Faecal carriage of optrA-positive enterococci in asymptomatic healthy humans in Hangzhou, China

A comprehensive One Health investigation of erythromycin and quinupristin/dalfopristin resistant Enterococcus spp. in Iran

Enterococci, flagged by the WHO as a rising cause of antibiotic-resistant infections, make surveillance crucial to control resistant strains. We investigated the resistance to linezolid, quinupristin/dalfopristin (Q/D), and erythromycin in Enterococcus faecalis (n = 251) and Enterococcus faecium (n = 434) isolates collected from patients, healthy carriers, hospitals, poultry, livestock, and municipal wastewater in Ardabil, Iran. The isolates were tested for resistance using phenotypic and genotypic methods. Although none of the isolates were resistant to linezolid, 24.9% of E. faecium isolates were resistant to Q/D, particularly those from patients and poultry slaughterhouse wastewater effluent (P < 0.05). The Q/D resistance genes msrC and ermB were detected in 76.85 and 20.37% of E. faecium isolates, respectively. Erythromycin resistance was common in E. faecalis (51.8%) and E. faecium (37.5%), with no significant difference between sources. However, isolates from patients and livestock wastewater had higher erythromycin MICs. Erythromycin resistance genes, such as ermB, ermC, ermTR, and ermA, were found in 80.7, 41.2, 26.5, and 19% of E. faecium and 80.3, 51.6, 22.4, and 25.8% of E. faecalis isolates, respectively. In conclusion, linezolid is a viable treatment for enterococcal infections in Ardabil, but widespread erythromycin- and Q/D-resistant enterococci pose a public health risk.

Whole-Genome Sequencing of Linezolid-Resistant and Linezolid-Intermediate-Susceptibility Enterococcus faecalis Clinical Isolates in a Mexican Tertiary Care University Hospital

Linezolid-non-susceptible Enterococcus faecalis (LNSEf) has emerged as a critical clinical concern worldwide, yet data from Latin American settings remain scarce. This study aimed to investigate the molecular epidemiology and mechanisms underlying LNSEf in a Mexican tertiary care university hospital, focusing on clinical correlates and clonal relationships. A total of 392 non-duplicated E. faecalis isolates were collected over 12 months, of which 24 with minimum inhibitory concentrations ≥4 µg/mL underwent whole-genome sequencing to identify specific resistance determinants (optrA, cfrA, 23S rRNA mutations) and to perform multilocus sequence typing (MLST) and phylogenetic analyses. Of the 392 isolates, 6.12% showed linezolid non-susceptibility, predominantly linked to plasmid- or chromosomally encoded optrA; only two isolates carried cfrA. No mutations were detected in 23S rRNA domain V or ribosomal proteins L3/L4. Clinically, LNSEf strains were associated with immunosuppression, previous surgical interventions, and prolonged hospital stays. Although most LNSEf isolates retained susceptibility to ampicillin, vancomycin, and daptomycin, they exhibited high rates of resistance to other antibiotic classes, particularly aminoglycosides and fluoroquinolones. These findings underscore the emergence of LNSEf in this region, highlighting the need for robust genomic surveillance, strict infection control, and judicious antimicrobial stewardship to curb further dissemination.

Isolation of vanA-Mediated Vancomycin-Resistant Enterococcus faecalis (ST1912/CC116) and Enterococcus faecium (ST80/CC17), optrA-Positive Linezolid-Resistant E. faecalis (ST32, ST1902) from Human Clinical Specimens in Bangladesh

BACKGROUND/OBJECTIVES

Enterococcus is one of the major nosocomial pathogens. The present status of antimicrobial resistance determinants and virulence factors was analyzed for current Enterococcus causing infectious diseases in Bangladesh.

METHODS

Clinical isolates of Enterococcus recovered from various specimens in a tertiary care hospital were analyzed. Antimicrobial susceptibility was measured by a broth microdilution test, and resistance genes/virulence factors were detected by uniplex/multiplex PCR, along with sequencing analysis as required. The sequence type (ST) of E. faecalis and E. faecium was identified based on a multilocus sequence typing (MLST) scheme.

RESULTS

For a one-year period, a total of 143 isolates (135 E. faecalis, 7 E. faecium, and 1 E. hirae) were collected. Although all E. faecalis isolates were susceptible to penicillin, high resistance rates were noted against erythromycin (87%) and levofloxacin (62%). High-level resistance to gentamicin was detected in 30% of E. faecalis and 86% of E. faecium. Vancomycin resistance due to vanA was identified in one isolate each of E. faecalis (ST1912, CC116) and E. faecium (ST80, CC17). Three E. faecalis isolates (2.2%) with ST32 or ST1902 were resistant to linezolid, harboring optrA-fexA.

CONCLUSIONS

The present study identifies the vancomycin-resistant Enterococcus harboring vanA from humans in Bangladesh and shows the potential spread of optrA in multiple lineages of E. faecalis.

The healthy human gut can take it all: vancomycin-variable, linezolid-resistant strains and specific bacteriocin-species interplay in Enterococcus spp

Enterococcus spp. are opportunistic human pathogens colonizing the human gut and a significant reservoir for the continuous adaptation of hospital clones. However, studies on the features of enterococci species co-colonizing healthy individuals are scarce. We investigated the prevalence, antibiotic resistance, and bacteriocin profiles of Enterococcus species in fecal samples from healthy adults in Portugal using culture-based methods, WGS, and bacteriocin inhibition assays. Results were compared with data from a 2001 study in the same region. Enterococcus spp. (n = 315; 24% MDR) were recovered from all volunteers. Enterococcus lactis was the prevalent species (75%), followed by Enterococcus faecalis (65%) and Enterococcus faecium (47%). E. lactis prevalence increased 2.5-fold since 2001. Linezolid resistance genes (optrA/poxtA) were detected in E. faecium and Enterococcus thailandicus isolates, while a vancomycin-variable E. faecium was also identified. Virulence and plasmid profiles were diverse across species, with evidence of exchange of virulence markers and plasmid replicons between E. faecium and E. lactis. Bacteriocin gene repertoires were extensive and species-specific. Higher numbers of bacteriocin genes were associated with stronger inhibition profiles, and 25% of E. faecium and E. lactis isolates were capable of inhibiting relevant VRE clones. This study unveils the co-occurrence and ecological dynamics of Enterococcus species in the healthy human gut, reinforcing its role as a reservoir for key antibiotic resistance genes and potentially pathogenic strains. The shift toward E. lactis prevalence and the detection of linezolid resistance genes in healthy individuals underscore the need for ongoing surveillance of the gut microbiome to guide public health strategies and antibiotic stewardship efforts.IMPORTANCEThis study highlights the role of Enterococcus species in the healthy human gut, revealing important insights into their prevalence and antibiotic resistance. It emphasizes that the human gut serves as a significant reservoir for antibiotic-resistant strains and shows a notable increase and prevalence of Enterococcus lactis, which has been underappreciated due to identification challenges. The research also underscores the bacteriocins' role in microbial competition, where commensal strains inhibit clinical VRE, potentially aiding the restoration of the gut microbiota, after antibiotic treatment. The findings accentuate the need for ongoing surveillance to track changes in gut bacteria, especially with the emergence of resistance genes to last resort antibiotics. Such monitoring is crucial for shaping public health strategies and managing the growing threat of antibiotic-resistant infections. Profiling bacteriocins at the species and strain level can identify ecological adaptation factors and inform strategies to target high-risk clones.

The prevalence of optrA-carrying Enterococci in the vaginal micro-ecology of pregnant women in late pregnancy

The colonization of Enterococcus in the female vagina leads to neonatal and pediatric enterococcal septicemia. Linezolid (LZD) is a kind of mainstream drug for treating multidrug-resistant Gram-positive infections. OptrA is the main LZD-resistance gene at Enterococci in human isolates. It is essential to explore the prevalence of optrA-carrying Enterococcus in vaginal secretions of late pregnant women and the drug resistance of optrA. From May to June 2023, this study recruited 340 volunteers in late pregnancy (35-40 weeks of pregnancy) to provide non-repetitive vaginal discharge samples. Luria-Bertani broth and florfenicol (10 µg/mL) were used to enrich bacteria. Enterococci was identified through time-of-flight mass spectrometry. Additionally, antimicrobial susceptibility, polymerase chain reaction, and next-generation sequencing assays were applied to this study. Fifty-four optrA-carrying Enterococcus strains were obtained, the proportion of the whole vagina of late pregnant women was 15.88% (54 out of 340), and Enterococcus faecalis account the highest proportion. All optrA-carrying Enterococcus were resistant to at least three drugs. Tetracycline, chloramphenicol, erythromycin, and LZD have higher bacterial resistance rates. Genetic environment analysis revealed that IS1216E, fexA, and erm(A) may synergistically exert multidrug resistance with optrA. It is necessary to strengthen the surveillance of optrA-carrying Enterococcus in pregnant women. This study provides scientific support for controlling hospital infections and managing antibiotic-resistant bacteria, and provides a scientific basis for rational clinical medication.IMPORTANCEThe disruption of cervicovaginal microbiota homeostasis is considered a key factor in causing imbalance in the microenvironment, leading to inflammation, transmission of infections, and illness. Enterococcus is considered a major cause of healthcare-related infections globally. It has resistance to multiple antimicrobial drugs, which pose significant challenges for clinical treatment. Therefore, it is crucial to assess the prevalence of optrA-carrying Enterococcus in vaginal secretions of late pregnant women and the drug resistance of optrA. This study detected 15.88% of optrA-carried Enterococci in 340 pregnant women. Furthermore, we found that optrA-carrying Enterococcus strains are highly resistant to tetracycline, chloramphenicol, erythromycin, and Linezolid. Additionally, genetic environment analysis revealed that IS1216E, fexA, and erm(A) may synergistically exert multidrug resistance with optrA. This study provides scientific support for controlling hospital infections and managing antibiotic-resistant bacteria and provides a scientific basis for rational clinical medication.

Antimicrobial resistance, virulence gene profiles, and molecular epidemiology of enterococcal isolates from patients with urinary tract infections in Shanghai, China

Urinary tract infections (UTIs) are among the most prevalent infectious diseases, yet there is still limited understanding of the epidemiology of Enterococcal strains isolated from UTI patients in Shanghai. This study aims to elucidate the antimicrobial resistance profiles, virulence gene carriage, and molecular epidemiology of selected Enterococcal strains from UTI patients in Shanghai. A cohort of 80 Enterococcus faecalis and 40 Enterococcus faecium clinical isolates were randomly selected from UTI patients from October 2022 to March 2023. No vancomycin-resistant strains were identified based on minimum inhibitory concentration (MIC) testing. However, five strains of linezolid-resistant E. faecalis were identified, all of which were confirmed to be optrA-positive through whole-genome sequencing (WGS), with ST300 being reported as the first instance of this ST type in China. Polymerase chain reaction (PCR) assays were employed to ascertain the presence of virulence genes and multi-locus sequence type (MLST). In E. faecalis, the most common virulence genes were asal (75%), gelE (65%), esp (52.5%), and cylA (47.5%). In contrast, E. faecium primarily exhibited esp (65%) and hyl (12.5%). Among the E. faecalis strains, 21 distinct MLST types were identified, with ST16 and ST179 prevailing. Conversely, E. faecium exhibited only five MLST types, with ST78 being predominant. The prevalence of E. faecalis CC16 and E. faecium CC17 further complicates the treatment landscape for Enterococcal UTIs.

IMPORTANCE

This study highlighted the critical need to understand Enterococcal strains causing UTIs in Shanghai, given their high prevalence. By assessing antimicrobial resistance profiles, virulence gene presence, and molecular epidemiology, the research offered valuable insights into the local epidemiology of Enterococcus faecalis and Enterococcus. faecium. Identifying linezolid-resistant strains, all of which carry the optrA gene, including the first report of ST300 in China and recognizing dominant MLST types, such as ST16 and ST179 for E. faecalis and ST78 for E. faecium, are vital for guiding treatment and addressing the challenges these infections present. The data emphasize the need for ongoing surveillance and customized therapeutic approaches to combat emerging resistance and virulence factors in Enterococcal UTIs.

High prevalence and plasmidome diversity of optrA-positive enterococci in a Shenzhen community, China

Background

The emergence of optrA, which can confer resistance to phenicols and oxazolidinones in Enterococcus spp., poses a growing public health threat.

Methods

102 optrA-positive enterococci (OPEs) including various species were isolated from feces of 719 healthy volunteers in a Shenzhen community, China. Antimicrobial susceptibility of these isolates was tested. Whole-genome sequencing and bioinformatics analysis were performed to characterize molecular epidemiology of OPEs.

Results

Compared to optrA-negative enterococci (ONEs), antimicrobial resistance (linezolid, florfenicol, doxycycline, erythromycin and ciprofloxacin) and presence of antimicrobial resistance genes (ARGs) (fexA, cat, tet(M), erm(A), erm(B) and etc) were higher in OPEs. Phylogenetic analysis revealed that high similarly (19-338 SNPs) was observed between the optrA-positive E. faecalis from community and the strains from patients, animals, and environment. In 102 OPEs, the optrA gene was detected on the chromosome (n = 36), on plasmids (n = 62), or both (n = 4). A diverse range of optrA-carrying plasmid types was identified. The rep9-plasmid replicons were widely detected in E. faecalis (44/66), whereas repUS1-plasmid replicons were widely identified in other enterococcal species (7/66). Most of all ARGs harbored by isolates were co-existed on optrA-carrying plasmids, suggesting that the acquisition of optrA-carrying plasmids will pose a greater threat to public health. Notably, the pAD1 (rep9 family) + DOp1-type plasmids should receive more attention for the transfer of optrA given their high prevalence (36.36%), high number of co-located ARGs with optrA (83.87% of total ARGs) and presence in multiple sources. Tn6674, IS1216E, ISEnfa1 and ISEnfa5 are related to the transfer of chromosomal and plasmids-derived optrA, respectively. The bcrABDR gene cluster, fexA, and erm(A) were frequently identified surrounding optrA and may be transferred with optrA via IS1216E or ISEnfa1.

Conclusion

The transfer of optrA gene is related to a variety of mobile elements (including plasmids, insertion sequences, transposons), which will promote the horizontal transfer of optrA. Moreover, many ARGs co-exist with optrA and could co-transfer with optrA. The acquisition of OPEs and optrA-carrying plasmids will pose a greater threat to public health and should be obtained more attention, especially optrA-positive E. faecalis and pAD1 + DOp1-type plasmids.

Prevalence and genetic characterization of linezolid resistance gene reservoirs in hospital sewage from Zhejiang Province, China

Hospital sewage represented important hotspots for the aggregation and dissemination of clinically relevant pathogens and antimicrobial resistance genes. To investigate the prevalence and molecular epidemiology of linezolid resistance genes in hospital sewage, both influent and effluent samples from 11 hospitals in Zhejiang Province, China, were collected and analyzed for linezolid resistance gene carriers. Thirty colonies of putative isolates that grew on the selective media with 10 mg/L florfenicol were randomly picked per sample. A total of 420 Gram-positive isolates, including 330 from 11 influent samples and 90 from three effluent samples, were obtained. Each isolate carried at least one of the linezolid resistance genes, including optrA, poxtA, cfr, and cfr(D), and the optrA gene was highly dominant (388/420). Enterococci displayed predominance among the linezolid resistance gene carriers in the hospital sewage, exhibiting a resistance rate to linezolid of 77.8 %. The wild-type OptrA and OptrA variants KLDP, RDK, and KLDK, all associated with high linezolid MICs, were most frequently detected. Phylogenetic analysis revealed the multispecies and polyclonal distribution of linezolid-resistant bacteria in hospital sewage, while Enterococcus faecalis sequence types (STs) 16 and 179 demonstrated the widest dissemination across different hospitals. Despite generally high genetic diversity, phylogenetic analysis showed that 87 isolates, assigned to ten STs from both sewage and other sources, were genetically related. Moreover, the genetic environment of linezolid resistance genes in isolates from sewage was similar to that from animals, humans, or the environment, with "Tn554-fexA-optrA" as the most common structure. These findings revealed the potential risk of the transmission of linezolid resistance genes through hospital sewage to other environments.

Prevalence and genetic diversity of optrA-positive enterococci isolated from patients in an anorectal surgery ward of a Chinese hospital

Linezolid-resistant enterococci have increased in recent years due to the worldwide spread of acquired resistance genes (cfr, optrA, and poxtA) in clinical, animal, and environmental settings. This study investigated the carriage of optrA-positive enterococci among patients in the anorectal surgery ward in Hangzhou, China, and characterized the genetic context of optrA. A total of 173 wound secretion samples were obtained to screen optrA-positive enterococci. Of the 173 samples, 15 (8.67%) were positive for optrA, including 12 Enterococcus faecalis, two E. faecium, and one E. hirae. Multilocus sequence type analysis revealed that 12 optrA-positive E. faecalis isolates belonged to eight different sequence types (STs), of which ST16 was the main type. Eight optrA variants were identified, whose optrA flanking regions with a fexA gene downstream were bounded by different mobile genetic elements. Furthermore, the optrA gene in 8 out of 15 optrA-positive enterococci could be successfully transferred through conjugation. The findings revealed a high carriage rate of optrA in enterococci from one anorectal surgery ward in China. The dissemination of optrA-positive enterococci isolates in clinical settings should be continually monitored.

Phenotypic and genotypic characterization of Enterococcus faecalis and Enterococcus faecium isolated from fish, vegetables, and humans

Enterococci, common hospital-acquired infections in immunocompromised patients, have garnered attention in clinical microbiology. To determine the clinical relevance of enterococci as food-borne pathogens, 116 fish, 90 vegetables, and 120 human diarrheal samples were tested for E. faecalis and E. faecium pathogenicity. Conventionally, 69 of 326 (21.17%) samples were positive for Enterococcus species, 52 (15.95%) of which were molecularly classified as E. faecalis and 13 (3.99%) as E. faecium. The E. faecalis contamination percentage of fresh fish (19.70%) was higher than frozen fish (4%). Cauliflower had the highest E. faecalis percentage (16.67%) when fish and vegetable samples didn't harbor the E. faecium atpA gene. 23.33% and 10.83% of participants' samples were molecularly confirmed as E. faecalis and E. faecium positive, respectively. E. faecalis isolates had all virulence genes, with gels being the most common (65.38%), while cylA and asa1 genes couldn't be detected in E. faecium isolates. E. faecalis showed the highest resistance against vancomycin and tetracycline (69.23%), whereas E. faecium extremely resisted tetracycline (76.92%) and erythromycin (69.23%) with the recognition of MDR among 44.2% of E. faecalis and 38.5% of E. faecium isolates. The great similarity of our isolates showed the clinical importance of food-borne antibiotic-resistant enterococci.

Antibiotic resistance of Escherichia coli isolated from food and clinical environment in China from 2001 to 2020

Antibiotics are widely used in China's aquaculture, agricultural, and clinical settings and can lead to antibiotic resistance in various pathogens. Although the pooled prevalence estimate (PPE) and antibiotic resistance of Escherichia coli (E. coli) in food and clinical settings has been extensively studied, a comprehensive analysis of the published literature is lacking. We conducted a comprehensive search for research indicators for 2001-2020 in eight major Chinese and English literature databases. Antibiotic PPE and resistance trends of 5933 and 29,451 E. coli isolates were screened and analysed in 35 food studies (total 1821) and 62 clinical studies (total 5159). E. coli strains derived from food had the highest antibiotic resistance rate to tetracycline (TET, 71.3 %), followed by trimethoprim-sulfamethoxazole (SXT, 62.5 %) and cefazolin (CFZ, 36.2 %). E. coli strains isolated from clinical environments were highly resistant to piperacillin (PIP, 71.7 %), TET (68.3 %) and CFZ (60.9 %), consistent with foodborne E. coli drug resistance patterns. E. coli strains isolated from food and clinical samples collected in laboratories carry multiple antibiotic resistance genes (ARGs), such as blaTEM, gryA, gryB, sul1, and tetA, making E. coli a reservoir of ARGs. This study highlights the presence of drug-resistant E. coli pathogens and ARGs in food and clinical environments.

Enterococci as a One Health indicator of antimicrobial resistance

The rapid increase of antimicrobial-resistant bacteria in humans and livestock is concerning. Antimicrobials are essential for the treatment of disease in modern day medicine, and their misuse in humans and food animals has contributed to an increase in the prevalence of antimicrobial-resistant bacteria. Globally, antimicrobial resistance is recognized as a One Health problem affecting humans, animals, and environment. Enterococcal species are Gram-positive bacteria that are widely distributed in nature. Their occurrence, prevalence, and persistence across the One Health continuum make them an ideal candidate to study antimicrobial resistance from a One Health perspective. The objective of this review was to summarize the role of enterococci as an indicator of antimicrobial resistance across One Health sectors. We also briefly address the prevalence of enterococci in human, animal, and environmental settings. In addition, a 16S RNA gene-based phylogenetic tree was constructed to visualize the evolutionary relationship among enterococcal species and whether they segregate based on host environment. We also review the genomic basis of antimicrobial resistance in enterococcal species across the One Health continuum.

Genomic Characterization and Phylogenetic Analysis of Linezolid-Resistant Enterococcus from the Nostrils of Healthy Hosts Identifies Zoonotic Transmission

Linezolid resistance in Enterococcus spp. is increasingly considered critically important and a public health threat which mandates the need to understand their genomic contents and dissemination patterns. Here, we used whole-genome sequencing to characterize the resistome, virulome and mobile genetic elements of nine linezolid-resistant (LZDR) enterococci (seven optrA-E. faecalis, one poxtA-E. faecium and one optrA-E. casseliflavus) previously obtained from the nares of healthy dogs, pigs, pig farmers and tracheal samples of nestling storks in Spain. Also, the relatedness of the isolates with publicly available genomes was accessed by core-genome single nucleotide polymorphism (SNP) analysis. The optrA gene of the E. faecalis and E. casseliflavus isolates was located downstream of the fexA gene. The optrA gene in the E. casseliflavus isolate was carried in a plasmid (pURX4962), while those in the seven E. faecalis isolates were chromosomally located. The OptrA proteins were mostly variants of wild type (DP-2: Y176D/T481P; RDK: I104R/Y176D/E256K; DD-3: Y176D/G393D; and EDD: K3E/Y176D/G393D), except two that were wild type (one E. faecalis and one E. casseliflavus). The poxtA gene in the E. faecium isolate was found alone within its contig. The cfrD was upstream of ermB gene in the E. casseliflavus isolate and flanked by ISNCY and IS1216. All the LZDR enterococci carried plasmid rep genes (2-3) containing tetracycline, chloramphenicol and aminoglycoside resistance genes. All isolates except E. casseliflavus carried at least one intact prophage, of which E. faecalis-ST330 (X4957) from a pig carried the highest (n = 5). Tn6260 was associated with lnuG in E. faecalis-ST330 while Tn554 was with fexA in E. feaecalis-ST59 isolates. All except E. casseliflavus (n = 0) carried at least two metal resistance genes (MRGs), of which poxtA-carrying E. faecium-ST1739 isolate contained the most (arsA, copA, fief, ziaA, znuA, zosA, zupT, and zur). SNP-based analyses identified closely related optrA-E. faecalis isolates from a pig and a pig farmer on the same farm (SNP = 4). Moreover, optrA- carrying E. faecalis-ST32, -ST59, and -ST474 isolates from pigs were related to those previously described from humans (sick and healthy) and cattle in Spain, Belgium, and Switzerland (SNP range 43-86). These findings strongly suggest the transmission of LZDR-E. faecalis between a pig and a pig farmer and potential inter-country dissemination. These highlight the need to strengthen molecular surveillance of LZDR enterococci in all ecological niches and body parts to direct appropriate control strategies.

Genomic epidemiology reveals multiple mechanisms of linezolid resistance in clinical enterococci in China

BACKGROUND

Infections caused by linezolid-resistant enterococci (LRE) are clinically difficult to treat and threaten patient health. However, there is a lack of studies on long time-span LRE strains in China. For this reason, our study comprehensively revealed the resistance mechanisms of LRE strains collected in a Chinese tertiary care hospital from 2011 to 2022.

METHODS

Enterococcal strains were screened and verified after retrospective analysis of microbial data. Subsequently, 65 LRE strains (61 Enterococcus faecalis and 4 Enterococcus faecium, MIC ≥ 8 µg/ml), 1 linezolid-intermediate Enterococcus faecium (MIC = 4 µg/ml) and 1 linezolid-susceptible Enterococcus faecium (MIC = 1.5 µg/ml) were submitted for whole-genome sequencing (WGS) analysis and bioinformatics analysis.

RESULTS

The optrA gene was found to be the most common linezolid resistance mechanism in our study. We identified the wild-type OptrA and various OptrA variants in 98.5% of LRE strains (61 Enterococcus faecalis and 3 Enterococcus faecium). We also found one linezolid-resistant Enterococcus faecium strain carried both optrA and cfr(D) gene, while one linezolid-resistant Enterococcus faecium only harbored the poxtA gene. Most optrA genes (55/64) were located on plasmids, with impB-fexA-optrA, impB-fexA-optrA-erm(A), fexA-optrA-erm(A), and fexA-optrA segments. A minority of optrA genes (9/64) were found on chromosomes with the Tn6674-like platform. Besides, other possible linezolid resistance-associated mechanisms (mutations in the rplC and rplD genes) were also found in 26 enterococcal strains.

CONCLUSIONS

Our study suggested that multiple mechanisms of linezolid resistance exist among clinical LRE strains in China.

Genomic insights into linezolid-resistant Enterococci revealed its evolutionary diversity and poxtA copy number heterogeneity

OBJECTIVES

This study aimed to determine the molecular mechanisms of linezolid-resistant enterococci (LRE) in swine slaughterhouses in China and apply the "One Health" perspective to analyse the evolutionary dynamics of poxtA-positive E. faecium in clinical and non-clinical settings worldwide.

METHODS

The phenotypic and genomic characteristics of multiple LRE isolates were systematically investigated using antimicrobial susceptibility testing, transfer assays, evolutionary experiments, quantitative RT-PCR assays, whole-genome sequencing, and bioinformatics analyses.

RESULTS

Swine faeces served as a significant reservoir for LRE isolates, and optrA and poxtA were the primary contributors to linezolid resistance. Co-occurrence network analysis revealed a significant interconnection between optrA and several other ARGs. The poxtA copy number heterogeneity and polymorphism were initially observed in E. faecium parental and evolved isolates. The poxtA-carrying tandem repeat region exhibits high mobility and has undergone extensive duplication owing to linezolid pressure. The poxtA copy number varies from four copies on the plasmid of E. faecium IC25 to 11 copies on the plasmid and six copies on the chromosome in the evolved isolate IC25-50_poxtA. Furthermore, phylogenetic analysis of 185 poxtA-positive E. faecium strains worldwide found that one isolate from a French patient in 2018 shared only two SNPs with CC17 E. faecium isolates IC25 and IC7-2 from this study, highlighting the potential global transmission of CC17 poxtA-positive E. faecium between humans and animals.

CONCLUSION

This study identified amplification of poxtA as a response of E. faecium to linezolid pressure. Phylogenetic analysis shed light on the potential global transmission of hospital-associated CC17 poxtA-positive E. faecium in clinical and non-clinical settings.

Genome characteristics of the optrA-positive Clostridium perfringens strain QHY-2 carrying a novel plasmid type

Clostridium perfringens is a bacterial species of importance to both public and animal health. The gene optrA is the first gene that confers resistance to the tedizolid, a last-resort antimicrobial agent in human medicine. Herein, we whole-genome sequenced and analyzed one optrA-positive C. perfringens strain QHY-2 from Tibetan sheep in Qinghai province and identified one optrA plasmid pQHY-2. The plasmid shared similar structure with the optrA-positive plasmids p2C45 and p21-D-5b previously identified in C. perfringens, demonstrating the potential horizontal transmission of the optrA plasmids among C. perfringens strains. Annotation of the optrA-positive plasmids showed optrA and erm(A) located on a segment flanked by IS element IS1216E, and fexA, optrA, and erm(A) located on a segment flanked by IS element ISVlu1, which revealed the possible dissemination mechanism. Additionally, a Tn6218-like transposon carrying aac(6')-aph(2″) and erm(B) was also detected on pQHY-2, demonstrating the transposition of Tn6218 and spread of antibiotic resistance among Clostridium bacteria. Molecular analysis indicated the optrA-positive plasmids belonged to a plasmid type distinct from the pCW3-like plasmids, pCP13-like plasmids, or pIP404-like plasmids. Further structure analysis showed they might be formed by inserting segments into plasmid pCPCPI53k-r1_1, which coexist with two pCW3-like plasmids and one pCP13-like plasmid in C. perfringens strain CPI 53k-r1 isolated from a healthy human in Finland. IMPORTANCE Antimicrobial resistance is now a global concern posing threats to food safety and public health. The pCW3-like plasmids can encode several main toxin genes and three antibiotic resistance genes (ARGs), including tetA(P), tetB(P), and erm(B), which used to be considered as the main carrier of ARGs in Clostridium perfringens. In this study, we found the optrA plasmids, which belonged to a novel plasmid type, could also harbor many other ARGs, indicating this type of plasmid might be the potential repository of ARGs in C. perfringens. Additionally, this type of plasmid could coexist with the pCW3-like plasmids and pCP13-like plasmids that encoded toxin genes associated with gastrointestinal diseases, which showed the potential threat to public health.

Oxazolidinone resistance genes in florfenicol-resistant enterococci from beef cattle and veal calves at slaughter

Background

Linezolid is a critically important oxazolidinone antibiotic used in human medicine. Although linezolid is not licensed for use in food-producing animals, the use of florfenicol in veterinary medicine co-selects for oxazolidinone resistance genes.

Objective

This study aimed to assess the occurrence of cfr, optrA, and poxtA in florfenicol-resistant isolates from beef cattle and veal calves from different herds in Switzerland.

Methods

A total of 618 cecal samples taken from beef cattle and veal calves at slaughter originating from 199 herds were cultured after an enrichment step on a selective medium containing 10 mg/L florfenicol. Isolates were screened by PCR for cfr, optrA, and poxtA which are genes known to confer resistance to oxazolidinones and phenicols. One isolate per PCR-positive species and herd was selected for antimicrobial susceptibility testing (AST) and whole-genome sequencing (WGS).

Results

Overall, 105 florfenicol-resistant isolates were obtained from 99 (16%) of the samples, corresponding to 4% of the beef cattle herds and 24% of the veal calf herds. Screening by PCR revealed the presence of optrA in 95 (90%) and poxtA in 22 (21%) of the isolates. None of the isolates contained cfr. Isolates included for AST and WGS analysis were Enterococcus (E.) faecalis (n = 14), E. faecium (n = 12), E. dispar (n = 1), E. durans (n = 2), E. gallinarum (n = 1), Vagococcus (V.) lutrae (n = 2), Aerococcus (A.) urinaeequi (n = 1), and Companilactobacillus (C.) farciminis (n = 1). Thirteen isolates exhibited phenotypic linezolid resistance. Three novel OptrA variants were identified. Multilocus sequence typing identified four E. faecium ST18 belonging to hospital-associated clade A1. There was a difference in the replicon profile among optrA- and poxtA-harboring plasmids, with rep9 (RepA_N) plasmids dominating in optrA-harboring E. faecalis and rep2 (Inc18) and rep29 (Rep_3) plasmids in poxtA-carrying E. faecium.

Conclusion

Beef cattle and veal calves are reservoirs for enterococci with acquired linezolid resistance genes optrA and poxtA. The presence of E. faecium ST18 highlights the zoonotic potential of some bovine isolates. The dispersal of clinically relevant oxazolidinone resistance genes throughout a wide variety of species including Enterococcus spp., V. lutrae, A. urinaeequi, and the probiotic C. farciminis in food-producing animals is a public health concern.

Identification of plasmids co-carrying cfr(D)/optrA and cfr(D2)/poxtA linezolid resistance genes in two Enterococcus avium isolates from swine brain

Oxazolidinones are critically important antibiotics to treat human infections caused by multidrug-resistant bacteria, therefore the occurrence of linezolid-resistant enterococci from food-producing animals poses a serious risk to human health. In this study, Enterococcus avium 38157 and 44917 strains, isolated from the brain of two unrelated piglets, were found to carry the linezolid resistance genes cfr(D)-optrA, and cfr(D2)-poxtA, respectively. Whole genome sequencing analysis of E. avium 38157 revealed that the genes were co-located on the 36.5-kb pEa_cfr(D)-optrA plasmid showing high identity with the pAT02-c of Enterococcus faecium AT02 from pet food. The optrA region, was 99% identical to the one of the pAv-optrA plasmid from a bovine Aerococcus viridans strain, whereas the cfr(D) genetic context was identical to that of the plasmid 2 of E. faecium 15-307.1. pEa_cfr(D)-optrA was not transferable to enterococcal recipients. In E. avium 44917 a cfr(D)-like gene, named cfr(D2), and the poxtA gene were co-located on the transferable 42.6-kb pEa-cfr(D2)-poxtA plasmid 97% identical to the Tn6349 transposon of the human MRSA AOUC-0915. The cfr(D2) genetic context, fully replaced the Tn6644 that in S. aureus AOUC-0915 harbor the cfr gene. In conclusion, this is, the best of our knowledge, the first report of the new cfr(D2) gene variant. The occurrence of plasmids co-carrying two linezolid resistance genes in enterococci from food-producing animals needs close surveillance to prevent their spread to human pathogens.

Bile Carriage of optrA-Positive Enterococcus faecium in a Patient with Choledocholith

We isolated one Enterococcus faecium isolate SZ21B15 from a bile sample of a patient with choledocholith in Shenzhen, China in 2021. It was positive for oxazolidinone resistance gene optrA and was intermediate to linezolid. The whole genome of E. faecium SZ21B15 was sequenced by Illumina Hiseq. It belonged to ST533 within the clonal complex 17. The optrA gene and additional two resistance genes fexA and erm(A) were located within a 25,777-bp multiresistance region, which was inserted into the chromosomal radC gene, being chromosomal intrinsic resistance genes. The chromosomal optrA gene cluster found in E. faecium SZ21B15 was closely related to the corresponding regions of multiple optrA-carrying plasmids or chromosomes from Enterococcus, Listeria, Staphylococcus, and Lactococcus strains. It further highlights the ability of the optrA cluster that transfers between plasmids and chromosomes and evolves by a series of molecular recombination events. IMPORTANCE Oxazolidinone are effective antimicrobial agents for the treatment of infections caused by multidrug-resistant Gram-positive bacteria, including vancomycin-resistant enterococci. The emergence and global spread of transferable oxazolidinone resistance genes such as optrA is worrisome. Enterococcus spp. can become causes of hospital-associated infections and are also widely distributed in the gastrointestinal tracts of animals and the natural environment. In this study, one E. faecium isolate from bile sample carried chromosomal optrA, being intrinsic resistance gene. optrA-positive E. faecium in bile not only makes the treatment of gallstones difficult, but also may become a reservoir of resistance genes in the body.

High occurrence of Enterococcus faecalis, Enterococcus faecium, and Vagococcus lutrae harbouring oxazolidinone resistance genes in raw meat-based diets for companion animals - a public health issue, Switzerland, September 2018 to May 2020

IntroductionEnterococci harbouring genes encoding resistance to florfenicol and the oxazolidinone antimicrobial linezolid have emerged among food-producing animals and meat thereof, but few studies have analysed their occurrence in raw meat-based diets (RMBDs) for pets.AimWe aimed to examine how far RMBDs may represent a source of bacteria with oxazolidinone resistance genes.MethodsFifty-nine samples of different types of RMBDs from 10 suppliers (three based in Germany, seven in Switzerland) were screened for florfenicol-resistant Gram-positive bacteria using a selective culture medium. Isolates were phenotypically and genotypically characterised.ResultsA total of 27 Enterococcus faecalis, Enterococcus faecium, and Vagococcus lutrae isolates were obtained from 24 of the 59 samples. The optrA, poxtA, and cfr genes were identified in 24/27, 6/27 and 5/27 isolates, respectively. Chloramphenicol and linezolid minimum inhibitory concentrations (MICs) ranged from 24.0 mg/L-256.0 mg/L, and 1.5 mg/L-8.0 mg/L, respectively. According to the Clinical and Laboratory Standards Institute (CLSI) breakpoints, 26 of 27 isolates were resistant to chloramphenicol (MICs ≥ 32 mg/L), and two were resistant to linezolid (MICs ≥ 8 mg/L). Multilocus sequence typing analysis of the 17 E. faecalis isolates identified 10 different sequence types (ST)s, with ST593 (n = 4 isolates) and ST207 (n = 2 isolates) occurring more than once, and two novel STs (n = 2 isolates). E. faecium isolates belonged to four different STs (168, 264, 822, and 1846).ConclusionThe high occurrence in our sample of Gram-positive bacteria harbouring genes encoding resistance to the critical antimicrobial linezolid is of concern since such bacteria may spread from companion animals to humans upon close contact between pets and their owners.

Various mobile genetic elements carrying optrA in Enterococcus faecium and Enterococcus faecalis isolates from swine within the same farm

OBJECTIVES

In this study, the distribution of the oxazolidinone/phenicol resistance gene optrA and the mobile genetic elements involved in its dissemination were analysed among enterococcal isolates from a farrow-to-finish swine farm.

METHODS

Enterococcus faecium and Enterococcus faecalis isolates were obtained from all pig production stages in the farm. The optrA-carrying E. faecium and E. faecalis isolates were subjected to PFGE and antimicrobial susceptibility testing. Complete sequences of the genetically unrelated optrA-carrying E. faecium and E. faecalis isolates were determined using Illumina HiSeq and MinION platforms.

RESULTS

The optrA gene was present in 12.2% (23/188) of the E. faecium and E. faecalis isolates, most of which originated from nursery and finishing stages. The 23 optrA-positive Enterococcus isolates represented 15 PFGE types. WGS of representative isolates of the 15 PFGE types showed that optrA was carried by diverse genetic elements either located in the chromosomal DNA or on plasmids. A novel optrA-bearing genetic element was identified on two distinct multi-resistance plasmids from E. faecium. Two new hybrid plasmids carrying several resistance genes were found in two E. faecalis isolates. pC25-1-like plasmids and chromosomally integrated Tn6674 and Tn6823-like transposons were prevalent in the remaining Enterococcus isolates.

CONCLUSIONS

The gene optrA was found in genetically unrelated E. faecium and E. faecalis isolates from the same farm. Analysis of the genetic contexts of optrA suggested that horizontal transfer including different plasmids and transposons played a key role in the dissemination of optrA in this farm.

Antimicrobial Resistance, Virulence Factors, and Genotypes of Enterococcus faecalis and Enterococcus faecium Clinical Isolates in Northern Japan: Identification of optrA in ST480 E. faecalis

Enterococcus faecalis and E. faecium are the major pathogens causing community- and healthcare-associated infections, with an ability to acquire resistance to multiple antimicrobials. The present study was conducted to determine the prevalence of virulence factors, drug resistance and its genetic determinants, and clonal lineages of E. faecalis and E. faecium clinical isolates in northern Japan. A total of 480 (426 E. faecalis and 54 E. faecium) isolates collected over a four-month period were analyzed. Three virulence factors promoting bacterial colonization (asa1, efaA, and ace) were more prevalent among E. faecalis (46-59%) than E. faecium, while a similar prevalence of enterococcal surface protein gene (esp) was found in these species. Between E. faecalis and E. faecium, an evident difference was noted for resistance to erythromycin, gentamicin, and levofloxacin and its responsible resistance determinants. Oxazolidinone resistance gene optrA and phenicol exporter gene fexA were identified in an isolate of E. faecalis belonging to ST480 and revealed to be located on a cluster similar to those of isolates reported in other Asian countries. The E. faecalis isolates analyzed were differentiated into 12 STs, among which ST179 and ST16 of clonal complex (CC) 16 were the major lineage. Nearly all the E. faecium isolates were assigned into CC17, which consisted of 10 different sequence types (STs), including a dominant ST17 containing multidrug resistant isolates and ST78 with isolates harboring the hyaluronidase gene (hyl). The present study revealed the genetic profiles of E. faecalis and E. faecium clinical isolates, with the first identification of optrA in ST480 E. faecalis in Japan.

An Optimized Screening Approach for the Oxazolidinone Resistance Gene optrA Yielded a Higher Fecal Carriage Rate among Healthy Individuals in Hangzhou, China

The linezolid resistance mediated by optrA has exhibited an increasing trend among Gram-positive bacteria, which greatly limits the treatment options for severe bacterial infections. However, the prevalence of optrA was usually underestimated based on the existing screening methods. In this study, we used a traditional method and an improved method that included a high-salinity condition treatment after enrichment to screen for optrA-carrying bacteria from stool samples from 1,018 healthy donors in Hangzhou, China. The fecal carriage rate of optrA-carrying bacteria was 19.25% when screened by the improved method (196/1,018), which was much higher than that of the traditional method at 5.89% (60/1,018). Enterococci were the majority of the optrA-positive isolates, while five nonenterococcal isolates were also obtained, including two Streptococcus gallolyticus, one Vagococcus lutrae, one Lactococcus garvieae, and one Lactococcus formosensis isolate. Whole-genome sequencing analysis identified four novel OptrA variants, IDKKGPM, IDKKGP, KLDK, and EYDDI, in these isolates, whose optrA-flanking regions with a fexA gene downstream were bounded by different insertion sequences. In conclusion, our optimized method displayed high sensitivity in the detection of optrA-positive bacteria in fecal samples and revealed a high carriage rate in a healthy population. Although enterococci are dominant, multiple optrA-carrying Gram-positive bacteria were also found. IMPORTANCE This study represented an optimized screening approach for the optrA gene, which is an important mechanism of antimicrobial resistance to linezolid as a last resort for the treatment of infections caused by multiresistant Gram-positive bacteria. We revealed a high fecal carriage rate of the optrA gene among adults by this method and reported the first identification of optrA in Lactococcus formosensis as well as the identification of this gene in Vagococcus lutrae and of the poxtA gene in Ligilactobacillus salivarius of human origin, suggesting the wide spread of the optrA gene in the Gram-positive bacterial community.

Persistence of transferable oxazolidinone resistance genes in enterococcal isolates from a swine farm in China

The appearance of transferable oxazolidinone resistance genes poses a major challenge to public health and environmental safety. These genes not only lead pathogenic bacteria to become resistant to linezolid but also reduce sensitivity to florfenicol, which is widely used in the veterinary field. To verify the dissemination of oxazolidinone resistance genes in enterococcal isolates from pigs at different production stages in a swine farm in China, we collected 355 enterococcal isolates that were resistant to florfenicol from 600 (150 per stage) fresh fecal swabs collected from a swine farm. Through initial PCR screening and whole-genome sequencing, 175 isolates harboring different oxazolidinone resistance genes were identified. All isolates carried the optrA gene. A total of 161 (92%, 161/175) isolates carried only the optrA gene. Three (1.71%, 3/175) isolates carried both the optrA and poxtA genes, and 11 (3.1%, 11/175) isolates contained the optrA gene and poxtA2 and cfr(D) variants. A total of 175 isolates that harbored oxazolidinone resistance genes included 161 E. faecalis, 6 E. faecium, and 8 E. hirae. By sequencing the whole genomes, we found that the 161 isolates of E. faecalis belonged to 28 different STs, including 8 new STs, and the 6 isolates of E. faecium belonged to four different STs, including one new ST. The phylogenetic tree based on SNPs of the core genome showed that both clonal spread and horizontal transfer mediated the diffusion of oxazolidone resistance genes in enterococcal isolates at specific stages in pig farms. Moreover, enterococcal isolates carrying oxazolidone resistance genes could spread from breeding pigs to fattening pigs, while transferable oxazolidone resistance genes in enterococcal isolates could persist on a pig farm throughout all production stages. Representative enterococcal isolates with different oxazolidinone resistance genes were further studied through Nanopore sequencing. We identified a novel plasmid, pM4-80 L4 (15,008 bp), carrying the poxtA2 and cfr(D) genes in enterococcal isolates at different stages. We also found three different plasmids harboring the poxtA gene with high genetic variation, and all poxtA genes were flanked by two copies of IS1216E elements. In addition, four genetically distinct plasmids carrying the optrA gene were identified, and Tn554 was found to mediate chromosome-localized optrA gene transfer. Our study highlighted that transferable oxazolidinone resistance genes in enterococcal isolates could persist throughout all production stages on a pig farm, and the prevalence and dissemination of oxazolidinone resistance genes in enterococcal isolates from animal farms should be continually monitored.

Faecal carriage of enterococci harbouring oxazolidinone resistance genes among healthy humans in the community in Switzerland

Objectives

This study aimed to investigate the faecal carriage of enterococci harbouring oxazolidinone resistance genes among healthy humans in Switzerland and to genetically characterize the isolates.

Methods

A total of 399 stool samples from healthy individuals employed in different food-processing plants were cultured on a selective medium containing 10 mg/L florfenicol. Resulting enterococci were screened by PCR for the presence of cfr, optrA and poxtA. A hybrid approach combining short-read and long-read WGS was used to analyse the genetic context of the cfr, optrA and poxtA genes.

Results

Enterococcus faecalis (n = 6), Enterococcus faecium (n = 6), Enterococcus gallinarum (n = 1) and Enterococcus hirae (n = 2) were detected in 15/399 (3.8%) of the faecal samples. They carried cfr + poxtA, optrA, optrA + poxtA or poxtA. Four E. faecalis harbouring optrA and one E. faecium carrying poxtA were resistant to linezolid (8 mg/L). In most optrA-positive isolates, the genetic environments of optrA were highly variable, but often resembled previously described platforms. In most poxtA-positive isolates, the poxtA gene was flanked on both sides by IS1216E elements and located on medium-sized plasmids.

Conclusions

Faecal carriage of Enterococcus spp. harbouring cfr, optrA and poxtA in healthy humans associated with the food-production industry demonstrates the possibility of spread of oxazolidinone resistance genes into the community. Given the importance of linezolid as a last-resort antibiotic for the treatment of serious infections caused by Gram-positive pathogens, the detection of the oxazolidinone resistance determinants in enterococci from healthy humans is of concern for public health.

Dissemination of Quinupristin-Dalfopristin and Linezolid resistance genes among hospital environmental and healthy volunteer fecal isolates of Enterococcus faecalis and Enterococcus faecium

BACKGROUND

Streptogramins and linezolid are important in the treatment of infections caused by vancomycin-resistant enterococci.

PURPOSE

Then, we aimed to evaluate the resistance rates against these drugs and the prevalence of genes involved in hospital environmental and fecal normal-flora isolates of Enterococcus faecalis and Enterococcus faecium.

METHODS AND RESULTS

The strains were isolated from the stool samples and hospital environments by culturing on M-Enterococcus (ME) agar, and identified by phenotypic and genotypic microbiological tests. The disk agar diffusion method was used to identify the antimicrobial susceptibility pattern of the isolates. The genomic DNA extraction was done by the alkaline lysis method, and the PCR test was used to detect the resistance genes. A total of 145 enterococci isolates were taken, from which 84 (57.9%) isolates were detected as E. faecalis and 61 (42.06%) isolates were E. faecium. Moreover, 70 (83.33), 4 (4.76%), 1 (1.19%), and 40 (47.61%) isolates of E. faecalis and 20 (32.78%), 1 (1.63%), 4 (6.55%), and 26 (42.62%) E. faecium isolates were resistant against quinupristin-dalfopristin, linezolid, vancomycin, and erythromycin, respectively. Also, 112 (77.24%), 50 (34.48%), 39 (26.89%), 27 (18.62%), 19 (13.1%), 4 (2.75%), and 1 (0.68%) isolates were contained LsaA, vatD, vgbB, vatE, cfr, lsaE and optrA genes, respectively. None of the isolates carried the vgbA gene.

CONCLUSIONS

High-level streptogramin resistance rate and high prevalence of resistance genes in enterococci isolated from the stool of healthy persons and the hospital environment indicates the importance of possible transmission of resistance genes from these isolates to clinical ones.

Antimicrobial Susceptibility of Enterococcus Isolates from Cattle and Pigs in Portugal: Linezolid Resistance Genes optrA and poxtA

Enterococci are part of the commensal gut microbiota of mammals, with Enterococcus faecalis and Enterococcus faecium being the most clinically relevant species. This study assesses the prevalence and diversity of enterococcal species in cattle (n = 201) and pig (n = 249) cecal samples collected in 2017. Antimicrobial susceptibility profiles of E. faecium (n = 48) and E. faecalis (n = 84) were assessed by agar and microdilution methods. Resistance genes were screened through PCR and nine strains were analyzed by Whole Genome Sequencing. A wide range of enterococci species was found colonizing the intestines of pigs and cattle. Overall, the prevalence of resistance to critically important antibiotics was low (except for erythromycin), and no glycopeptide-resistant isolates were identified. Two daptomycin-resistant E. faecalis ST58 and ST93 were found. Linezolid-resistant strains of E. faecalis (n = 3) and E. faecium (n = 1) were detected. Moreover, oxazolidinone resistance determinants optrA (n = 8) and poxtA (n = 2) were found in E. faecalis (ST16, ST58, ST207, ST474, ST1178) and E. faecium (ST22, ST2138). Multiple variants of optrA were found in different genetic contexts, either in the chromosome or plasmids. We highlight the importance of animals as reservoirs of resistance genes to critically important antibiotics.

Fattening Pigs Are a Reservoir of Florfenicol-Resistant Enterococci Harboring Oxazolidinone Resistance Genes

ABSTRACT

The use of florfenicol in farm animals may select enterococci that carry resistance genes that confer resistance to linezolid, a critically important oxazolidinone antibiotic used in human medicine. This cross-sectional study aimed to assess the occurrence of oxazolidinone resistance genes in florfenicol-resistant enterococci from fattening pigs in Switzerland and to characterize a subset of the isolates using whole genome sequencing. A total of 31 florfenicol-resistant enterococcal isolates were obtained from 27 (5%) of 565 cecal samples of fattening pigs from seven (11%) of 62 farms. Screening by PCR revealed the presence of cfr-poxtA in 1 of 31, optrA in 15 of 31, and poxtA in 15 of 31 enterococcal isolates. One randomly selected isolate per PCR-positive Enterococcus species and positive farm was selected for further analysis (n = 10). In nine of the 10 isolates, the presence of oxazolidinone resistance genes did not result in phenotypic resistance. Whole genome sequencing analysis showed the presence of E. faecalis (n = 1), E. faecium (n = 1), and E. hirae (n = 1), harboring optrA18, optrA7, and a new optrA allele, respectively. E. durans (n = 1), E. faecium (n = 4), and E. hirae (n = 1) carried the wild-type poxtA, and E. faecalis (n = 1) coharbored cfr(D) and poxtA2. Except for optrA7, all oxazolidinone resistance genes were found on plasmids. Multilocus sequence typing analysis identified E. faecalis ST19 and ST376, E. faecium ST80 belonging to hospital-adapted clade A1, and E. faecium ST21, ST55, ST269, and ST416 belonging to clade A2, which represents human commensals and animal strains. The occurrence of cfr(D), optrA, and poxtA in various porcine Enterococcus spp. demonstrates the spread of oxazolidinone resistance genes among enterococci from fattening pigs in Switzerland. The presence in one sample of poxtA-carrying E. faecium ST80 emphasizes the potential risk to human health through dissemination of strains carrying oxazolidinone resistance genes into the food chain.

HIGHLIGHTS

Emergence of optrA-Mediated Linezolid Resistance in Enterococcus faecium: A Molecular Investigation in a Tertiary Hospital of Southwest China from 2014-2018

Purpose

To investigate the potential mechanism and molecular characteristics of linezolid-non-sensitive Enterococcus faecium from a tertiary hospital in southwest China and characterize the relevant plasmids.

Patients and Methods

Linezolid-non-sensitive Enterococcus faecium (LNSEFM) isolates collected from January 2014 to December 2018 were screened for resistant genes 23s rRNA, rplC, rplD, rplV, optrA, cfr, poxtA, by PCR. Molecular epidemiological analysis was performed by multilocus sequence typing (MLST). The optrA-and-poxtA co-harboring strain EFM_7150 was subjected to the whole genome sequencing (WGS) by Illumina HiSeq and Oxford Nanopore MinION.

Results

A total of 15 LNSEFM with linezolid MICs ranging from 4 to 16 mg/L were identified. About 66.7% (10/15) of isolates were linezolid-resistant. About 46.7% (7/15) of strains were positive for optrA. Two types of optrA variants (P and EYDNDM) were identified. About 13.3% (2/15) of isolates had poxtA. 1 harbored a L22 protein alteration (Ser77Thr). One isolate coharbored optrA (EYDNDM variant) and poxtA. There was no mutation in the gene that encoded the ribosomal protein L3/L4 or the domain V of 23S rRNA. No cfr gene was detected. Based on WGS data, optrA was associated with Tn558 inserted to radC gene and poxtA was flanked by IS1216E.

Conclusion

OptrA is primary mechanism in linezolid-resistant Enterococcus faecium. This is the first report ofoptrA variants P and EYDNDM identified in Enterococcus faecium and optrA-and-poxtA co-harboring Enterococcus faecium clinically in southwest China. Besides, Tn558 and IS1216Es may play an important role in the dissemination of optrA and poxtA, respectively. The findings revealed the potential threat to nosocomial infection by optrA and coexistence of optrA and poxtA in Enterococcus faecium. Thus, clinical surveillance of linezolid-resistant Enterococcus is urgently needed.

Mobile Oxazolidinone Resistance Genes in Gram-Positive and Gram-Negative Bacteria

Seven mobile oxazolidinone resistance genes, including cfr, cfr(B), cfr(C), cfr(D), cfr(E), optrA, and poxtA, have been identified to date. The cfr genes code for 23S rRNA methylases, which confer a multiresistance phenotype that includes resistance to phenicols, lincosamides, oxazolidinones, pleuromutilins, and streptogramin A compounds. The optrA and poxtA genes code for ABC-F proteins that protect the bacterial ribosomes from the inhibitory effects of oxazolidinones. The optrA gene confers resistance to oxazolidinones and phenicols, while the poxtA gene confers elevated MICs or resistance to oxazolidinones, phenicols, and tetracycline. These oxazolidinone resistance genes are most frequently found on plasmids, but they are also located on transposons, integrative and conjugative elements (ICEs), genomic islands, and prophages. In these mobile genetic elements (MGEs), insertion sequences (IS) most often flanked the cfr, optrA, and poxtA genes and were able to generate translocatable units (TUs) that comprise the oxazolidinone resistance genes and occasionally also other genes. MGEs and TUs play an important role in the dissemination of oxazolidinone resistance genes across strain, species, and genus boundaries. Most frequently, these MGEs also harbor genes that mediate resistance not only to antimicrobial agents of other classes, but also to metals and biocides. Direct selection pressure by the use of antimicrobial agents to which the oxazolidinone resistance genes confer resistance, but also indirect selection pressure by the use of antimicrobial agents, metals, or biocides (the respective resistance genes against which are colocated on cfr-, optrA-, or poxtA-carrying MGEs) may play a role in the coselection and persistence of oxazolidinone resistance genes.

From farm to fork: identical clones and Tn6674-like elements in linezolid-resistant Enterococcus faecalis from food-producing animals and retail meat

OBJECTIVES

Increasing numbers of linezolid-resistant Enterococcus carrying optrA are being reported across different niches worldwide. We aimed to characterize the first optrA-carrying Enterococcus faecalis obtained from food-producing animals and retail meat samples in Tunisia.

METHODS

Seven optrA-carrying E. faecalis obtained from chicken faeces (n=3, August 2017) and retail chicken meat (n=4, August 2017) in Tunisia were analysed. Antimicrobial susceptibility was determined by disc diffusion, broth microdilution and Etest against 13 antibiotics, linezolid and tedizolid, respectively (EUCAST/CLSI). optrA stability (∼600 bacterial generations), transfer (filter mating) and location (S1-PFGE/hybridization) were characterized. WGS (Illumina-HiSeq) was done for four representatives that were analysed through in silico and genomic mapping tools.

RESULTS

Four MDR clones carrying different virulence genes were identified in chicken faeces (ST476) and retail meat (the same ST476 clone plus ST21 and ST859) samples. MICs of linezolid and tedizolid were stably maintained at 8 and 1-2 mg/L, respectively. optrA was located in the same transferable chromosomal Tn6674-like element in ST476 and ST21 clones, similar to isolates from pigs in Malaysia and humans in China. ST859 carried a non-conjugative plasmid of ∼40 kb with an impB-fexA-optrA segment, similar to plasmids from pigs and humans in China.

CONCLUSIONS

The same chromosomal and transferable Tn6674-like element was identified in different E. faecalis clones from humans and animals. The finding of retail meat contaminated with the same linezolid-resistant E. faecalis strain obtained from a food-producing animal highlights the potential role of the food chain in the worrisome dissemination of optrA that can be stably maintained without selective pressure over generations.

Excellent performance of CHROMagarTM LIN-R to selectively screen for linezolid-resistant enterococci and staphylococci

The increasing number of nosocomial pathogens with resistances against last resort antibiotics like linezolid leads to a pressing need for the reliable detection of these drug-resistant bacteria. National guidelines on infection prevention, e.g., in Germany, have already recommend screening for linezolid-resistant bacteria, although a corresponding screening agar medium has not been provided. In this study we analyzed the performance and reliability of a commercial, chromogenic linezolid screening agar. The medium was capable to predict more than a hundred linezolid-resistant isolates of E. faecium, E. faecalis, S. aureus, S. epidermidis, and S. hominis with excellent sensitivity and specificity. All isolates were collected at the National Reference Centre between 2010 and 2020.

Emergence of optrA-Mediated Linezolid-Nonsusceptible Enterococcus faecalis in a Tertiary Care Hospital

This study investigated resistance mechanisms and epidemiology of emerging linezolid-nonsusceptible Enterococcus faecalis (LNSEF) in a tertiary care hospital. LNSEF isolated from clinical samples were collected from November 2017 to June 2019. The isolates were investigated for linezolid resistance and the associated molecular mechanisms, including mutations of 23S rRNA domain V and acquisition of the cfr or optrA resistance gene. We used pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing for the molecular typing of the isolates. Among 4,318 E. faecalis isolates, 10 (0.23%) were linezolid-nonsusceptible. All LNSEF isolates were optrA-positive and cfr-negative. Of these isolates, five were sequence type (ST) 476, two ST585, one ST16, one ST16-like, and one ST480. Six LNSEF isolates obtained in the first year clustered to three types in the PFGE analysis: two ST476 isolates of type A, two ST585 isolates of type B, and two ST16 or ST16-like isolates of type C. Seven cases were of community-onset and three were hospital acquired, but total of eight were healthcare-associated including five community-onset. None of the patients had a history of linezolid treatment, and in one patient, we detected linezolid-susceptible E. faecalis one month before LNSEF detection. In conclusion, heterogenous clones of optrA-positive LNSEF emerged in the hospital mainly via community-onset.

Detection of Oxazolidinone Resistance Genes and Characterization of Genetic Environments in Enterococci of Swine Origin, Italy

One hundred forty-five florfenicol-resistant enterococci, isolated from swine fecal samples collected from 76 pig farms, were investigated for the presence of optrA, cfr, and poxtA genes by PCR. Thirty florfenicol-resistant Enterococcus isolates had at least one linezolid resistance gene. optrA was found to be the most widespread linezolid resistance gene (23/30), while cfr and poxtA were detected in 6/30 and 7/30 enterococcal isolates, respectively. WGS analysis also showed the presence of the cfr(D) gene in Enterococcus faecalis (n = 2 isolates) and in Enterococcus avium (n = 1 isolate). The linezolid resistance genes hybridized both on chromosome and plasmids ranging from ~25 to ~240 kb. Twelve isolates were able to transfer linezolid resistance genes to enterococci recipient. WGS analysis displayed a great variability of optrA genetic contexts identical or related to transposons (Tn6628 and Tn6674), plasmids (pE035 and pWo27-9), and chromosomal regions. cfr environments showed identities with Tn6644-like transposon and a region from p12-2300 plasmid; cfr(D) genetic contexts were related to the corresponding region of the plasmid 4 of Enterococcus faecium E8014; poxtA was always found on Tn6657. Circular forms were obtained only for optrA- and poxtA-carrying genetic contexts. Clonality analysis revealed the presence of E. faecalis (ST16, ST27, ST476, and ST585) and E. faecium (ST21) clones previously isolated from humans. These results demonstrate a dissemination of linezolid resistance genes in enterococci of swine origin in Central Italy and confirm the spread of linezolid resistance in animal settings.

Molecular characteristics of optrA-carrying Enterococcus faecalis from chicken meat in South Korea

The purpose of this study was to identify the genetic environment of optrA gene in linezolid (LZD)-resistant Enterococcus faecalis from chicken meat and to describe the probable mechanism of dissemination of the optrA gene through plasmid or chromosomal integration. Whole genome sequencing and analysis revealed that all 3 E. faecalis isolates confirmed as LZD- and chloramphenicol-resistant carried fexA adjacent to the optrA gene as well as a variety of resistance genes for macrolides, tetracyclines, and aminoglycosides, simultaneously. But, the other genes conferring LZD resistance, cfr and poxtA, were not detected in those strains. Two isolates harboring the optrA gene in their chromosomal DNA showed >99% similarity in arrangement to the transposon Tn6674 and the transposase genes, tnpA, tnpB, and tnpC and were located in the first open reading frame for transposase. One isolate harboring an optrA-carrying plasmid also showed >99% similarity with the previously reported pE439 plasmid but had 2 amino acid changes (Thr96Lys and Tyr160Asp) and a higher minimum inhibitory concentration against LZD of 16 mg/L than that of pE439 (8 mg/L). Mobile genetic elements such as transposons or plasmids flanking the optrA gene conduct a crucial role in the dissemination of antimicrobial resistance genes. Further investigations are required to identify the way by which optrA is integrated into chromosomal DNA and plasmids.

[National surveillance of clinical isolates of Enterococcus faecalis resistant to linezolid carrying the optrA gene in Colombia, 2014-2019]

OBJECTIVE

To describe the epidemiological, phenotypical and genetic characteristics of clinical isolates carrying the optrA gene identified in antimicrobial resistance surveillance by the laboratory of the National Institute of Health of Colombia.

METHODS

Between October 2014 and February 2019, 25 isolates of Enterococcus spp. resistant to linezolid were received. Antimicrobial identification and sensitivity were determined using Vitek 2 and the minimum inhibitory concentration (MIC) to linezolid was established with E-test. The optrA gene was detected by PCR, and the genetic diversity of optrA-positive isolates was tested with Diversilab®. Six isolates were selected to perform whole genome sequencing.

RESULTS

The optrA gene was confirmed in 23/25 isolates of E. faecalis from seven departments in Colombia. The isolates presented a MIC to linezolid between 8 and >256µg/mL. Typing by Diversilab® showed a wide genetic variability. All the isolates analyzed by whole genome sequencing showed the resistance genes fexA, ermB, lsaA, tet(M), tet(L) and dfrG in addition to optrA and were negative for other mechanisms of resistance to linezolid. Three type sequences and three optrA variants were identified: ST16 (optrA-2), ST476 (optrA-5) and ST618 (optrA-6). The genetic environment of the optrA-2 (ST16) isolates presented the impB, fex, optrA segment, associated with plasmid, while in two isolates (optrA-6 and optrA-5) the transferable chromosomal element Tn6674-like was found.

CONCLUSION

OptrA-positive clinical isolates present a high genetic diversity, with different optrA clones and variants related to two types of structures and different mobile genetic elements.

Drug Resistance Determinants in Clinical Isolates of Enterococcus faecalis in Bangladesh: Identification of Oxazolidinone Resistance Gene optrA in ST59 and ST902 Lineages

Enterococcus faecalis is one of the major causes of urinary tract infection, showing acquired resistance to various classes of antimicrobials. The objective of this study was to determine the prevalence of drug resistance and its genetic determinants for E. faecalis clinical isolates in north-central Bangladesh. Among a total of 210 E. faecalis isolates, isolated from urine, the resistance rates to erythromycin, levofloxacin, and gentamicin (high level) were 85.2, 45.7, and 11.4%, respectively, while no isolates were resistant to ampicillin, vancomycin and teicoplanin. The most prevalent resistance gene was erm(B) (97%), and any of the four genes encoding aminoglycoside modifying enzyme (AME) were detected in 99 isolates (47%). The AME gene aac(6′)-Ie-aph(2”)-Ia was detected in 46 isolates (21.9%) and was diverse in terms of IS256-flanking patterns, which were associated with resistance level to gentamicin. Tetracycline resistance was ascribable to tet(M) (61%) and tet(L) (38%), and mutations in the quinolone resistance-determining region of both GyrA and ParC were identified in 44% of isolates. Five isolates (2.4%) exhibited non-susceptibility to linezolide (MIC, 4 μg/mL), and harbored the oxazolidinone resistance gene optrA, which was located in a novel genetic cluster containing the phenicol exporter gene fexA. The optrA-positive isolates belonged to ST59, ST902, and ST917 (CC59), while common lineages of other multiple drug-resistant isolates were ST6, ST28, CC16, and CC116. The present study first revealed the prevalence of drug resistance determinants of E. faecalis and their genetic profiles in Bangladesh.

Mechanisms of Linezolid Resistance Among Enterococci of Clinical Origin in Spain-Detection of optrA- and cfr(D)-Carrying E. faecalis

The mechanisms of linezolid resistance among 13 E. faecalis and 6 E. faecium isolates, recovered from six Spanish hospitals during 2017–2018, were investigated. The presence of acquired linezolid resistance genes and mutations in 23S rDNA and in genes encoding for ribosomal proteins was analyzed by PCR and amplicon sequencing. Moreover, the susceptibility to 18 antimicrobial agents was investigated, and the respective molecular background was elucidated by PCR-amplicon sequencing and whole genome sequencing. The transferability of the linezolid resistance genes was evaluated by filter-mating experiments. The optrA gene was detected in all 13 E. faecalis isolates; and one optrA-positive isolate also carried the recently described cfr(D) gene. Moreover, one E. faecalis isolate displayed the nucleotide mutation G2576T in the 23S rDNA. This mutation was also present in all six E. faecium isolates. All linezolid-resistant enterococci showed a multiresistance phenotype and harbored several antimicrobial resistance genes, as well as many virulence determinants. The fexA gene was located upstream of the optrA gene in 12 of the E. faecalis isolates. Moreover, an erm(A)-like gene was located downstream of optrA in two isolates recovered from the same hospital. The optrA gene was transferable in all but one E. faecalis isolates, in all cases along with the fexA gene. The cfr(D) gene was not transferable. The presence of optrA and mutations in the 23S rDNA are the main mechanisms of linezolid resistance among E. faecalis and E. faecium, respectively. We report the first description of the cfr(D) gene in E. faecalis. The presence of the optrA and cfr(D) genes in Spanish hospitals is a public health concern.

Dispersal of linezolid-resistant enterococci carrying poxtA or optrA in retail meat and food-producing animals from Tunisia

OBJECTIVES

The epidemiology of Enterococcus resistant to priority antibiotics including linezolid has mainly been investigated in developed countries and especially in hospitals. We aimed to evaluate the contribution of different non-human reservoirs for the burden of MDR enterococci in Tunisia, where scarce data are available.

METHODS

Samples (n = 287) were collected from urban wastewater (n = 57), retail meat (n = 29; poultry/bovine/ovine), milk (n = 89; bovine/ovine), farm animal faeces (n = 80; poultry/bovine/ovine) and pets (n = 32; rabbit/dogs/cats/birds) in different Tunisian regions (2014-17). They were plated onto Slanetz-Bartley agar after pre-enrichment without antibiotics. Standard methods were used for bacterial identification and characterization of antibiotic resistance and virulence genes (PCR), antibiotic susceptibility testing (disc diffusion/broth microdilution; EUCAST/CLSI) and clonality (SmaI-PFGE/MLST).

RESULTS

All samples carried Enterococcus (n = 377 isolates) resistant to antibiotics considered to be critical or highly important by WHO. Even without antibiotic selection, 38% of Enterococcus faecalis (Efs) and 22% of Enterococcus faecium (Efm) were identified as MDR. Linezolid-resistant isolates (5%; MIC = 8 mg/L) comprised six poxtA-carrying Efm (cow milk), seven optrA-carrying Efs (chicken faeces/meat) and five Efm lacking cfr/optrA/poxtA (poultry/bovine/ovine/wastewater). Clinically relevant Efm clones (clade A1) were identified in animal/meat sources. Ampicillin resistance (1%) was confined to ST18/ST78-like MDR Efm clones from bovine meat/milk samples carrying relevant virulence markers (e.g. ptsD/IS16).

CONCLUSIONS

This study provides evidence of the contribution of livestock and foodstuffs to the dispersal of acquired linezolid resistance genes including poxtA and optrA. We report the first poxtA-carrying Efm in Tunisia, and for the first time in bovine samples, stressing the urgent need for alternative measures to counteract the spread of linezolid-resistant enterococci globally.

Comparative genomics of global optrA-carrying Enterococcus faecalis uncovers a common chromosomal hotspot for optrA acquisition within a diversity of core and accessory genomes

Linezolid-resistant Enterococcus faecalis (LREfs) carrying optrA are increasingly reported globally from multiple sources, but we lack a comprehensive analysis of human and animal optrA-LREfs strains. To assess if optrA is dispersed in isolates with varied genetic backgrounds or with common genetic features, we investigated the phylogenetic structure, genetic content [antimicrobial resistance (AMR), virulence, prophages, plasmidome] and optrA-containing platforms of 27 publicly available optrA-positive E. faecalis genomes from different hosts in seven countries. At the genome-level analysis, an in-house database with 64 virulence genes was tested for the first time. Our analysis showed a diversity of clones and adaptive gene sequences related to a wide range of genera from Firmicutes. Phylogenies of core and accessory genomes were not congruent, and at least PAI-associated and prophage genes contribute to such differences. Epidemiologically unrelated clones (ST21, ST476-like and ST489) obtained from human clinical and animal hosts in different continents over eight years (2010–2017) could be phylogenetically related (3–126 SNPs difference). optrA was located on the chromosome within a Tn6674-like element (n=10) or on medium-size plasmids (30–60 kb; n=14) belonging to main plasmid families (RepA_N/Inc18/Rep_3). In most cases, the immediate gene vicinity of optrA was generally identical in chromosomal (Tn6674) or plasmid (impB-fexA-optrA) backbones. Tn6674 was always inserted into the same ∆radC integration site and embedded in a 32 kb chromosomal platform common to strains from different origins (patients, healthy humans, and animals) in Europe, Africa, and Asia during 2012–2017. This platform is conserved among hundreds of E. faecalis genomes and proposed as a chromosomal hotspot for optrA integration. The finding of optrA in strains sharing common adaptive features and genetic backgrounds across different hosts and countries suggests the occurrence of common and independent genetic events occurring in distant regions and might explain the easy de novo generation of optrA-positive strains. It also anticipates a dramatic increase of optrA carriage and spread with a serious impact on the efficacy of linezolid for the treatment of Gram-positive infections.

Analysis of combined resistance to oxazolidinones and phenicols among bacteria from dogs fed with raw meat/vegetables and the respective food items

The gene optrA is the first gene that confers resistance to the oxazolidinone tedizolid, a last resort antimicrobial agent in human medicine. In this study we investigated the presence of optrA and the multi-resistance genes poxtA and cfr in enterococci and staphylococci from (i) pet animals known to be fed raw meat and vegetables and (ii) the respective food items. We examined 341 bacterial isolates from cats and dogs, 195 bacterial isolates from supermarket food items and only one E. faecium collected from industrial food in Beijing during 2016. Thirty-five (6.5%) of the 537 isolates, including 31/376 (8.2%) enterococci and 4/161 (2.5%) staphylococci, were positive for optrA, while all isolates were negative for poxtA and cfr. S1-nuclease pulsed-field gel electrophoresis (PFGE) and Southern blotting confirmed that optrA was located in the chromosomal DNA of 19 isolates and on a plasmid in the remaining 16 isolates. Whole genome sequencing revealed several different genetic environments of optrA in plasmid- or chromosome-borne optrA genes. PFGE, multilocus sequence typing (MLST) and/or SNP analysis demonstrated that the optrA-carrying Staphylococcus and Enterococcus isolates were genetically heterogeneous. However, in single cases, groups of related isolates were identified which might suggest a transfer of closely related optrA-positive E. faecalis isolates between food items and dogs.

Tn6674 Is a Novel Enterococcal optrA-Carrying Multiresistance Transposon of the Tn554 Family

The novel 12,932-bp nonconjugative multiresistance transposon Tn6674 was identified in the chromosomal DNA of a porcine Enterococcus faecalis strain. Tn6674 belongs to the Tn554 family of transposons. It shares the same arrangement of the transposase genes tnpA, tnpB, and tnpC with Tn554 However, in addition to the Tn554-associated resistance genes spc and erm(A), Tn6674 harbored the resistance genes fexA and optrA Circular forms of Tn6674 were detected and suggest the functional activity of this transposon.

Detection of transferable oxazolidinone resistance determinants in Enterococcus faecalis and Enterococcus faecium of swine origin in Sichuan Province, China

OBJECTIVES

The aim of this study was to detect transferable oxazolidinone resistance determinants (cfr, optrA and poxtA) in Enterococcus faecalis and Enterococcus faecium isolates of swine origin in Sichuan Province, China.

METHODS

A total of 158 enterococcal isolates (93 E. faecalis and 65 E. faecium) isolated from 25 large-scale swine farms (2016-2017) were screened for the presence of cfr, optrA and poxtA by PCR. The genetic environments of cfr, optrA and poxtA were characterised by whole-genome sequencing. Transfer of oxazolidinone resistance determinants was determined by conjugation or electrotransformation experiments.

RESULTS

The transferable oxazolidinone resistance determinants cfr, optrA and poxtA were detected in zero, six and one enterococcal isolates, respectively. The poxtA gene in one E. faecalis isolate was located on a 37 990-bp plasmid that co-harboured fexB, cat, tet(L) and tet(M) and could be conjugated to E. faecalis JH2-2. One E. faecalis isolate harboured two different OptrA variants, including one variant with a single substitution (Q219H) that has not been reported previously. Two optrA-carrying plasmids, pC25-1 (45 581bp) and pC54 (64 500bp), shared a 40 494-bp identical region containing the genetic context IS1216E-fexA-optrA-erm(A)-IS1216E that could be electrotransformed into Staphylococcus aureus. Four different chromosomal optrA gene clusters were found in five strains, in which optrA was associated with Tn554 or Tn558 inserted into the radC gene.

CONCLUSION

This study highlights the fact that mobile genetic elements, such as plasmids, IS1216E, Tn554 and Tn558, may facilitate the horizontal transmission of optrA and poxtA genes.