A newly discovered Bacteroides conjugative transposon, CTnGERM1, contains genes also found in gram-positive bacteria.

The human intestinal tract is thought to be a site in which horizontal transfer of genes might occur fairly often. The concentration of bacteria in the colon is high, and factors such as the availability of abundant nutrients and surfaces on which biofilms could form would seem to be conducive to gene transfer (8, 11). It has been difficult to test this hypothesis, however, because of limited information about gene transfer elements in the predominant genera of colon bacteria. The numerically predominant bacteria in the colon are all obligate anaerobes, including Bacteroides species and a mixture of gram-positive genera (8). Virtually nothing is known about gene transfer elements of the gram-positive anaerobes, but some of the Bacteroides gene transfer elements have been characterized (13). Accordingly, attempts to assess the extent to which horizontal gene transfer occurs in the colon have focused on Bacteroides species. Many Bacteroides strains carry plasmids, but a family of self-transmissible integrated elements called conjugative transposons (CTns) seems to be particularly widespread and may be the primary drivers of horizontal gene transfer in this genus (13, 18). These CTns are exemplified by CTnDOT and CTnERL, two CTns that are virtually identical except for a 13-kbp region in CTnDOT that contains an erythromycin resistance gene, ermF (23). Both CTnERL and CTnDOT carry the tetracycline resistance gene tetQ, and transfer of both of these CTns is stimulated 1,000- to 10,000-fold by exposure of donors to tetracycline (12, 13, 24). More than 80% of Bacteroides strains isolated since 1980 harbor one or more of these CTns, compared to 22 to 30% of strains isolated prior to 1970 (18). Clearly, these CTns have been in Bacteroides strains for some time and are continuing to spread actively within the genus. These data do not, however, address the question of the extent to which horizontal gene transfer might be introducing new DNA elements into Bacteroides species. A recent survey of 289 Bacteroides isolates showed that, as expected, ermF was the predominant erythromycin (MLSB) resistance gene in Bacteroides species (18, 23). Nonetheless, nine isolates were identified that contained ermG—a class of erm gene that has been found previously only in a gram-positive bacterium, Bacillus sphaericus (7). The DNA sequences of the ermG genes found in the Bacteroides isolates were over 99% identical to the DNA sequence of the Bacillus sphaericus gene (3). Finding virtually identical antibiotic resistance genes in two such distantly related genera suggested that ermG is being transmitted horizontally, possibly from gram-positive bacteria into Bacteroides species. Moreover, since the ermG was found primarily in Bacteroides strains isolated after 1990, it was clear that ermG had come into Bacteroides species fairly recently—presumably from some other genus (3). Broad host range transfers usually occur by conjugation, a process thought to help DNA escape restriction modification systems in the recipients, but whether ermG was on a conjugative element was not known. We report here that the ermG gene is carried on an integrated conjugative element, a CTn, which is different from CTnERL and CTnDOT and may have entered Bacteroides species much more recently than did the CTnERL- and CTnDOT-type elements. Our findings provide additional evidence which suggests that the exchange of DNA in the human colon may involve genera other than Bacteroides.