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Antimicrob Agents Chemother. Mar 2003; 47(3): 878–882.
PMCID: PMC149302

Prevalence of Tetracycline Resistance Genes in Oral Bacteria

Abstract

Tetracycline is a broad-spectrum antibiotic used in humans, animals, and aquaculture; therefore, many bacteria from different ecosystems are exposed to this antibiotic. In order to determine the genetic basis for resistance to tetracycline in bacteria from the oral cavity, saliva and dental plaque samples were obtained from 20 healthy adults who had not taken antibiotics during the previous 3 months. The samples were screened for the presence of bacteria resistant to tetracycline, and the tetracycline resistance genes in these isolates were identified by multiplex PCR and DNA sequencing. Tetracycline-resistant bacteria constituted an average of 11% of the total cultivable oral microflora. A representative 105 tetracycline-resistant isolates from the 20 samples were investigated; most of the isolates carried tetracycline resistance genes encoding a ribosomal protection protein. The most common tet gene identified was tet(M), which was found in 79% of all the isolates. The second most common gene identified was tet(W), which was found in 21% of all the isolates, followed by tet(O) and tet(Q) (10.5 and 9.5% of the isolates, respectively) and then tet(S) (2.8% of the isolates). Tetracycline resistance genes encoding an efflux protein were detected in 4.8% of all the tetracycline-resistant isolates; 2.8% of the isolates had tet(L) and 1% carried tet(A) and tet(K) each. The results have shown that a variety of tetracycline resistance genes are present in the oral microflora of healthy adults. This is the first report of tet(W) in oral bacteria and the first report to show that tet(O), tet(Q), tet(A), and tet(S) can be found in some oral species.

Tetracycline is a broad-spectrum antibiotic which is used in the treatment of bacterial infections in humans and animals, as well as protozoal infections in humans; it is also used as a growth promoter in animals and aquaculture and as an immunosuppressor in humans (8, 31). Tetracycline is commonly used in dental practice as a prophylactic agent and for treatment of oral infections (8). The wide use of tetracycline has had the effect of exposing commensal as well as pathogenic bacteria from different ecosystems to the drug, resulting in a major increase in the rate of tetracycline resistance among bacteria since this antibiotic was first used in the 1950s (8, 30). Resistance to tetracycline is commonly mediated by efflux of the drug or ribosomal protection; however, there has also been one example of a gene encoding tetracycline inactivation (9, 35). So far eight classes of genes encoding ribosome protection proteins have been described: tet(M), tet(O), tetB(P), tet(Q), tet(S), tet(W), tet(T), and tet(32). The most common of these is tet(M) (8, 31). One of the reasons for the success of this gene is the fact that it is commonly contained within conjugative transposons, which have an extraordinarily broad host range (8, 28, 35). The tet(Q) gene is also common and is contained within a conjugative transposon, and there is also some evidence that tet(W) and tet(32) are contained within conjugative chromosomal elements, although these are not yet well characterized (15, 17, 33). Thirteen genes encoding tetracycline efflux pumps have also been described: tet(A), tet(B), tet(C), tet(D), tet(E), tet(G), tet(H), tet(J), tet(Y), tet(Z), tet(30), tet(K), and tet(L) (2, 8, 20, 30). Efflux genes are found in both gram-positive and gram-negative species. The efflux genes from gram-negative organisms are widely distributed and are associated with large plasmids, most of which are conjugative (8). Efflux genes [tet(K) and tet(L)] from gram-positive organisms are usually found on small transmissible plasmids (4). A novel tet gene, tet(34), which apparently is different from the three known groups, has recently been described (22).

tet(M) has been found in many different bacterial genera in the oral cavity (12); tet(O) and tet(Q) have been isolated from gram-positive and gram-negative oral species, respectively (21, 24, 25); tet(K) and tet(L) have been isolated from Streptomyces species (26); and several efflux genes have been isolated from gram-negative oral species (32). However, the prevalence of the newly discovered tetracycline resistance genes in the oral cavity has not been investigated. Furthermore, oral species from healthy adults have not been investigated for the presence of the different tetracycline resistance genes. Therefore, the aim of this work was to investigate the prevalence of tetracycline resistance genes in the oral microflora, a complex ecosystem containing many different genera and species of bacteria where there is ample opportunity for genetic exchange (11, 18). Oral bacteria also have the opportunity to come into contact with bacteria from other body sites (18). In this study we screened the tetracycline-resistant oral microflora for the presence of 12 different tetracycline resistance genes and show, for the first time, that tet(W) is common in oral bacteria and that tet(O), tet(Q), tet(A), and tet(S) were found for the first time in some oral species.

MATERIALS AND METHODS

Sampling.

Saliva and plaque samples were obtained from 20 healthy adults attending the Orthodontic Dentistry Clinic at the Eastman Dental Hospital. All the individuals were medically healthy and not suffering from any oral diseases. Furthermore, the individuals had not received any antibiotics during the previous 3 months. A saliva sample (2 ml) was collected in a sterile container. Plaque samples were collected with a sterile swab, and subgingival plaque samples were collected from four different sites with sterile paper points; both samples were pooled into 4 ml of Ringer's solution and mixed with the saliva sample.

Culture.

The bacteria were resuspended, and serial 10-fold dilutions were made in Todd-Hewitt broth (Oxoid). They were held in this medium for the briefest possible time to ensure that the ratios of the different bacteria remained unchanged. Each sample from each individual was plated on duplicate blood agar plates (with 5% horse blood) containing tetracycline-HCl (Sigma) at a concentration of 8 μg/ml (19) as well as antibiotic-free medium to determine the total viable count. One set of plates was incubated anaerobically in an anaerobic chamber (10% CO2, 10% H2, 80% N2) for 7 days at 37°C, and the other was incubated in 5% CO2-air for 2 days at 37°C. Resistant isolates were enumerated according to their morphologies, and one representative colony of each colony type was subcultured for further identification. The MIC for each isolate was determined by an agar dilution assay on Iso-Sensitest agar plates (Oxoid) with an inoculum of 104 CFU/spot. The plates were read after 24 -h incubation at 37°C aerobically or anaerobically for 72 h, according to the atmospheric requirement of the isolate. The MIC was defined as the concentration that prevented growth. All of the isolates were stored in a Microbank cryovial (Pro-Lab Diagnostics) at −70°C.

Detection of tetracycline resistance determinants.

All of the isolates were tested for the presence of tetracycline resistance genes by PCR. A multiplex PCR assay (Tables (Tables11 and and2)2) was used by the method described by Ng et al. (20). Individual PCRs were performed for tet(Q), tet(W), tet(L), and tet(T); the primers and PCR conditions used were those described by Aminov et al. (1). All PCRs were performed with DNA isolated by use of the Puregen kit (Gentra system; Flowgen). Some of the PCR results were confirmed, when required, by DNA sequencing. DNA sequencing was carried out with the Big Dye Terminator ready reaction mixture (PE Biosystems, Warrington, United Kingdom) and analyzed on a 310 genetic analyzer (PE Biosystems).

TABLE 1.
Description of primers used to detect each of the tetracycline resistance genes
TABLE 2.
PCR primers targeting tet genes

Tet(W) was further analyzed and compared with the tet(W) gene from Butyrivibrio fibrisolvens by using primers specific for the whole gene (Fig. (Fig.1,1, primers AF and AR,) as well as primers specific for the whole gene and the downstream region (Fig. (Fig.1,1, primers AF and CR). Both sets of primers were provided by C. M. Melville (primer AF, 5′-TTG GGG CTG TAA AGG GAG GAC-3′; primer AR, 5′-CAT CGG TGC TCC ATA AC-3′; primer CR, 5′-GTG TTG CTG CAA TAA CAC CAT C-3′) (R. Brunel and K. P. Scott, unpublished data).

FIG. 1.
Diagram showing the tet(W) gene. The thick line represents tet(W) DNA, and the thin line represents flanking sequences. The binding sites of the primers used in the PCR experiments are shown as arrows, with the direction of priming shown by the point ...

Bacterial identification.

Preliminary characterization of all isolates was performed and consisted of Gram stain, catalase, and oxidase tests. Additionally, a number of strains were further characterized and in most cases were identified to the species level by partial 16S rRNA gene sequencing (14) and analysis by use of the Ribosomal Database Project (16) and BLAST databases.

RESULTS AND DISCUSSION

On average, 11% of the cultivable microflora from the 20 samples was found to be tetracycline resistant. A representative set of 105 tetracycline-resistant bacteria was isolated from the 20 samples for further study. Most of the isolates carried tetracycline resistance genes encoding a ribosomal protection protein. The most common tet gene identified was tet(M), which was found in 79% of all the tetracycline-resistant isolates. Sixty percent of the tet(M) genes were found on their own, and 19% were found with one or more different tet genes (Table (Table3).3). The second most common gene identified was tet(W), which was found in 21% of all the isolates, followed by tet(O) and tet(Q) (10.5 and 9.5% of the isolates, respectively) and then tet(S) (2.8% of the isolates). Tetracycline resistance genes encoding an efflux protein were detected in 4.8% of all the tetracycline-resistant isolates; 2.8% of the isolates had tet(L), and 1% of the isolates carried tet(A) and tet(K) each (Table (Table33).

TABLE 3.
Distribution of tetracycline resistance genes among cultivable oral bacteria

Tet(M) was the most common tetracycline resistance gene isolated; it was isolated from every sample. Among the tetracycline-resistant bacteria from this group of 20 individuals, between 14.8 and 100% (mean, 81.1%; standard deviation [SD], 22.7%) harbored the tet(M) gene. The wide distribution of tet(M) probably reflects the fact that it is contained within broad-host-range conjugative transposons which have previously been found to be common in oral bacteria (29).

This is the first report of the isolation of the tet(W) gene from the human oral cavity, and this gene was found to be the second most common tetracycline resistance gene in the cultivable bacteria. Among the tetracycline-resistant bacteria from this group of 20 individuals, between 0.0 and 71.5% (mean, 16%; SD, 20%) harbored the tet(W) gene. This study has shown for the first time that the tet(W) gene is present in Veillonella, Prevotella, Streptococcus, Staphylococcus, Streptomyces, Lactobacillus, and Neisseria spp. (Table (Table4).4). This gene was originally identified from the bovine rumen anaerobe Butyrivibrio fibrisolvens and subsequently in human fecal anaerobes, pigs, and recently, a facultative anaerobe, Arcanobacterium pyogenes (3, 5, 33). In B. fibrisolvens, tet(W) is contained within a large conjugative transposon, TnB1230 (3), which is capable of high-frequency conjugative transfer among B. fibrisolvens species. In A. pyogenes, tet(W) was associated with a mob gene, which is not found in TnB1230, and was capable of conjugative transfer at low frequency (5), indicating that at least two different genetic elements are responsible for the spread of this resistance gene. However, the genetic elements carrying tet(W) have not been fully characterized.

TABLE 4.
Tetracycline resistance genes isolated for the first time from oral species

The tet(W) genes found in this study were compared with the original tet(W) gene from B. fibrisolvens. Of 22 isolates tested, the DNA sequence of the PCR product produced by primers BF and BR was almost identical to that of the PCR product of the original tet(W) gene (Fig. (Fig.1).1). However, only 12 isolates produced a PCR product with primers AF and AR as well as primers AF and CR, and all of the isolates gave a PCR product when primers AF and BR were used. These data indicate that some of the tet(W) genes are different from that originally found in B. fibrisolvens. It is possible that these variants are a new subgroup of tet(W) or a new tet gene with a high degree of homology to tet(W) when primers AF and BR are used (Fig. (Fig.1).1). It is also possible that the tet(W) gene in these samples may be closely related to the newly discovered A. pyogenes tet(W) gene (5).

Between 0.0 and 50% (mean, 8.3%; SD, 3.6%) of the tetracycline-resistant bacteria in the 20 samples harbored the tet(Q) gene. Tet(Q) was mainly found in gram-negative organisms (Table (Table3).3). Tet(Q) has been reported to be common in gram-negative oral bacteria that are associated with periodontal destruction (13, 24). However, this study has shown for the first time that tet(Q) is present in commensal Neisseria spp. (Table (Table4).4). Previous work has shown that the tet(Q) gene is frequently associated with conjugative transposons in Bacteroides and Prevotella (15, 23, 34). These elements were found to have a broad host range and may be responsible for the wide dissemination of the tet(Q) gene in oral bacteria.

The tet(O) gene was the fourth most commonly isolated gene in this study (Table (Table3).3). Between 0.0 and 87% (mean, 10.4%; SD, 22.5%) of the tetracycline-resistant bacteria from the 20 individuals harbored the tet(O) gene. The tet(O) gene has been found in different species (8), but it has been isolated only from gram-positive species (Lactobacillus, Enterococcus, Staphylococcus, and Streptococcus spp.) from the oral and respiratory tracts (32). However, in this study tet(O) was found for the first time in one Neisseria sp. (Table (Table44).

Of the 105 tetracycline-resistant isolates, the tet(S) gene was found in only 3 different gram-negative anaerobes from three different samples; it is the first time that tet(S) has been isolated from gram-negative anaerobes (Table (Table3).3). Tet(S) was first detected in Listeria monocytogenes BM4210, where it is carried by self-transferable plasmids (6). It has also been found in Enterococcus faecalis, where it is integrated into the chromosomal DNA (7, 10), and in Lactococcus spp., where it is located on a conjugative plasmid (27).

Tet(A), tet(S), and tet(W) were isolated from Veillonella spp. for the first time (Table (Table4).4). Previously, the only tet genes isolated from this organism were tet(L), tet(M), and tet(Q) (8). This shows that Veillonella spp. may be an important reservoir for different tetracycline resistance genes.

We found that the carriage of more than one tetracycline resistance gene was common (Table (Table3).3). However, this did not have any effect on the MIC, indicating that a need for an increased level of tetracycline resistance is not the selective pressure for carriage of more than one gene. Rather, this could be because some of the tet genes are contained within conjugative transposons [i.e., tet(M), tet(Q), and tet(W); furthermore, tet(S) may also be on a conjugative transposon]. The possession of one conjugative transposon is not a barrier to that same cell being able to receive other related or unrelated conjugative transposons (8, 18). It has also been shown that some bacteria (Neisseria, Haemophilus, and Streptococcus spp.) are naturally competent, which could help with the further dissemination of the tetracycline resistance genes (32).

In conclusion this study has shown that the bacteria in the oral cavity contain a variety of tetracycline resistance genes, indicating that oral bacteria have access to and/or are a reservoir of resistance genes. In future it will be important to determine the genetic supports for these resistance genes in order to understand how they disseminate.

Acknowledgments

We thank Jean Marc Collard, Patrice Courvalin, and R. I. Mackie for the provision of tetracycline-resistant strains as well as C. M. Melville for providing us with the primer sequences.

This work is supported by project grant G99000875 from the Medical Research Council.

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