Identification of a new ribosomal protection type of tetracycline resistance gene,tet(36), from swine manure pits

Previously, only one ribosome protection type of a tetracycline resistance gene, tetQ, had been identified in Bacteroides spp. During an investigation of anaerobic bacteria present in swine feces and manure storage pits, a tetracycline-resistant Bacteroides strain was isolated. Subsequent analysis showed that this new Bacteroides strain, Bacteroides sp. strain 139, did not contain tetQ but contained a previously unidentified tetracycline resistance gene. Sequence analysis showed that the tetracycline resistance gene from Bacteroides sp. strain 139 encoded a protein (designated Tet 36) that defines a new class of ribosome protection types of tetracycline resistance. Tet 36 has 60% amino acid identity over 640 aa to TetQ and between 31 and 49% amino acid identity to the nine other ribosome protection types of tetracycline resistance genes. The tet(36) region was not observed to transfer from Bacteroides sp. strain 139 to another Bacteroides sp. under laboratory conditions. Yet tet(36) was found in other genera of bacteria isolated from the same swine manure pits and from swine feces. Phylogenetic analysis of the tet(36)-containing isolates indicated that tet(36) was present not only in the Cytophaga-Flavobacter-Bacteroides group to which Bacteroides sp. strain 139 belongs but also in gram-positive genera and gram-negative proteobacteria, indicating that horizontal transfer of tet(36) is occurring between these divergent phylogenetic groups in the farm environment.     

The tetracycline resistance gene tet39 is present in both Gram-negative and Gram-positive bacteria from a polluted river, Southwestern Nigeria

Aim:  Previous analysis of tet39 suggests it may be present in other bacterial species. Hence, we investigated the host range of tet39 among bacterial from a poultry waste polluted river in Southwestern Nigeria.
Methods and Results:  Thirteen resistant bacterial isolated from the water and sediment of the polluted river was investigated for the presence of tetracycline resistance genes tetA , tetB , tetC , tet39 and the transposon integrase gene of the Tn916/1545 family by PCR. While tetA , tetB , tetC and integrase genes cannot be detected in any of the organisms, tet39 was detected in eight of the tested organisms including three Gram-positive species. Sequence analysis showed the genes have high sequence identities (≥99%) with tet39 of Acinetobacter sp. LUH5605, the first and only bacterial genus from which the gene has been reported to date. This is a novel observation.
Conclusions:  This study shows that apart from Acinetobacter , tet39 is present in other bacterial species tested in this study.
Significance and Impact of the Study:  This study adds to available information on the occurrence and distribution of tet39 among environmental bacteria and suggests that the gene has a broader host range than previously reported.     

Identification and characterization of antibiotic resistance genes in Lactobacillus reuteri and Lactobacillus plantarum

Aims:  The study aimed to identify the resistance genes mediating atypical minimum inhibitory concentrations (MICs) for tetracycline, erythromycin, clindamycin and chloramphenicol within two sets of representative strains of the species Lactobacillus reuteri and Lactobacillus plantarum and to characterize identified genes by means of gene location and sequencing of flanking regions.
Methods and Results:  A tet (W) gene was found in 24 of the 28 Lact. reuteri strains with atypical MIC for tetracycline, whereas four of the six strains with atypical MIC for erythromycin were positive for erm (B) and one strain each was positive for erm (C) and erm (T). The two Lact. plantarum strains with atypical MIC for tetracycline harboured a plasmid-encoded tet (M) gene. The majority of the tet (W)-positive Lact. reuteri strains and all erm -positive Lact. reuteri strains carried the genes on plasmids, as determined by Southern blot and a real-time PCR method developed in this study.
Conclusions:  Most of the antibiotic-resistant strains of Lact. reuteri and Lact. plantarum harboured known plasmid-encoded resistance genes. Examples of putative transfer machineries adjacent to both plasmid- and chromosome-located resistance genes were also demonstrated.
Significance and Impact of the Study:  These data provide some of the knowledge required for assessing the possible risk of using Lact. reuteri and Lact. plantarum strains carrying antibiotic resistance genes as starter cultures and probiotics.     

A conjugative macrolide resistance gene, mef(A), in environmental Clostridium perfringens carrying multiple macrolide and/or tetracycline resistance genes

Aims:  To determine if environmental Clostridium perfringens carry antibiotic resistance genes and if the genes are mobile.
Methods and Results:  Clostridium perfringens from water, soil and sewage (2003–2006) were screened for the tetracycline and macrolide resistance genes previously described in animal and human C. perfringens [ erm (B), erm (Q), tetA (P), tetB (P) and tet (M) genes] and the macrolide resistance mef (A) gene. Of the 160 isolates, 108 (67·5%) carried ≥1 of the six antibiotic resistance gene(s). The tetA (P), tetB (P) and tet (M) genes were in 53%, 22% and 8%, and the erm (B), erm (Q) and mef (A) genes in 26%, 1% and 18% of the isolates, respectively. The mef (A) gene and flanking regions were sequenced. The tet (M), erm (B), erm (Q) and mef (A) genes transfer independently from C. perfringens donors to the Enterococcus faecalis recipient.
Conclusions:  Six resistance genes were found in the environmental C. perfringens with the most common being the tetA (P) gene and the erm (Q) gene the least common.
Significance and Impact of the Study:  This is the first time conjugal transfer of macrolide resistance genes and/or the tet (M) gene from C. perfringens has been demonstrated. The data presented supports the hypothesis that antibiotic-resistant environmental C. perfringens are capable of acting as reservoirs for these antibiotic resistance genes.     

Influence of oral doxycycline therapy on the diversity and antibiotic susceptibility of human intestinal bifidobacterial population

Aim:  To evaluate the influence of doxycycline therapy on the composition and antibiotic susceptibility of intestinal bifidobacteria.
Methods and Results:  Faecal samples were collected from nine subjects receiving doxycycline therapy and ten control subjects, and analysed for bifidobacteria by culturing and PCR–DGGE (denaturing gradient gel electrophoresis). A marked decrease in the diversity (average number of amplicons detected by PCR–DGGE 0·8 in the antibiotic vs 4·3 in the control group) of Bifidobacterium populations was observed during doxycycline therapy. The proportion of a tetracycline-resistant bifidobacterial population was higher in the antibiotic group than in the control group (83% vs 26%). Based on the tet gene PCR, resistance could be associated with the presence of tet (W). In two subjects, strains representing highly similar genetic fingerprints but different tetracycline susceptibilities were detected. A mutation causing lack of functionality in the tet (W) was observed in one of the susceptible strains.
Conclusions:  Doxycycline therapy had a drastic effect on the diversity and tetracycline susceptibility of intestinal Bifidobacterium populations.
Significance and Impact of the Study:  The use of broad-spectrum antibiotics increased the pool of tetracycline-resistant commensal bacteria in the intestine. The detection of resistance genes alone is not sufficient for the evaluation of bacterial antibiotic resistance.     

Erythromycin- and tetracycline-resistant lactobacilli in Italian fermented dry sausages

Aims:  To assess the frequency of erythromycin- and tetracycline-resistant lactobacilli in Italian fermented dry sausages.
Methods and Results:  We isolated lactobacilli colonies from 20 salami from the north of Italy (Piacenza province) using selective medium supplemented with erythromycin or tetracycline; we determined the minimum inhibitory concentration and searched for selected erythromycin and tetracycline resistance genes. A total of 312 lactobacilli colonies were genetically ascribed to 60 different strains belonging to seven Lactobacillus species. Lactobacillus sakei , Lactobacillus curvatus and Lactobacillus plantarum were the most frequently found species. Thirty strains (50%) were phenotypically resistant to erythromycin, 45 (75%) to tetracycline and 27 (45%) were resistant to both. The most frequently detected resistance genes were tet (M) and erm (B).
Conclusions:  This study provides evidence of the presence of tetracycline- and, to a lesser extent, erythromycin-resistant lactobacilli in fermented dry sausages produced in northern Italy.
Significance and Impact of the Study:  Although these antibiotic-resistant lactobacilli could serve as reservoir organisms, in our study, 16 of 20 salami could be considered safe in regard to possible antibiotic resistance gene transfer to pathogens, whereas 4 of 20 could represent a borderline situation.     

Evidence for recent intergeneric transfer of a new tetracycline resistance gene, tet(W), isolated from Butyrivibrio fibrisolvens, and the occurrence of tet(O) in ruminal bacteria

We have previously reported high-frequency transfer of tetracycline resistance between strains of the rumen anaerobic bacterium Butyrivibrio fibrisolvens . Donor strains were postulated to carry two Tc^R genes, one of which is transferred on a novel chromosomal element. It is shown here that coding sequences within the non-transmissible gene in B. fibrisolvens 1.230 are identical to those of the Streptococcus pneumoniae tet (O) gene. This provides the first evidence for genetic exchange between facultatively anaerobic bacteria and rumen obligate anaerobes. In contrast, the product of the transmissible Tc^R gene shares only 68% amino acid sequence identity with the TetO and TetM proteins and represents a new class of ribosome protection tetracycline resistance determinant, designated Tet W. The tet (W) coding region shows a higher DNA G + C content (53%) than other B. fibrisolvens genes or other ribosome protection-type tet genes, suggesting recent acquisition from a high G + C content genome. Tet (W) genes with almost identical sequences are also shown to be present in Tc^R strains of B. fibrisolvens from Australian sheep and in Tc^R strains of two other genera of rumen obligate anaerobes, Selenomonas ruminantium and Mitsuokella multiacidus . This provides compelling evidence for recent intergeneric transfer of resistance genes between ruminal bacteria. Tet (W) is not restricted to ruminal bacteria, as it was also present in a porcine strain of M. multiacidus .     

Analysis of a Bacteroides conjugative transposon using a novel "targeted capture" model system

Large conjugative transposons (CTn's) are widespread among Bacteroides spp. and they are responsible for the high rates of Bacteroides tetracycline resistance, which is mediated by the tetQ gene. These elements are self-transmissible and conjugation can be induced up to 1000-fold by the addition of tetracycline to cultures prior to mating. In addition to self-transfer, the Bacteroides CTn's, such as CTn341, are able to mobilize unlinked genetic elements such as plasmids and mobilizable transposons in a tetracycline-inducible manner. To study the molecular properties of these unique elements, a vector was designed to capture CTn's for analysis in heterologous hosts. This plasmid, pFD670, consisted of the low-copy vector pWSK29, the RK2 oriT, an ermF gene, and a tetQ gene fragment containing the N-terminus and promoter. The vector was transferred into Bacteroides recipients containing CTn341 where it integrated into the tetQ gene by homologous recombination. This integrated construct then was transferred back into an Escherichia coli host where it replicated as a plasmid, pFD699, about 56 kb in size. Further analysis showed that pFD699 could be transferred into Bacteroides hosts where it displayed the same tetracycline-inducible properties as the native CTn341. The captured element appeared to utilize a circular intermediate in both transfer and transposition, and integration into the chromosome seemed to be random. Hybridization studies with a range of Bacteroides CTn's encoding tetracycline resistance revealed a great deal of homology between most of the CTn's but there was much variation seen in the restriction patterns of these elements, suggesting great diversity among this group.     

The identification of a tetracycline resistance gene tet(M), on a Tn916-like transposon,in the Bacillus cereus group

In order to investigate whether resistance genes present in bacteria in manure could transfer to indigenous soil bacteria, resistant isolates belonging to the Bacillus cereus group (Bacillus cereus, Bacillus anthracis and Bacillus thuringiensis) were isolated from farm soil (72 isolates) and manure (12 isolates) samples. These isolates were screened for tetracycline resistance genes (tet(K), tet(L), tet(M), tet(O), tet(S) and tet(T)). Of 88 isolates examined, three (3.4%) isolates carried both tet(M) and tet(L) genes, while four (4.5%) isolates carried the tet(L) gene. Eighty-one (92.1%) isolates did not contain any of the tested genes. All tet(M) positive isolates carried transposon Tn916 and could transfer this mobile DNA element to other Gram-positive bacteria.     

Conjugative chromosomal gene transfer in Bacillus subtilis

Conjugative transfer of 20-kb chromosomal fragment carrying genes encoding tetracycline (tet(r)) and lincomycin (lin(r)) resistance in the soil strain Bacillus subtilis 19 is described. Transfer was preceded by this fragment insertion into the large conjugative pl9cat plasmid producing a hybrid plasmid. Insertion frequency was 10(-4)-10(-5). Then genes tet(r) and lin(r) were transferred to the recipient strains. The transfer of chromosomal genes inserted into the plasmid and plasmid gene cat occurred sequentially and resembled sexduction, which represents chromosomal gene transfer by F'- and R' plasmids during conjugation in Escherichia coli and other gram negative bacteria.     

Isolation and Characterization of BTF-37: Chromosomal DNA Captured from Bacteroides fragilis That Confers Self-Transferability and Expresses a Pilus-Like Structure in Bacteroides spp. and Escherichia coli

We report the isolation and preliminary characterization of BTF-37, a new 52-kb transfer factor isolated from Bacteroides fragilis clinical isolate LV23. BTF-37 was obtained by the capture of new DNA in the nonmobilizable Bacteroides-Escherichia coli shuttle vector pGAT400DeltaBglII using a functional assay. BTF-37 is self-transferable within and from Bacteroides and also self-transfers in E. coli. Partial DNA sequencing, colony hybridization, and PCR revealed the presence of Tet element-specific sequences in BTF-37. In addition, Tn5520, a small mobilizable transposon that we described previously (G. Vedantam, T. J. Novicki, and D. W. Hecht, J. Bacteriol. 181:2564-2571, 1999), was also coisolated within BTF-37. Scanning and transmission electron microscopy of Tet element-containing Bacteroides spp. and BTF-37-harboring Bacteroides and E. coli strains revealed the presence of pilus-like cell surface structures. These structures were visualized in Bacteroides spp. only when BTF-37 and Tet element strains were induced with subinhibitory concentrations of tetracycline and resembled those encoded by E. coli broad-host-range plasmids. We conclude that we have captured a new, self-transferable transfer factor from B. fragilis LV23 and that this new factor encodes a tetracycline-inducible Bacteroides sp. conjugation apparatus.     

Occurrence of tetracycline resistance genes tet(M) and tet(S) in bacteria from marine aquaculture sites

Occurrence of tetracycline resistance genes encoding ribosomal protection proteins was examined in 151 tetracycline-resistant bacterial isolates from fish and seawater at coastal aquaculture sites in Japan and Korea. The tet(M) gene was detected in 34 Japanese and Korean isolates, which included Vibrio sp., Lactococcus garvieae, Photobacterium damsela subsp. piscicida, and unidentified Gram-positive bacteria. The majority of these bacterial isolates displayed high-level resistance with a minimum inhibitory concentrations (MICs) equal to or greater than 250 microg/ml of oxytetracycline and only four isolates had MICs less than 31.3 microg/ml. 16S rDNA RFLP typing of tet(M)-positive Vibrio isolates suggests that these are clonal populations of the same phylotype specific to a particular location. One Vibrio clone (phylotype III), however, is widely disseminated, being detected during different sampling years, at different locations, and in different fish species in both Japan and Korea. The tet(S) gene was detected in L. garvieae from yellowtail in Japan and in Vibrio sp. from seawater in Korea. This is the first report of tet(S) occurrence in Gram-negative facultative anaerobes. These results suggest that tet(M) and tet(S) genes are present in fish intestinal and seawater bacteria at aquaculture sites and could be an important reservoir of tetracycline resistance genes in the marine environment.     

Characterization of pRAS1-like plasmids from atypical North American psychrophilic Aeromonas salmonicida

Atypical psychrophilic Aeromonas salmonicida isolates were obtained from farmed and wild fish in Northeastern North America. These bacteria were isolated between 1992 and 2001 and carried tetracycline resistance (Tc(r)) plasmids of approximately 58 kb. The nine isolates had plasmids which could be divided into four groups based on the specific tetracycline resistance (tet) gene carried [tet(A) or tet(B)], incompatibility of the plasmid [IncU or other], whether the plasmid carried the IS6100 sequences, the sul1 gene, coding for sulfonamide resistance, the dfrA16 gene, coding for trimethoprim resistance, and/or carried a complete Tn1721, and their ability to transfer their Tc(r) plasmids to an Escherichia coli recipient at 15 degrees C. Five of the isolates, with genetically related Tc(r) plasmids, were able to transfer their plasmids to an E. coli recipient at frequencies ranging from 5.7x10(-4) to 2.8x10(-6) per recipient. The 1992 isolate carried a genetically distinct plasmid, which transferred at a slightly higher rate. The three remaining isolates carried one of two genetically different plasmids, which were unable to transfer to an E. coli recipient. Conjugal transfer at 15 degrees C is the lowest temperature that has been documented in bacteria.     

Molecular ecology of a facultative swine waste lagoon

Aims:  To investigate the microbial ecology of three facultative swine waste lagoons.
Methods and Results:  Phylogenetic analysis of sequences in a 16S rRNA gene clone library and fluorescence in situ hybridization (FISH) analyses were used to assess bacterial diversity in a swine waste lagoon. FISH analysis and Gram-staining were used to compare the microbial communities of all three swine waste lagoons. Six operational taxonomic units were in high relative abundance and corresponded to the following phylotypes; Thiolamprovum , Verrucomicrobia , Acholeplasma , Turicibacter , Clostridium and Bacteroides . PCR was employed to detect the genes apsA and dsrAB which encode for enzymes specifically associated with dissimilatory sulfate-reduction within sulfate-reducing bacteria (SRB). Amplification of these genes confirmed their presence within the lagoons.
Conclusions:  All lagoons were dominated by purple sulfur bacteria, affiliated to Thiolamprovum pedioforme . The molecular identification of fermentative bacteria and SRB indicate the following metabolic processes within such facultative ponds: sulfur-cycling, fermentation, inter-species hydrogen transfer and carbon cycling.
Significance and Impact of the Study:  This study provides the first molecular evidence for the existence of a sulfur cycle which is linked to phototrophic sulfide oxidation by purple bacteria and organotrophic sulfate-reduction by SRB.     

Cloning, expression in Escherichia coli and nucleotide sequence of a tetracycline-resistance gene from Streptomyces rimosus

Determinants of tetracycline resistance have been cloned from two different tetracycline-producing industrial strains of Streptomyces into Streptomyces lividans using the plasmid vector pUT206. Three plasmids, pUT250 and pUT260 with a 9.5 and a 7.5 kb insert respectively of Streptomyces rimosus DNA, and pUT270 with a 14.0 kb insert of Streptomyces aureofaciens DNA, conferring resistance to tetracycline, have been isolated. By in vitro sub-cloning, a similar fragment of 2.45 kb containing the tetracycline resistance gene (tet347) was further localized on these plasmids. The S. rimosus gene has been cloned into Escherichia coli and expressed under the control of lambda pL or Lpp promoters. Differential protein extraction of E. coli cells revealed the presence of an additional membrane-embedded protein in tetracycline-resistant cells. On the basis of available restriction endonuclease maps, the tet347 gene is probably identical to the tetB gene from S. rimosus recently identified by T. Ohnuki and co-workers as responsible for the reduced accumulation of tetracycline. The nucleotide sequence of a 2052 bp DNA fragment containing the TcR structural gene from S. rimosus has been determined. The amino acid sequence of the tet347 protein (Mr35818) deduced from the nucleotide sequence shows a limited but significant homology to other characterized tetracycline transport acting determinants from pathogenic bacteria.     

Mobilization function of the pBHR1 plasmid, a derivative of the broad-host-range plasmid pBBR1

The pBHR1 plasmid is a derivative of the small (2.6-kb), mobilizable broad-host-range plasmid pBBR1, which was isolated from the gram-negative bacterium Bordetella bronchiseptica (R. Antoine and C. Locht, Mol. Microbiol. 6:1785-1799, 1992). Plasmid pBBR1 consists of two functional cassettes and presents sequence similarities with the transfer origins of several plasmids and mobilizable transposons from gram-positive bacteria. We show that the Mob protein specifically recognizes a 52-bp sequence which contains, in addition to the transfer origin, the promoter of the mob gene. We demonstrate that this gene is autoregulated. The binding of the Mob protein to the 52-bp sequence could thus allow the formation of a protein-DNA complex with a double function: relaxosome formation and mob gene regulation. We show that the Mob protein is a relaxase, and we located the nic site position in vitro. After sequence alignment, the position of the nic site of pBBR1 corresponds with those of the nick sites of the Bacteroides mobilizable transposon Tn4555 and the streptococcal plasmid pMV158. The oriT of the latter is characteristic of a family of mobilizable plasmids that are found in gram-positive bacteria and that replicate by the rolling-circle mechanism. Plasmid pBBR1 thus appears to be a new member of this group, even though it resides in gram-negative bacteria and does not replicate via a rolling-circle mechanism. In addition, we identified two amino acids of the Mob protein necessary for its activity, and we discuss their involvement in the mobilization mechanism.     

Acquired tetracycline and/or macrolide-lincosamides-streptogramin resistance in anaerobes

In general bacterial antibiotic resistance is acquired on mobile elements such as plasmids, transposons and/or conjugative transposons. This is also true for many antibiotic resistant anaerobic species described in the literature. Of the 23 different tetracycline resistant efflux genes identified, tet(B), tet(K), tet(L), and tetA(P) have been found in anaerobic species and six of the ten tetracycline resistant genes coding for ribosomal protection proteins, tet(M), tet(O), tetB(P), tet(Q), tet(W), and tet(32), have been identified in anaerobes. There are now three enzymes which inactivate tetracycline, of which the tet(X) has been identified in Bacteroides though is not functional under anaerobic growth conditions. A similar situation exists with the genes conferring macrolide-lincosamide-streptogramin (MLS) resistance. Of the 26 rRNA methylase MLS resistant genes characterized, five genes; erm(B), erm(C), erm(F), erm(G), and erm(Q), have been identified in anaerobes. In contrast, no genes coding for MLS resistant efflux proteins or inactivating enzymes have been described in anaerobic species. This mini-review will summarize what is known about tetracycline and MLS resistance in genera with anaerobic species and the mobile elements associated with acquired tetracycline and/or MLS resistance genes.     

Update on acquired tetracycline resistance genes

This mini-review summarizes the changes in the field of bacterial acquired tetracycline resistance (tet) and oxytetracycline (otr) genes identified since the last major review in 2001. Thirty-eight acquired tetracycline resistant (Tc(r)) genes are known of which nine are new and include five genes coding for energy-dependent efflux proteins, two genes coding for ribosomal protection proteins, and two genes coding for tetracycline inactivating enzymes. The number of inactivating enzymes has increased from one to three, suggesting that work needs to be done to determine the role these enzymes play in bacterial resistance to tetracycline. In the same time period, 66 new genera have been identified which carry one or more of the previously described 29 Tc(r) genes. Included in the new genera is, for the first time, an obligate intracellular pathogen suggesting that this sheltered group of bacteria is capable of DNA exchange with non-obligate intracellular bacteria. The number of genera carrying ribosomal protection genes increased dramatically with the tet(M) gene now identified in 42 genera as compared with 24 and the tet(W) gene found in 17 new genera as compared to two genera in the last major review. New conjugative transposons, carrying different ribosomal protection tet genes, have been identified and an increase in the number of antibiotic resistance genes linked to tet genes has been found. Whether these new elements may help to spread the tet genes they carry to a wider bacterial host range is discussed.     

Similarities between the regulatory sequences of the unrelated tetracycline genes of pBR322 and Tn10

The regulatory regions of the tetracycline genes present in pBR322 (pSC101) and in the transposon Tn10 are compared. They show a low degree of nucleotide sequence similarity but a high level of structure similarity. Furthermore, analyses of RNAs transcribed in the opposite direction of the pBR322 tet gene show that there are two mRNA initiation sites separated by 29 nucleotides. This suggests the existence of two promoters for the tet repressor gene in Tn10. These features reveal a strong resemblance of the mode of regulation between the tet operons of Tn10 and pSC101.     

The gut as reservoir of antibiotic resistance: microbial diversity of tetracycline resistance in mother and infant

The microbiota in the human gastrointestinal tract (GIT) is highly exposed to antibiotics, and may be an important reservoir of resistant strains and transferable resistance genes. Maternal GIT strains can be transmitted to the offspring, and resistances could be acquired from birth. This is a case study using a metagenomic approach to determine the diversity of microorganisms conferring tetracycline resistance (Tc(r)) in the guts of a healthy mother-infant pair one month after childbirth, and to investigate the potential for horizontal transfer and maternal transmission of Tc(r) genes. Fecal fosmid libraries were functionally screened for Tc(r), and further PCR-screened for specific Tc(r) genes. Tc(r) fosmid inserts were sequenced at both ends to establish bacterial diversity. Mother and infant libraries contained Tc(r), although encoded by different genes and organisms. Tc(r) organisms in the mother consisted mainly of Firmicutes and Bacteroidetes, and the main gene detected was tet(O), although tet(W) and tet(X) were also found. Identical Tc(r) gene sequences were present in different bacterial families and even phyla, which may indicate horizontal transfer within the maternal GIT. In the infant library, Tc(r) was present exclusively in streptococci carrying tet(M), tet(L) and erm(T) within a novel composite transposon, Tn6079. This transposon belongs to a family of broad host range conjugative elements, implying a potential for the joint spread of tetracycline and erythromycin resistance within the infant's gut. In addition, although not found in the infant metagenomic library, tet(O) and tet(W) could be detected in the uncloned DNA purified from the infant fecal sample. This is the first study to reveal the diversity of Tc(r) bacteria in the human gut, to detect a likely transmission of antibiotic resistance from mother to infant GITs and to indicate the possible occurrence of gene transfers among distantly related bacteria coinhabiting the GIT of the same individual.