjugative transposons have a mechanism of excision and integration that resemble some features of both plasmids and bacteriophage (231). Conjugative transposons are practically ubiquitous among the Bacteroides: over 80% of Bacteroides strains contain at least one conjugative transposon (250).…”

The Gut Microbiota: Ecology and Function

“…Studies have shown that several antibiotic resistance genes are stably maintained in the GI tract once they are acquired (Shoemaker et al, 2001). These genes are often carried on conjugative elements and, under selection pressure, they have the potential to be transferred between different species within the densely populated gut microbial community.…”

Diverse Tetracycline Resistance Genotypes of Megasphaera elsdenii Strains Selectively Cultured from Swine Feces

“…Escherichia coli and Bacteroides and Enterococcus spp. have been used to monitor the flow and persistence of antibiotic resistance determinants within and between intestinal ecosystems (1,8,22,23,28,29). M. elsdenii is a common anaerobe in the intestinal tracts of both ruminant and nonruminant mammals, including humans.…”