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Appl Environ Microbiol. Nov 2003; 69(11): 6455–6463.
PMCID: PMC262298

A New Bacteroides Conjugative Transposon That Carries an ermB Gene


The erythromycin resistance gene ermB has been found in a variety of gram-positive bacteria. This gene has also been found in Bacteroides species but only in six recently isolated strains; thus, the gene seems to have entered this genus only recently. One of the six Bacteroides ermB-containing isolates, WH207, could transfer ermB to Bacteroides thetaiotaomicron strain BT4001 by conjugation. WH207 was identified as a Bacteroides uniformis strain based on the sequence of its 16S rRNA gene. Results of pulsed-field gel electrophoresis experiments demonstrated that the transferring element was normally integrated into the Bacteroides chromosome. The element was estimated from pulsed-field gel data to be about 100 kb in size. Since the element appeared to be a conjugative transposon (CTn), it was designated CTnBST. CTnBST was able to mobilize coresident plasmids and the circular form of the mobilizable transposon NBU1 to Bacteroides and Escherichia coli recipients. A 13-kb segment that contained ermB was cloned and sequenced. Most of the open reading frames in this region had little similarity at the amino acid sequence level to any proteins in the sequence databases, but a 1,723-bp DNA segment that included a 950-bp segment downstream of ermB had a DNA sequence that was virtually identical to that of a segment of DNA found previously in a Clostridium perfringens strain. This finding, together with the finding that ermB is located on a CTn, supports the hypothesis that CTnBST could have entered Bacteroides from some other genus, possibly from gram-positive bacteria. Moreover, this finding supports the hypothesis that many transmissible antibiotic resistance genes in Bacteroides are carried on CTns.

Erythromycin resistance was once rare in Bacteroides strains, and few strains isolated before 1970 carried any of the erm genes that encode this type of resistance. The first erm gene found in a Bacteroides strain was an ermF gene, which was located on a self-transmissible plasmid (14). The erm genes encode enzymes that modify the 23S rRNA by adding one or two methyl groups. This modification reduces the binding to the ribosome of three classes of antibiotics, macrolides, lincosamides, and streptogramin B, resulting in resistance to all three classes. Two genes with ≥80% amino acid identity are assigned the same class, which is given a letter designation. More recently, the ermG gene has been found in Bacteroides. Some ermF genes are located on plasmids, but both ermF and ermG genes have also been found on integrated self-transmissible elements called conjugative transposons (CTns) (8). In a recent survey of Bacteroides resistance genes, it was noted for the first time that a third type of erm gene, ermB, had begun to appear in Bacteroides strains (18). This observation was of interest because ermB genes have been found mainly in gram-positive bacteria.

Earlier studies had revealed that CTns carrying ermF and the tetracycline resistance gene tetQ were all closely related to a CTn called CTnDOT. CTnDOT-type CTns have now spread so widely among Bacteroides strains that over 80% of recent isolates carry a CTn of this type (18). Yet this type of CTn seems to be transferring mainly among members of the Bacteroides phylogenetic group, which includes Bacteroides species and Porphyromonas species. Finding ermB genes in Bacteroides species raised the possibility that transmissible elements might also be transferring to Bacteroides species from much more distantly related phylogenetic groups, such as gram-positive bacteria.

Since the human colon is thought to be one of the sites where horizontal gene transfer is likely to occur, it is of interest to learn more about the diversity of transmissible elements found in the species that numerically predominate in the colon, such as Bacteroides species, and about the extent to which such elements are actually transferring in that setting. In this paper, we report that the ermB gene in one of the Bacteroides strains found in our survey can be transferred by conjugation and is located on a CTn that is unrelated to any of the Bacteroides CTns described so far.


Bacterial strains and growth conditions.

The bacterial strains and plasmids used in this study are listed in Table Table1.1. Bacteroides strains were grown anaerobically at 37°C in prereduced Trypticase-yeast extract-glucose (TYG) broth (9) or TYG agar plates incubated in BBL GasPak jars. Escherichia coli strains were grown aerobically at 37°C in Luria-Bertani (LB) broth or plated on LB agar. Bacteroides strains BT4001 and BT4100 are spontaneous mutants of Bacteroides thetaiotaomicron 5482A (ATCC 29148) that are rifampin resistant and trimethoprim resistant, respectively. E. coli strain DH5αMCR (Gibco BRL) was used as a host for plasmid construction and, in many cases, as a recipient in filter mating assays.

Bacterial strains and plasmids used in this study

DNA isolation and molecular methods.

DNA preparations were made by a modification of the method of Saito and Miura (15). Plasmid DNA was isolated from E. coli and Bacteroides either by the method of Ish-Horowitz as described by Sambrook et al. (17) or with the QIAGEN plasmid midi kit. Restriction digests and ligations were performed as specified by the manufacturer (New England Biolabs, Promega, or Invitrogen).

Filter mating assays.

The filter mating assays have been described previously in detail (20). For matings between Bacteroides donors and Bacteroides recipients, the filters were incubated anaerobically overnight at 37°C. When E. coli was the donor, the filters were incubated aerobically. Bacteroides transconjugants were selected on TYG medium containing relevant antibiotics under anaerobic conditions. E. coli transconjugants were incubated on LB selective plates aerobically. The antibiotics used and their concentrations were as follows: ampicillin, 100 μg/ml; erythromycin, 3 μg/ml; gentamicin, 200 μg/ml; rifampin, 10 μg/ml; streptomycin, 100 μg/ml; tetracycline, 10 μg/ml; trimethoprim, 100 or 300 μg/ml; and cefoxitin, 20 μg/ml. During experiments to test for tetracycline induction or erythromycin induction, the Bacteroides strains were grown in TYG medium containing 2 μg of tetracycline/ml or 3 μg of erythromycin/ml.

In experiments to determine whether natural isolates of ermB-carrying Bacteroides strains could transfer ermB by conjugation, thymidine-requiring spontaneous mutants of the donors were selected so that the auxotrophy could be used in the mating experiment to select against donors. These thymidine-requiring spontaneous mutants were also resistant to trimethoprim. BT4001, whose DNA does not hybridize to known Bacteroides CTns and which is rifampin resistant (Rifr), was the recipient. Transconjugants were selected as Rifr, erythromycin-resistant (Emr) colonies. There is one report in the literature that claims that BT4001, a derivative of ATCC 29148, contains a copy of ermB (7). We saw no evidence of the presence of ermB sequences based on DNA-DNA hybridization or PCR. Moreover, the genome sequence of BT4001, which was completed recently (25), contains no ermB gene. The ermB-containing isolates were the donors in separate filter mating experiments.

Some elements that appear to be self-transmissible are actually mobilized by self-transmissible elements present in the donor strain. Accordingly, BT4001 transconjugants isolated in the original mating, designated BT4020, were then used as donors in a second mating experiment with BT4100 (trimethoprim resistant [Tpr], thymidine deficient, and erythromycin sensitive [Ems]) as the recipients. Transconjugants were selected as Tpr Emr colonies and were checked to make sure that they were Thy Rifr.

Species identification of WH207.

WH207, the community isolate that contained the transmissible ermB gene, had not been identified to the species level. To determine its species identification, a universal forward bacterial 16S rRNA primer, 5′AGAGTTTGAT(C/T)(A/C)TGGC3′ (22), and a Bacteroides specific reverse primer, 5′CCTTGTTACGACTTAGCCC3′ (13), were used to amplify by PCR a 1.4-kb segment of the 16S rRNA gene. The cycling conditions were as follows: 95°C for 5 min; 25 cycles of 94°C for 1 min, 50°C for 1 min, and 72°C for 2 min; followed by 72°C for 5 min. The product was gel purified and sequenced.

Southern blot, dot blot, and colony hybridization procedures.

Colony hybridization, Southern hybridization, and dot blot analyses were done according to the methods of Sambrook et al. (17) and the Renaissance kit protocols (PerkinElmer Life Sciences). Probes were labeled with fluorescein-dUTP by the random primer method as outlined in the PerkinElmer Life Sciences Renaissance kit protocol. Hybridization was carried out at 42°C overnight with denatured probes. Nonspecifically hybridized probes were removed with two washes of increasing stringency. To prevent nonspecific protein binding, the membrane was treated with the blocking reagent provided in the kit. The conditions used were calculated to allow hybridization of the probe only to DNA that shared more than 85% sequence identity with it.

Isolation of DNA flanking the ermB gene.

Genomic DNA preparations were made from WH207 (Table (Table1),1), and aliquots were digested with various restriction enzymes. The ermB gene was amplified by PCR as described in a prior publication (18), and a probe was constructed. Southern blot analysis, using the ermB probe, identified a 3-kb ermB-containing band in the HindIII-digested WH207 genomic DNA lane. HindIII fragments between 2 and 4 kb were isolated by extraction from an agarose gel. These fragments were then ligated with HindIII-digested pUC19 and transformed into DH5αMCR-competent cells. The ampicillin-resistant transformants were screened by colony hybridization with the ermB probe. Plasmids from positive colonies were analyzed by restriction digestion and Southern blotting. A plasmid, pAG1, was shown to contain a 3-kb HindIII fragment that hybridized to the ermB probe. Then, a probe made from the cloned fragment in pAG1 hybridized to a 3-kb HindIII fragment of WH207. The 3-kb fragment in pAG1 was sequenced.

Plasmid rescue technique to obtain DNA adjacent to the ermB gene.

A plasmid rescue technique was employed to clone more of the sequence from the ermB element (Fig. (Fig.1).1). A 1.5-kb fragment containing N-terminal ermB sequence and upstream sequence was cloned into the tetracycline-resistant (Tcr) Bacteroides suicide vector, pLYL001. The clone was then mobilized into BT4020, a transconjugant containing the ermB element, and Tcr colonies were isolated. The Tcr insertion mutants obtained after mating were analyzed by Southern hybridization to confirm that the recombination had occurred within the 1.5-kb region. Various restriction enzymes were then chosen to cut the genomic DNA of the insertion mutant so that the plasmid DNA remained intact. A Southern experiment with an internal-region probe helped to determine the size of the rescued piece for each enzyme. A PstI fragment containing pLYL001 and 12 kb of the ermB element was identified. The genomic DNA from the insertion mutant was cut with PstI, and ligation reactions were set up to favor monomeric circularization. This mixture was used to transform E. coli DH5αMCR, and cells with the intact vector were obtained by selecting for ampicillin resistance. The clone was named pAG4, and the rescued fragment was then sequenced from this clone.

FIG. 1.
Schematic representation of the plasmid rescue experiment done to obtain additional flanking sequences and thus reach an end. BT4020 is the transconjugant obtained by mating WH207 with BT4001. CTnBST is the only known mobile element in this strain.


DNA sequencing was done at the W. M. Keck Center for Comparative and Functional Genomics at the University of Illinois, Urbana, with the Applied Biosystems model 373A version 2.0.1S dye terminator sequencing system. Primer walking was done to obtain 13 kb of sequence from CTnBST.


For pulsed-field gel electrophoresis (PFGE), DNA samples were prepared, digested with restriction enzymes in agarose plugs, and electrophoresed in 1% agarose gels as described in the Bio-Rad CHEF-DRIII PFGE system manual. The DNA was electrophoresed in 0.5× Tris-borate-EDTA (0.045 M Tris, 0.045 M borate, 0.001 M EDTA, pH 8.3) at 14°C. The voltage was 6 V/cm, and the field angle was 120°. The pulse parameters and run times were varied as needed. The Bio-Rad lambda ladder and the Saccharomyces cerevisiae ladder were used to estimate the sizes of restriction fragments.

Preparation of constructs for plasmid mobilization experiments.

To determine whether the CTnBST could mobilize coresident plasmids, the plasmid of interest had to be mobilized into a Bacteroides strain containing CTnBST from E. coli by triparental matings. Three plasmids were tested; pLYL01 had the mob and oriT region from Bacteroides plasmid pBI143, pT-COW had the mob and oriT region from Bacteroides plasmid pB8-51, and pLYL11 contained the mob and oriT region from Bacteroides mobilizable transposon of NBU1.

For the triparental matings, the donors were DH5αMCR cells containing the relevant plasmid and HB101 containing the IncPα plasmid RP1. The B. thetaiotaomicron strain BT4020 (Emr Rifr), which carried a single copy of CTnBST, was the recipient. The E. coli-to-Bacteroides sp. matings were done as described earlier (20).

Plasmid mobilization experiments.

To test whether CTnBST could mobilize plasmids or NBU1, conjugation experiments in which BT4020 isolates carried each of the plasmids as a donor were done. The recipient was E. coli HB101 (streptomycin resistant [Smr]). Only if CTnBST is able to supply the transfer functions can these plasmids transfer from Bacteroides to E. coli. Transconjugants were selected on LB agar plates containing ampicillin and streptomycin. Plasmid preparations were made from transconjugants to confirm that the correct plasmid was present.

Nucleotide sequence accession numbers.

The new sequences for ermB from WH207 and CTnBST identified as a result of the present study were submitted to the GenBank database under the accession numbers AY349154 and AY345595, respectively.


Characteristics of the six ermB-containing isolates.

Five of the six isolates whose DNA hybridized with the ermB gene probe were phenotypically erythromycin resistant (at an erythromycin concentration of 3 to 10 μg/ml) as detected by the broth dilution technique. The exception was WH202, which contained ermB sequences but was sensitive to erythromycin. Two of the six isolates had two erm genes, DNA from Bov7991 hybridized to ermB and ermG probes and DNA from BF8371 hybridized to ermB and ermF probes. All six isolates were resistant to tetracycline, and DNA from these strains hybridized to the tetQ gene, a gene that is carried by many Bacteroides CTns. All six isolates hybridized to probes that detected the ends of CTnDOT (Table (Table22).

The six ermB-containing strains found in the strain collection of the Salyers laboratory

To determine whether all of these isolates carried identical or near-identical ermB genes, the ermB gene in each isolate was amplified by PCR and sequenced. WH207 and Bov7991 contained ermB genes with identical sequences. The ermB gene in WH202 was identical to the ermB gene in BF8371. Since BF8371 was resistant to erythromycin, the sensitivity of WH202 was presumably due to the failure of the promoter to function in Bacteroides spp. The sequences of all of the other ermB genes were nearly identical except for single-base-pair substitutions at positions 380, 397, and 469. The species identities of two of the ermB-containing isolates had been determined previously; Bov7991 is a B. ovatus strain, and BF8371 is a B. fragilis strain (S. M. Finegold, Wadsworth VA Hospital Anaerobe Laboratory, Los Angeles, Calif., personal communication). Thus, at least two different Bacteroides species were represented in the collection of ermB-containing isolates. The fact that ermB genes found in different Bacteroides species are virtually identical supports the hypothesis that the ermB genes have been moving into Bacteroides species by horizontal gene transfer.

The ermB sequences from Bov7991 and WH202 were identical to the sequence of an ermB(P) gene found previously in Clostridium perfringens (1), and all of the Bacteroides ermB genes had sequences that were virtually identical to those of ermB genes found in gram-positive cocci such as Streptococcus pneumoniae (19). Our finding of ermB genes in Bacteroides was the first time that this type of erm gene had been found in a gram-negative anaerobe. All of the ermB-containing isolates had been isolated after 1990. This finding raises the possibility that ermB has entered Bacteroides strains only recently.

The ermB gene in strain WH207 is on a transmissible element.

To test whether any of the ermB genes found in the Bacteroides strains were carried on a transmissible element that is still active in Bacteroides, mating experiments were done with BT4001 as the recipient. All of the ermB-containing strains except WH202, which was phenotypically erythromycin sensitive, were tested. Transconjugants were obtained only when WH207 was the donor, at a transfer frequency of 10−5 transconjugants/recipient. All transconjugants obtained (designated BT4020) hybridized with an ermB gene probe, confirming that erythromycin resistance cotransferred with ermB. None of these transconjugants contained tetQ, indicating that tetQ was not on the same transmissible element as ermB. Moreover, DNA from the transconjugants did not hybridize with DNA from the ends of CTnDOT, indicating that ermB was carried on some other transmissible element.

Although ermB was not carried on CTnDOT, it was possible that the element that carried ermB was being mobilized out of WH207 by CTnDOT, which is known to be capable of mobilizing coresident elements such as mobilizable plasmids and mobilizable transposons that are otherwise incapable of conjugal transfer. To determine whether the ermB-carrying element could retransfer from BT4020, a mating experiment was done in which BT4020 was used as the donor and BT4100, a Thy Tpr derivative of BT4000, was used as the recipient. The transfer frequency in this mating experiment was again 10−5 transconjugants/recipient. The fact that the transfer frequency remained the same whether WH207 or BT4020 was the donor indicates that the ermB-containing element is self-transmissible and its frequency is not stimulated by some other transmissible element present in WH207.

Transfer of the CTnDOT-type elements is enhanced 1,000-fold by the exposure of the donor to tetracycline; exposure of the donor to tetracycline did not affect the transfer frequency of the ermB element (Table (Table3).3). The transfer frequency was also unaffected by exposure of the WH207 or BT4020 donors to erythromycin (Table (Table33).

Transfer frequencies obtained in filter mating assays.

WH207 was isolated in Woods Hole, Mass., from the feces of a healthy volunteer but had not been identified to the species level. To determine the species of WH207, its 16S rRNA gene was amplified and sequenced. One of the two closest matches was to a 16S rRNA gene from a rumen bacterium that had been isolated in Australia but not given a species name (99% identity; GenBank accession no. AF139524). The other sequence match (99% identity) was to the 16S rRNA gene from a B. uniformis strain (accession no. AB050110). Thus, WH207 appears to be a B. uniformis strain. With WH207, the number of different Bacteroides species in which ermB has been found amounts to three.

The ermB element is a CTn.

Plasmid preparations were made from the six original ermB isolates and from the transconjugant (BT4020) to determine if the ermB element was a plasmid. BT4001 has a 30-kb plasmid. This plasmid was present in BT4020, but no additional plasmids were seen. No plasmids were found in BF8371 and DH3760, two of the six original ermB-containing strains. Plasmid preparations from all of the isolates were probed on Southern blots with the ermB probe. None of the isolates hybridized with this probe (data not shown). Thus, it seemed likely that the ermB element was integrated into the chromosome.

To determine whether the ermB element in BT4020 is integrated into the chromosome, PFGE experiments were done (2). DNA from recipient strain BT4001 and three independent isolates was digested with I-CeuI. Comparison of the fragment patterns showed that a 300-kb segment of DNA from BT4001 shifted to 400 kb in DNA from the transconjugants (Fig. (Fig.2A).2A). Thus, the element that transfers ermB is normally integrated into the chromosome and is about 100 kb in size (Fig. (Fig.2A).2A). PFGE was also done with DNA restricted with NotI and SfiI (data not shown). Several differences were observed between the BT4001 pattern and the pattern for BT4020, indicating that there are NotI and SfiI sites within the element.

FIG. 2.
(A) PFGE of I-CeuI-digested recipient (lanes 3) and transconjugant (lanes 4 to 6) DNA was carried out in a 1% agarose-0.5× Tris-borate-EDTA gel run at 6 V/cm, with an initial pulse of 10 s and a final pulse of 120 s, for 30 h with a Bio-Rad CHEF-DR ...

To confirm that the band shift seen in DNA digested with I-CeuI was due to the ermB element, a Southern blot of the gel shown in Fig. Fig.2A2A was probed with a 3-kb HindIII segment that contains the ermB gene. The probe hybridized to the shifted band, proving that the band shift observed was due to the integration of the ermB element (Fig. (Fig.2B).2B). Taken together, our results support the hypothesis that the ermB gene from WH207 is carried on a 100-kb CTn. This CTn has been designated CTnBST.

Analysis of the region around the ermB gene.

Using the plasmid rescue approach, we cloned a total of 13 kb of DNA from the ermB region. A map of this cloned region is shown in Fig. Fig.3.3. The ermB gene from Bacteroides was virtually identical to genes from gram-positive bacteria such as C. perfringens (GenBank accession no. X58285 and U18931) (1), Enterococcus faecalis (accession no. Y00116, AE016833, M11180, M36722, X92945, and U86375) (4), Streptococcus pyogenes (accession no. X64695 and X66468) (5, 6), Streptococcus pneumoniae (accession no. X52632) (19), Clostridium difficile (accession no. AJ294529), Staphylococcus aureus (accession no. Y13600), Bacillus cereus (accession no. AF480455 to AF480460), Staphylococcus intermedius (accession no. AF239773 and AF299292) (3), Streptococcus agalactiae (accession no. X72021), Enterococcus faecium (accession no. AF507977, AF516335, AF229200, and AF242872), Enterococcus hirae (accession no. AF406971 and X81655), Lactobacillus reuteri (accession no. AF080450, AF205068, and AY082384), Staphylococcus lentus (accession no. U35228) (23), and Lactobacillus fermentum (accession no. U48430). The region downstream of ermB in CTn-BST was also 99% identical to a sequence that is located next to an ermB gene found in C. perfringens, although some DNA seems to have been deleted in CTnBST in this region. No more sequence from the C. perfringens strain was available in the databases, so we were unable to determine if the identity extended beyond this point (1).

FIG. 3.
Matches obtained when a BLAST search was performed with the 13 kb of DNA sequence obtained from CTnBST. The 1.7-kb sequence from the right end is 99% identical to a sequence found for C. perfringens (accession no. ...

Figure Figure33 shows the results of BLAST searches of the rest of the cloned 13-kb region. Six low-similarity matches were found to be putative genes by BLASTX. Only two of these are complete open reading frames (ORFs), the ssr (site-specific recombinase) and tpn (transposase) genes. The remaining four, designated met (methylase), rep (replication protein), mob (mobilization protein), and mte (methyltransferase) in Fig. Fig.3,3, were truncated in CTnBST. The deduced amino acid sequence of the protein encoded by the CTnBST homolog of ssr had 27% identity to a site-specific recombinase, the DNA invertase Pin homolog of Clostridium acetobutylicum (GenBank accession no. AAK79913). The closest match to the protein encoded by tpn, with 29% amino acid identity, was a putative transposase in Deinococcus radiodurans (accession no. NP_051548), which is similar to IS10-like transposases based on homology searches. The DDE motif was found in the sequence of the tpn gene product. The transposase gene seems to have inserted into a truncated mte gene. The closest match to the protein encoded by mte, with 67% amino acid identity, is a hypothetical protein from Clostridium thermocellum ATCC 27405 (accession no. ZP_00061314). Also, the protein encoded by the mob gene shown in Fig. Fig.33 was similar to recombination and mobilization proteins, with 31% identity at the amino acid level to a recombination protein in Staphylococcus aureus (accession no. NP_052168). The region indicated as rep in Fig. Fig.33 had a corresponding amino acid sequence that is 37% identical to replication proteins in Staphylococcus haemolyticus (accession no. CAC16673).

Comparison of the ermB regions from the six original ermB isolates.

We used a 3-kb fragment from WH207 which contained the 738-bp ermB gene, 1,292 bp of upstream flanking DNA, 961 bp of downstream flanking DNA, and a 10-kb fragment that contained the rest of the cloned DNA (Fig. (Fig.3)3) to probe DNA from the ermB isolates that did not transfer the gene under the conditions we used. As expected, the hybridization pattern seen with WH207 DNA was the same as that seen with DNA from two transconjugants (BT4020 and BT4021), except that there was a doublet in the WH207 DNA probed with the 3-kb ermB-containing probe that was not present in the transconjugants (Fig. (Fig.4A).4A). Thus, there could be a second copy of ermB in WH207. Both probes hybridized to different-size bands in the six isolates, and the intensity was different even though approximately the same amounts of DNA were loaded in all the lanes (Fig. (Fig.4).4). These results indicate that although the regions surrounding the ermB gene in all six strains were similar enough to cross-hybridize on a high-stringency Southern blot under conditions that should detect only DNA segments that were more than 85% identical to the probe, the regions were not identical.

FIG. 4.
Results of Southern hybridization with probes from CTnBST. The strain from which the DNA was obtained is indicated above each lane. The DNA was digested with HindIII and PstI in all the lanes. The probes used were a 3-kb HindIII region from CTnBST (A) ...

Dot blot analyses of strains in our collection that did not hybridize with the ermB probe identified 10 additional isolates that hybridized to the 10-kb probe; 3 of these were clinical isolates and 7 were community isolates. All of these isolates were isolated after 1980. DNA from five of these isolates was probed on a Southern blot with the 10-kb probe. Although three of the five isolates had similarly sized HindIII fragments, the other two had weakly hybridizing fragments of different sizes.

The ends of the CTn are more than 16 kb from the right end and 20 kb from the left end of the cloned region.

Although there were sequences within the cloned region that might have encoded an integrase, which would be expected to be located near one end of the CTn, the fact that the Southern hybridization patterns were the same in the original strain and in the transconjugants (Fig. (Fig.5)5) indicated that the cloned region was internal to the CTn. To estimate the distance between the ends of the cloned region and the ends of the element, we digested DNA from WH207 and two transconjugants, BT4020 and BT4001, with NsiI, KpnI, and SacII and probed these digests on Southern blots with the 3- and 10-kb cloned fragments (Fig. 5A and B). The positions of these sites within the cloned DNA are indicated in Fig. Fig.5C.5C. By analyzing the resulting Southern hybridization patterns, we deduced that the cloned region was more than 16 kb from the right end and at least 20 kb from the left end of the available sequence (Fig. (Fig.5C).5C). Since the sizes of these sequences do not add up to 100 kb, the size of the CTn estimated from PFGE analysis, it is clear that there is a large segment of the CTn that is not detected by probes made from the cloned DNA.

FIG. 5.
(A and B) Southern blot analysis to estimate the distance to the ends from the available sequence. Lanes 1, 4, and 9, WH207 DNA; lanes 2, 3, 5 to 8, and 9 to 13, transconjugant DNA digested with NsiI, SacII, or KpnI as indicated in the figure. The 3-kb ...

CTnBST can mobilize coresident elements.

We wanted to determine whether CTnBST, like CTnDOT, could mobilize Bacteroides plasmids and a Bacteroides mobilizable transposon (NBU1). pLYL01 contains the oriT and the mobilization gene of pBI143, a naturally occurring Bacteroides plasmid that is found in many strains. pT-COW carries the oriT and the mobilization gene of another widely dispersed Bacteroides plasmid, pB8-51. pLYL11 carries the oriT and the mobilization gene of the mobilizable transposon, NBU1. These plasmids cannot transfer themselves from Bacteroides strain BT4001 to E. coli. All three constructs were transferred from a derivative of BT4001 that contained CTnBST and the plasmid to E. coli recipients at a frequency of 10−6 to 10−7 transconjugants per recipient.


In this study, we demonstrated that an ermB gene that has only recently entered Bacteroides strains is carried on a 100-kb self-transmissible integrated element designated CTnBST. CTnBST is a new CTn in Bacteroides strains, and it does not belong to the predominant CTnDOT family of Bacteroides CTns. This element is also distinct from two ermG-containing CTns that have been found in Bacteroides species (8, 21). The initial picture of CTn diversity in Bacteroides species was that the CTns were all related to CTnDOT. Results of this study and other recent studies show that this is not the case and that there is a widely diverse set of CTns in Bacteroides species. It is noteworthy that most of the matches in the databases with the 13-kb cloned region from CTnBST are to genes from gram-positive bacteria. In particular, the ermB gene and a segment of downstream sequence were identical to sequences found previously in C. perfringens. In the human colon, the two types of bacteria that predominate numerically are the gram-negative Bacteroides species and a group of gram-positive anaerobes about which little is known (10, 11, 24). C. perfringens is not one of the numerically predominant species, yet a segment of DNA found in a clinical isolate is identical to that in CTnBST. This is the strongest evidence yet that there may be horizontal transfer events occurring between gram-positive and gram-negative bacteria in the human colon, although the direction of transfer is not known. The fact that only very recently isolated strains of Bacteroides carry ermB suggests that this gene has entered Bacteroides species from some other source.

The hypothesis that human colonic bacteria could be serving as reservoirs of antibiotic resistance genes, constantly acquiring and transmitting these genes and possibly passing them on to pathogenic bacteria, is an old one. Only recently, however, has evidence started to accumulate that this may be the case, especially for the species of bacteria that predominate numerically. It is important to know whether colonic bacteria serve such a function because if so, the effect on the colonic microflora of antibiotic treatment for other diseases or for prophylactic uses of antibiotics needs to be taken more seriously. It is now possible to begin to explore the ecology of antibiotic resistance genes in intestinal bacterial populations, although more will have to be learned about transmissible elements and genes in the gram-positive anaerobes to obtain a complete picture. Such information is important not only for antibiotic resistance concerns but also because the extent to which these genes are transferred is likely to be a good indicator of how readily other types of gene transfers occur in the same environment.

We were able to obtain a 13-kb section of CTnBST, and we used this segment as a probe to evaluate the distribution and diversity of related elements. Two results were notable. First, we found examples of strains whose DNA cross-hybridized with the probe and yet did not carry the ermB gene. This result indicates that ermB is entering these elements and perhaps, in some cases, being lost by them. That is, there is a fluid situation in the evolution of this group of CTns. Second, it is obvious from the variety of restriction patterns seen on Southern blots that this type of CTn has more than one form. It is not clear whether this diversity reflects rapid evolution of this type of element or repeated separate entries of such CTns into Bacteroides species.

Many of the ermB strains that contained CTnBST also contained a CTnDOT-type element. Thus, unlike plasmids, the CTns seem not to exclude each other. There was no evidence found that they are stimulating transfer of one another, however. Another activity that both types of CTns seem to be capable of is the mobilization of coresident plasmids and NBU1, which is a mobilizable transposon (12, 16). This type of activity significantly increases the potential for CTns to participate in the transfer of genes that they themselves do not carry. A trait not shared between CTnBST and CTnDOT is the stimulation of transfer by tetracycline. CTnBST transferred itself and coresident elements at the same frequency under all the conditions tested. This could mean that in the future CTnBST-type elements might one day come to rival the CTnDOT-type elements in distribution. A recent survey found that more than 80% of recently isolated strains from more than 10 different species carried a CTnDOT-type element (18). It remains to be seen whether CTnBST-type elements possess not only the transfer capacity under conditions within the colon but also the stability in a new host that causes them to be maintained in the absence of antibiotic selection. The fact that five of the six ermB-containing strains were not capable of transferring the resistance under laboratory conditions despite the fact that they contained at least a portion of CTnBST DNA might mean that these CTns, in contrast to the CTnDOT elements, readily lose their transfer capacity in Bacteroides strains. Alternatively, CTnBST may have acquired DNA that made it transmissible.


This work was supported by grant number AI22383 from the U.S. National Institutes of Health.


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