Identification of chromosomal antibiotic resistance genes in Streptococcus anginosus ("S. milleri")

Metagenomic investigation of antibiotic resistance genes and resistant bacteria contamination in pharmaceutical plant sites in China

Pharmaceutical plant sites play a significant role in the dissemination of antibiotic resistance genes (ARGs) into the environment. It is imperative to comprehensively monitor of ARGs across various environmental media at these sites. This study focused on three pharmaceutical plants, two located in North China and one in South China. Through metagenomic approaches, we examined the composition, mobility potential, and bacterial hosts of ARGs in diverse media such as process water, groundwater, topsoil, soil cores, and pharmaceutical fermentation residues across diverse environmental matrices, including topsoil, soil cores, process water, groundwater, and pharmaceutical fermentation residues. We identified a wide array of ARGs, comprising 21 types and 740 subtypes, with process water exhibiting the highest abundance and diversity. Treatment processes varied in their efficacy in eliminating ARGs, and the clinically relevant ARGs should also be considered when evaluating wastewater treatment plant efficiency. Geographical distinctions in groundwater ARG distribution between northern and southern regions were observed. Soil samples from the three sites showed minimal impact from pharmaceutical activity, with vancomycin-resistance genes being the most prevalent. High levels of ARGs in pharmaceutical fermentation residues underscore the necessity for improved waste management practices. Metagenomic assembly revealed that plasmid-mediated ARGs were more abundant than chromosome-mediated ARGs. Metagenome-assembled genomes (MAGs) analysis identified 166 MAGs, with 62 harboring multiple ARGs. Certain bacteria tended to carry specific types of ARGs, revealing distinct host-resistance associations. This study enhances our understanding of ARG dissemination across different environmental media within pharmaceutical plants and underscores the importance of implementing strict regulations for effluent and residue discharge to control ARG spread.

Urban green waste bulking agent is the major source of antimicrobial resistance genes persisted in home compost, not animal manure

Urban green waste and food waste are often used as bulking agents to prepare home compost in combination with animal manure in urban horticulture and community gardening. Although it is known that antimicrobial resistance genes (ARGs) persist in home compost, their origins have not been determined. In addition, the factors contributing to ARGs persistence remain unclear. In this study, we aim to (i) characterize the changes in the microbiome and antimicrobial resistome during the composting process of home compost using metagenomics shotgun sequencing, (ii) identify the source of the ARGs persisted in home compost using SourceTracker, and (iii) elucidate the collective effect of compost microbiome and environmental factors, including the physicochemical properties and antibiotics concentration of home compost, in contributing to ARG persistence using Procrustes analysis, co-occurrence network analysis, variation partitioning analysis, and structural equation modeling. SourceTracker analysis indicated that urban green waste bulking agent was the major source of the persisting ARGs in home compost instead of animal manure. Procrustes analysis and co-occurrence network analysis revealed a strong association between microbiome and antimicrobial resistome. Variation partitioning analysis and structural equation modeling suggested that physicochemical properties shaped the antimicrobial resistome directly and indirectly by influencing the microbiome. Our results indicated that the persistence of ARGs in home compost might be due to the succession of microbial species from the urban green waste bulking agent, and the physicochemical properties might have defined the compost environment to shape the microbiome in the compost, thus, in turn, the persisting antimicrobial resistome.

Streptococcus anginosus group infections in hospitalised children and young people

AIM

The Streptococcus anginosus group (SAG) comprises three bacterial species colonising the mouth and gastrointestinal and genitourinary tracts and capable of serious pyogenic infections. Although well-described in adults, studies in children are limited. Here, we characterise paediatric SAG infections from a single Australian centre.

METHODS

Hospitalised patients aged ≤18 years with positive SAG cultures from January 2009 to December 2019 were identified from Pathology Queensland's Gold Coast Laboratory database and their medical records were reviewed.

RESULTS

Two-hundred children (62% male), median age 12 years (interquartile range 6-16), with positive SAG cultures were identified. Overall, 90% received intravenous antibiotics, 89% underwent surgical drainage, 23% were readmitted and 15% required additional surgery. The most common sites were the abdomen (39%), soft tissues (36%) and head and neck regions (21%). Since 2011, Pathology Queensland reported SAG at the species level (n = 133). Of these, S. anginosus was the most prevalent (39%), then S. constellatus (34%) and S. intermedius (27%). Compared with the other two species, S. intermedius was most commonly associated with head and neck infections (relative risk (RR) = 2.2, 95% confidence interval (CI) 1.4-3.5), while S. constellatus (RR = 1.7, 95% CI 1.2-2.4) and S. anginosus (RR = 1.5, 95% CI 1.0-2.0) were each associated with a higher risk of intra-abdominal infection than S. intermedius. Since February 2015, the number of children admitted with SAG-associated intra-abdominal infection per 1000 hospitalisations increased by 29% annually compared with an annual decline of 8% in previous years.

CONCLUSIONS

SAG infections occur at various anatomical sites. Despite antibiotics and surgical management, almost one-quarter are re-hospitalised for further treatment.

Characterization and structure prediction of partial length protein sequences of pcoA, pcoR and chrB genes from heavy metal resistant bacteria from the Klip River, South Africa

The Klip River has suffered from severe anthropogenic effects from industrial activities such as mining. Long-term exposure to heavy metal pollution has led to the development of heavy metal resistant strains of Pseudomonas sp. KR23, Lysinibacillus sp. KR25, and E. coli KR29. The objectives of this study were to characterize the genetics of copper and chromate resistance of the isolates. Copper and chromate resistance determinants were cloned and sequenced. Open reading frames (ORFs) related to the genes CopA and CopR were identified in E. coli KR29, PcoA in Lysinibacillus sp. KR25 and none related to chromate resistance were detected. The 3D-models predicted by I-TASSER disclose that the PcoA proteins consist of β-sheets, which form a part of the cupredoxin domain of the CopA copper resistance family of genes. The model for PcoR_29 revealed the presence of a helix turn helix; this forms part of a DNA binding protein, which is part of a heavy metal transcriptional regulator. The bacterial strains were cured using ethidium bromide. The genes encoding for heavy metal resistance and antibiotic resistance were found to be located on the chromosome for both Pseudomonas sp. (KR23) and E. coli (KR29). For Lysinibacillus (KR25) the heavy metal resistance determinants are suspected to be located on a mobile genetic element, which was not detected using gel electrophoresis.

Variation on a theme; an overview of the Tn916/Tn1545 family of mobile genetic elements in the oral and nasopharyngeal streptococci

The oral and nasopharyngeal streptococci are a major part of the normal microbiota in humans. Most human associated streptococci are considered commensals, however, a small number of them are pathogenic, causing a wide range of diseases including oral infections such as dental caries and periodontitis and diseases at other body sites including sinusitis and endocarditis, and in the case of Streptococcus pneumoniae, meningitis. Both phenotypic and sequence based studies have shown that the human associated streptococci from the mouth and nasopharynx harbor a large number of antibiotic resistance genes and these are often located on mobile genetic elements (MGEs) known as conjugative transposons or integrative and conjugative elements of the Tn916/Tn1545 family. These MGEs are responsible for the spread of the resistance genes between streptococci and also between streptococci and other bacteria. In this review we describe the resistances conferred by, and the genetic variations between the many different Tn916-like elements found in recent studies of oral and nasopharyngeal streptococci and show that Tn916-like elements are important mediators of antibiotic resistance genes within this genus. We will also discuss the role of the oral environment and how this is conducive to the transfer of these elements and discuss the contribution of both transformation and conjugation on the transfer and evolution of these elements in different streptococci.

Genetic mechanisms of antimicrobial resistance identified in Salmonella enterica, Escherichia coli, and Enteroccocus spp. isolated from U.S. food animals

The prevalence of antimicrobial resistance (AR) in bacteria isolated from U.S. food animals has increased over the last several decades as have concerns of AR foodborne zoonotic human infections. Resistance mechanisms identified in U.S. animal isolates of Salmonella enterica included resistance to aminoglycosides (e.g., alleles of aacC, aadA, aadB, ant, aphA, and StrAB), β-lactams (e.g., bla_{CMY−2, TEM−1, PSE−1}), chloramphenicol (e.g., floR, cmlA, cat1, cat2), folate pathway inhibitors (e.g., alleles of sul and dfr), and tetracycline [e.g., alleles of tet(A), (B), (C), (D), (G), and tetR]. In the U.S., multi-drug resistance (MDR) mechanisms in Salmonella animal isolates were associated with integrons, or mobile genetic elements (MGEs) such as IncA/C plasmids which can be transferred among bacteria. It is thought that AR Salmonella originates in food animals and is transmitted through food to humans. However, some AR Salmonella isolated from humans in the U.S. have different AR elements than those isolated from food animals, suggesting a different etiology for some AR human infections. The AR mechanisms identified in isolates from outside the U.S. are also predominantly different. For example the extended spectrum β-lactamases (ESBLs) are found in human and animal isolates globally; however, in the U.S., ESBLs thus far have only been found in human and not food animal isolates. Commensal bacteria in animals including Escherichia coli and Enterococcus spp. may be reservoirs for AR mechanisms. Many of the AR genes and MGEs found in E. coli isolated from U.S. animals are similar to those found in Salmonella. Enterococcus spp. isolated from animals frequently carry MGEs with AR genes, including resistances to aminoglycosides (e.g., alleles of aac, ant, and aph), macrolides [e.g., erm(A), erm(B), and msrC], and tetracyclines [e.g., tet(K), (L), (M), (O), (S)]. Continuing investigations are required to help understand and mitigate the impact of AR bacteria on human and animal health.

Prevalence of β-hemolytic groups C and F streptococci in patients with acute pharyngitis

Background:

The roles of group C and F streptococci in causing endemic pharyngitis are still controversial, although group C streptococci are implicated in the outbreaks of pharyngitis and associated disorders.

Aim:

The aim of this study was to determine the prevalence and the role of these groups of β-hemolytic streptococci in acute pharyngitis with emphasis on the Streptococcus anginosus group. The antimicrobial susceptibility profile of these bacterial isolates and their ability to produce some virulence factors was also determined.

Materials and Methods:

Throat swab specimens were collected from 177 patients suffering from acute pharyngitis who had been admitted to the Hilla Teaching Hospital, Hilla, Iraq, during October 2009 to January 2010. The necessary biochemical tests were conducted and the organisms identified using standard procedures. Susceptibility of isolates pathogens to several antibiotics was examined using standard susceptibility testing. Virulence factors of these isolates were also determined using standard methods.

Results:

Results revealed that a total of 67 isolates belonged to β-hemolytic streptococci, of which 11(16.4%) isolates belonged to anginosus group streptococci, which possessed Lancefield group C and F antigens. Most of these bacterial isolates have the ability to produce more than one virulence factor such as capsule, hemolysin, CFA III, and lipase enzyme. The bacterial isolates were highly resistant to ampicillin, cefotaxime, and cefepime while they exhibited moderate resistance to tetracycline, ceftriaxone, and ciprofloxacin. On the other hand, they showed a high sensitivity to vancomycin, ofloxacin, and clindamycin.

Conclusion:

This study concluded that groups C and F Streptococci were implicated as a cause of acute pharyngitis in 6.2% of the specimens among other groups of streptococci. Most of these isolates have the ability to produce more than one virulence factor. There was a high rate of resistance among isolates for β-lactam antibiotics; however, they were highly susceptible to vancomycin, ofloxacin, and clindamycin.

Antimicrobial susceptibility and distribution of antimicrobial-resistance genes among Enterococcus and coagulase-negative Staphylococcus isolates recovered from poultry litter

Data on the prevalence of antimicrobial resistant enterococci and staphylococci from the poultry production environment are sparse in the United States. This information is needed for science-based risk assessments of antimicrobial use in animal husbandry and potential public-health consequences. In this study, we assessed the susceptibility of staphylococci and enterococci isolated from poultry litter, recovered from 24 farms across Georgia, to several antimicrobials of veterinary and human health importance. Among the 90 Enterococcus isolates recovered, E. hirae (46%) was the most frequently encountered species, followed by E. faecium (27%), E. gallinarum (12%), and E. faecalis (10%). Antimicrobial resistance was most often observed to tetracycline (96%), followed by clindamycin (90%), quinupristin-dalfopristin (62%), penicillin (53%), erythromycin (50%), nitrofurantoin (49%), and clarithromycin (48%). Among the 110 staphylococci isolates recovered, only coagulase-negative staphylococci (CNS) were identified with the predominant Staphylococcus species being S. sciuri (38%), S. lentus (21%), S. xylosus (14%) and S. simulans (12%). Resistance was less-frequently observed among the Staphylococcus isolates for the majority of antimicrobials tested, as compared with Enterococcus isolates, and was primarily limited to clarithromycin (71%), erythromycin (71%), clindamycin (48%), and tetracycline (38%). Multidrug resistance (MDR) phenotypes were prevalent in both Enterococcus and Staphylococcus; however, Enterococcus exhibited a statistically significant difference in the median number of antimicrobials to which resistance was observed (median = 5.0) compared with Staphylococcus species (median = 3.0). Because resistance to several of these antimicrobials in gram-positive bacteria may be attributed to the shuttling of common drug-resistance genes, we also determined which common antimicrobial-resistance genes were present in both enterococci and staphylococci. The antimicrobial resistance genes vat(D) and erm(B) were present in enterococci, vgaB in staphylococci, and mobile genetic elements Tn916 and pheromone-inducible plasmids were only identified in enterococci. These data suggest that the disparity in antimicrobial-resistance phenotypes and genotypes between enterococci and staphylococci isolated from the same environment is, in part, because of barriers preventing exchange of mobile DNA elements.

Molecular basis of resistance to macrolides and other antibiotics in commensal viridans group streptococci and Gemella spp. and transfer of resistance genes to Streptococcus pneumoniae

We assessed the mechanisms of resistance to macrolide-lincosamide-streptogramin B (MLS(B)) antibiotics and related antibiotics in erythromycin-resistant viridans group streptococci (n = 164) and Gemella spp. (n = 28). The macrolide resistance phenotype was predominant (59.38%); all isolates with this phenotype carried the mef(A) or mef(E) gene, with mef(E) being predominant (95.36%). The erm(B) gene was always detected in strains with constitutive and inducible MLS(B) resistance and was combined with the mef(A/E) gene in 47.44% of isolates. None of the isolates carried the erm(A) subclass erm(TR), erm(A), or erm(C) genes. The mel gene was detected in all but four strains carrying the mef(A/E) gene. The tet(M) gene was found in 86.90% of tetracycline-resistant isolates and was strongly associated with the presence of the erm(B) gene. The cat(pC194) gene was detected in seven chloramphenicol-resistant Streptococcus mitis isolates, and the aph(3')-III gene was detected in four viridans group streptococcal isolates with high-level kanamycin resistance. The intTn gene was found in all isolates with the erm(B), tet(M), aph(3')-III, and cat(pC194) gene. The mef(E) and mel genes were successfully transferred from both groups of bacteria to Streptococcus pneumoniae R6 by transformation. Viridans group streptococci and Gemella spp. seem to be important reservoirs of resistance genes.

Macrolide resistance phenotypes of commensal viridans group streptococci and Gemella spp. and PCR detection of resistance genes

One hundred and sixty viridans group streptococci (VGS) and 26 Gemella spp. resistant to erythromycin were studied to detect macrolide lincosamide and streptogramin B (MLS(B)) phenotypes and to investigate resistance rates to other antibiotics. The M phenotype was most prevalent in both bacterial groups (59.6% in VGS, 69.2% in gemellae) and the iMLS(B) phenotype was found least often (9.3 and 13.9%, respectively). All isolates with M phenotype had the mef(A/E) gene, being prevalent the mef(E) subclass. cMLS(B) and iMLS(B) strains contained the erm(B) gene, alone or in combination with the mef(A/E) gene. Thirteen isolates were intermediately resistant to quinupristin/dalfopristin and 11 strains showed low susceptibility to telithromycin. Linezolid was active against all the isolates tested and tetracycline resistance was the major one in VGS (41.6%) and Gemella spp. (46.2%).

Viridans group streptococci: a reservoir of resistant bacteria in oral cavities

The worldwide spread of erythromycin A-resistant streptococci, including Streptococcus pneumoniae, is of concern. Many studies have demonstrated that the viridans group streptococci can be a reservoir of erythromycin A resistance. Within oral streptoccoci, an important difference in the susceptibility pattern has been noted. The purpose of this short editorial is to highlight the importance of this group of bacteria as a reservoir of resistance to erythromycin A and the possible transfer of resistance to S. pneumoniae and S. pyogenes.

Prevalence of macrolide resistance genes in clinical isolates of the Streptococcus anginosus ("S. milleri") group

Twenty-two unrelated erythromycin-resistant anginosus group strains (3.2% resistance rate) were assessed for mechanisms of resistance. Streptococcus anginosus accounted for 16 of the 22 isolates. Fifteen isolates harbored the erm(B) gene. The erm(TR) and the mef(E) genes were carried by two isolates each. In three isolates, none of these resistance genes was detected by PCR.

A variety of gram-positive bacteria carry mobile mef genes

The mefE gene codes for a membrane bound efflux protein, which confers resistance to macrolides, and has been identified in Streptococcus pneumoniae. A variety of gram-positive organisms were examined. Twenty-six isolates of S. pneumoniae carried mefE and were resistant to erythromycin (MIC of 2-16 mg/L). Two additional isolates of Emr S. pneumoniae carried both ermB and mefE(MIC of 16-128 mg/L). One Micrococcus luteus, one Corynebacterium jeikeium, three Corynebacterium spp., two viridans streptococci and seven Enterocccus spp. also carried mef genes. It was possible to move the mef gene from all 11 S. pneumoniae tested to susceptible S. pneumoniae and/or Enterococcus faecalis recipients. The addition of DNase (1 g/L) did not affect the gene transfer. It was also possible to move the mef gene from donor Enterococcus spp., viridans streptococci, M. luteus, C. jeikeium and Corynebacterium spp. to E. faecalis recipients. Transconjugant isolates were resistant to erythromycin (MIC = 16 mg/L). Hybridization with a labelled mef oligonucleotide probe against Southern blots and bacterial dot blots confirmed the presence of the mef genes. This is the first time that a mobile mef gene has been identified in four different genera, from three distinct geographical locations.

Molecular characterization of multidrug resistance in Streptococcus mitis

Antimicrobial resistance was characterized for 14 strains of Streptococcus mitis. HinfI restriction fragment length mapping of gyrA PCR amplicons from three ciprofloxacin-resistant isolates correlated with mutations associated with such resistance in other organisms. By using PCR, seven erythromycin-resistant strains were found to possess either the mef or ermB gene. Hybridization revealed tet(M) in seven tetracycline-resistant isolates.

Presence of erm gene classes in gram-positive bacteria of animal and human origin in Denmark

A classification of the different erm gene classes based on published sequences was performed, and specific primers to detect some of these classes designed. The presence of ermA (Tn554), ermB (class IV) and ermC (class VI) was determined by PCR in a total of 113 enterococcal, 77 streptococcal and 68 staphylococcal erythromycin resistant isolates of animal and human origin. At least one of these genes was detected in 88% of the isolates. Four isolates contained more than one erm gene. ermB dominated among the enterococci (88%) and streptococci (90%) and ermC among staphylococci (75%) with ermA (Tn554) present in some isolates (16%). Variations in the presence of the different genes when comparing staphylococcal isolates of human and animal origin were observed.

Identification of streptococci to species level by sequencing the gene encoding the manganese-dependent superoxide dismutase

We have used a PCR assay based on the use of degenerate primers in order to characterize an internal fragment (sodA(int)) representing approximately 85% of the genes encoding the manganese-dependent superoxide dismutase in various streptococcal type strains (S. acidominimus, S. agalactiae, S. alactolyticus, S. anginosus, S. bovis, S. constellatus, S. canis, S. cricetus, S. downei, S. dysgalactiae, S. equi subsp. equi, S. equi subsp. zooepidemicus, S. equinus, S. gordonii, S. iniae, S. intermedius, S. mitis, S. mutans, S. oralis, S. parasanguis, S. pneumoniae, S. porcinus, S. pyogenes, S. salivarius, S. sanguis, S. sobrinus, S. suis, S. thermophilus, and S. vestibularis). Phylogenetic analysis of these sodA(int) fragments yields an evolutionary tree having a topology similar to that of the tree constructed with the 16S rRNA sequences. We have shown that clinical isolates could be identified by determining the positions of their sodA(int) fragments on the phylogenetic tree of the sodA(int) fragments of the type species. We propose this method for the characterization of strains that cannot be assigned to a species on the basis of their conventional phenotypic reactions.

New tetracycline resistance determinants coding for ribosomal protection in streptococci and nucleotide sequence of tet(T) isolated from Streptococcus pyogenes A498

An approach based on PCR has been developed to identify new members of the tet gene family in streptococci resistant to tetracycline and minocycline. Degenerate primers, corresponding to portions of the conserved domains of the proteins Tet(M), Tet(O), TeTB(P), Tet(Q), and Tet(S), all specifying the tetracycline-minocycline resistance phenotype, were used to selectively amplify DNA fragments within the coding sequences. Nine streptococcal strains which do not carry the genes tet(M), tet(O), tetB(P), tet(Q), or tet(S) were investigated. Four of them gave no detectable PCR products. The five remaining strains each yielded a PCR product of 1.1 kbp. DNA hybridization experiments showed that these putative Tet determinants fell into four new hybridization classes, of which one, Tet T, was further analyzed. The gene tet(T) was isolated from Streptococcus pyogenes A498, and the nucleotide sequence that was necessary and sufficient for the expression of tetracycline resistance in Escherichia coli was determined. The deduced Tet(T) protein consists of 651 amino acids. The protein most closely related to Tet(T) was Tet(Q), which has 49% identical amino acid residues. A phylogenetic analysis revealed that Tet T represents a novel branching order among the Tet determinants so far described.

Species belonging to the "Streptococcus milleri" group: antimicrobial susceptibility and comparative prevalence in significant clinical specimens

The association between the three species belonging to the "Streptococcus milleri" group and different sites of isolation was examined for 73 successive strains recovered from clinically significant, purulent infections. Susceptibility testing was performed on 64 of these strains. The present study supports the association of particular species with different clinical sources. Susceptibility data suggest that emerging penicillin resistance among Streptococcus anginosus and Streptococcus intermedius isolates may represent a potential clinical problem in the therapeutic management of infections caused by these species.

Detection of tet(M) and tet(O) using the polymerase chain reaction in bacteria isolated from patients with periodontal disease

The polymerase chain reaction was used to examine 114 tetracycline-resistant anaerobic and facultative anaerobic bacterial isolates from patients with periodontal disease for the tet(M) and tet(O) genes. A 740-base-pair fragment of the tet(M) gene was amplified from 84 of 114 isolates, and a 519-base-pair fragment of the tet(O) gene was amplified from 13 streptococcal isolates. Six of 7 tetracycline-resistant isolates of Veillonella spp. and tetracycline-resistant isolates of Eubacterium spp. (n = 3), Eubacterium saburreum (n = 1), Streptococcus intermedius (n = 5) and Gemella morbillorum (n = 2) all harbored the tet(M) gene. The tet(M) and tet(O) negative as well as selected positive isolates were tested for the tet(K) and tet(L) genes using DNA probes. All isolates of Staphylococcus spp. (n = 11) hybridized with the tet(K) probe. None of the isolates tested hybridized with the probe for tet(L). This is the first report of the tet(M) gene in the facultative bacterium G. morbillorum and in E. saburreum.

Detection and incidence of the tetracycline resistance determinant tet(M) in the microflora associated with adult periodontitis

Subgingival plaque samples were collected from 68 patients with adult periodontitis, enumerated on Trypticase-soy blood agar plates, with and without tetracycline at 4 micrograms/ml, and incubated anaerobically for 5 days. Each different colony morphotype was enumerated, and a representative colony was subcultured for identification and examined for the tetracycline resistance gene tet(M). Both PCR amplification and DNA hybridization, using a fragment of tet(M) from Tn1545, were used to detect tet(M). The PCR primers (5'-GACACGCCAGGACATATGG-3' and 5'-TGCTTTCCTCTTGTTCGAG-3') were chosen to amplify a 397 bp region of tet(M). Tetracycline-resistant bacteria represented approximately 12% of the total viable count. The percentage of tet(M)-positive bacteria in the tetracycline resistant microflora varied from < or = 0.05 to 83% (mean of 10%). tet(M) was detected in 60% of 204 tetracycline-resistant strains subcultured and identified. The tet(M) containing strains consisted of streptococci (55%, mainly S. intermedius, S. oralis, S. sanguis, and Streptococcus SM4), Actinomyces D01 (14%), Bifidobacterium D05 (11%), and Veillonella spp. (10%). Tetracycline-resistant strains in which tet(M) was not detected included the Prevotella and Bacteroides species (41%, mainly Bacteroides D28, P. intermedia, P. nigrescens, and P. oris). These results suggest that tet(M) is widely spread in the adult periodontal microflora, but it appears, with the exception of S. intermedius, to be mainly associated with microorganisms not considered to be periodontopathogens. Assessment of other tetracycline-resistant genes in oral organisms is needed to fully evaluate the nature of resistance to this antibiotic in the oral flora.

Sequencing of a tet(Q) gene isolated from Bacteroides fragilis 1126

Recently, Tet Q, a tetracycline resistance determinant that confers resistance by a ribosome protection mechanism, was described and added to the two previously described classes, Tet M and Tet O. The first representative of this class, tetA(Q)1, was isolated from Bacteroides thetaiotaomicron DOT. We report the sequencing of a gene isolated from B. fragilis 1126 which also confers tetracycline resistance. Because of its high degree of identity (97%) with the tetA(Q)1 gene, we defined it as tetA(Q)2. MIC studies revealed that tetA(Q)2 provides a low level of resistance to tetracycline when cloned into Escherichia coli. The extensive homology between tetA(Q)1 and tetA(Q)2 supports the idea of a recent horizontal transfer of tet(Q) genes among Bacteroides spp.

Natural occurrence of structures in oral streptococci and enterococci with DNA homology to Tn916

Seventeen oral streptococci and 18 enterococci were tested for the presence of DNA sequences homologous to the conjugative transposon Tn916 encoding tetracycline resistance. All the strains were resistant to tetracyclines, including minocycline, and most of them were resistant to other antibiotics. Tn916-like structures, identified by hybridization of HincII-digested DNA, were found on the chromosomes of 11 oral streptococci and four enterococci and on two plasmids, pIP1549 and pIP1440, one harbored by an Enterococcus hirae strain and the other harbored by an Enterococcus faecalis strain. Sequences homologous to Tn916, only some of which corresponded to its internal HincII structure (Tn916-modified elements), were chromosomally located in three oral streptococci and two enterococci and were plasmid borne in pIP614 harbored by an E. faecalis strain. Nine enterococci and three oral streptococci carried either the Tet M or the Tet O determinant chromosomally, but they carried no other sequences homologous to Tn916.

Tetracycline resistance heterogeneity in Enterococcus faecium

The tetracycline (Tet) determinants, which encode resistance either to tetracyclines without minocycline (Tcr) or to tetracyclines including minocycline (Tcr-Mnr), of 30 wild-type clinical isolates of Enterococcus faecium were identified and localized. The Tet determinants were transferred by conjugation into a plasmid-free Enterococcus faecalis recipient at frequencies of 10(-6) to 10(-9) transconjugants per donor, as follows: Tcr, 6 strains; Tcr-Mnr, 14 strains; both Tcr and Tcr-Mnr, 6 strains; no detectable transfer, 4 strains. Classes L (Tcr phenotype) and M and O (Tcr-Mnr phenotype) of the Tet determinants were identified by DNA-DNA hybridization experiments. The Tet L determinant was plasmid-borne in 18 strains and was chromosomal in 2 strains. Tet M was chromosomal in 27 strains and plasmid-borne (pIP1534) in 1 strain; pIP1534 also carried Tet L. Tet M was located on Tn916-like elements in 22 strains and on a Tn916-modified element in 1 strain. Tet O was detected in only one strain in which it was plasmid-borne. Both Tet L and Tet M determinants were carried by 19 strains. One strain carried, in addition to chromosomal nonconjugative Tet L and Tet M determinants, a conjugative Tcr-Mnr marker which did not correspond to any Tet determinant tested in this study. These results attest to the genetic complexity of tetracycline resistance in E. faecium strains.