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

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.

Transferable linezolid resistance genes (optrA and poxtA) in enterococci derived from livestock compost at Japanese farms

OBJECTIVES

Linezolid is a last-resort antimicrobial in human clinical settings to treat multidrug-resistant Gram-positive bacterial infections. Mobile linezolid resistance genes (optrA, poxtA, and cfr) have been detected in various sources worldwide. However, the presence of linezolid-not-susceptible bacteria and mobile linezolid resistance genes in Japan remains uncertain. Therefore, we clarified the existence of linezolid-not-susceptible bacteria and mobile linezolid resistance genes in farm environments in Japan.

METHODS

Enterococci isolates from faeces compost collected from 10 pig and 11 cattle farms in Japan in 2021 were tested for antimicrobial susceptibility and possession of mobile linezolid resistance genes. Whole-genome sequencing of optrA and/or poxtA genes positive-enterococci was performed.

RESULTS

Of 103 enterococci isolates, 12 from pig farm compost were not-susceptible (2 resistant and 10 intermediate) to linezolid. These 12 isolates carried mobile linezolid resistance genes on plasmids or chromosomes (5 optrA-positive Enterococcus faecalis, 6 poxtA-positive E. hirae or E. thailandicus, and 1 optrA- and poxtA-positive E. faecium). The genetic structures of optrA- and poxA-carrying plasmids were almost identical to those reported in other countries. These plasmids were capable of transferring among E. faecium and E. faecalis strains. The optrA- and poxtA-positive E. faecium belonged to ST324 (clade A2), a high-risk multidrug-resistant clone. The E. faecalis carrying optrA gene on its chromosome was identified as ST593.

CONCLUSIONS

Although linezolid is not used in livestock, linezolid-not-susceptible enterococci could be indirectly selected by frequently used antimicrobials, such as phenicols. Moreover, various enterococci species derived from livestock compost may serve as reservoirs of linezolid resistance genes carried on globally disseminated plasmids and multidrug-resistant high-risk clones.

phytools 2.0: an updated R ecosystem for phylogenetic comparative methods (and other things)

Phylogenetic comparative methods comprise the general endeavor of using an estimated phylogenetic tree (or set of trees) to make secondary inferences: about trait evolution, diversification dynamics, biogeography, community ecology, and a wide range of other phenomena or processes. Over the past ten years or so, the phytools R package has grown to become an important research tool for phylogenetic comparative analysis. phytools is a diverse contributed R library now consisting of hundreds of different functions covering a variety of methods and purposes in phylogenetic biology. As of the time of writing, phytools included functionality for fitting models of trait evolution, for reconstructing ancestral states, for studying diversification on trees, and for visualizing phylogenies, comparative data, and fitted models, as well numerous other tasks related to phylogenetic biology. Here, I describe some significant features of and recent updates to phytools, while also illustrating several popular workflows of the phytools computational software.

An extraction-free method for rapid detection of CYP2C19 * 2/3/17 polymorphisms in one tube using melting curve analysis

Drug-metabolizing enzymes play an important role in the metabolism of drugs in vivo. Their activity is an important factor affecting the rate of drug metabolism, which directly determines the intensity and persistence of drug action. Patients taking medication can be divided into different metabolic types through detection of CYP2C19 drug-metabolizing enzyme gene polymorphisms, which can then be used for medication guidance for clopidogrel. Here, we describe a detection method based on real-time polymerase chain reaction (PCR). This method uses multicolor melting curve analysis to accurately identify different mutation sites and genotypes of CYP2C19 * 2, CYP2C19 * 3, and CYP2C19 * 17. The detection limit of plasmid samples was 1 copies μL-1 ; that of genomic samples was 0.1 ng μL-1 . The system can detect nine types of CYP2C19 * 2/3/17 at three sites in one tube, quickly achieving detection within 1 h. Combined with the sample release agent, sample extraction was completed in 5 s, achieving rapid diagnosis without extraction for timely diagnosis and treatment. Furthermore, the system is not limited to blood samples and can also be applied to oropharyngeal and saliva samples, increasing sampling diversity and convenience. When using clinical blood samples (n = 93), the detection system we established was able to quickly and accurately identify different genotypes, and the accuracy and effectiveness of the detection were confirmed by Sanger sequencing. Due to its accuracy, rapidity, simple operation, and low cost, detection technology based on real-time polymerase amplification combined with melting curve analysis is expected to become a powerful tool for detecting and guiding clopidogrel use in countries with limited resources.

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.

Antibiotic Resistance among Gastrointestinal Bacteria in Broilers: A Review Focused on Enterococcus spp. and Escherichia coli

Chickens can acquire bacteria at different stages, and bacterial diversity can occur due to production practices, diet, and environment. The changes in consumer trends have led to increased animal production, and chicken meat is one of the most consumed meats. To ensure high levels of production, antimicrobials have been used in livestock for therapeutic purposes, disease prevention, and growth promotion, contributing to the development of antimicrobial resistance across the resident microbiota. Enterococcus spp. and Escherichia coli are normal inhabitants of the gastrointestinal microbiota of chickens that can develop strains capable of causing a wide range of diseases, i.e., opportunistic pathogens. Enterococcus spp. isolated from broilers have shown resistance to at least seven classes of antibiotics, while E. coli have shown resistance to at least four. Furthermore, some clonal lineages, such as ST16, ST194, and ST195 in Enterococcus spp. and ST117 in E. coli, have been identified in humans and animals. These data suggest that consuming contaminated animal-source food, direct contact with animals, or environmental exposure can lead to the transmission of antimicrobial-resistant bacteria. Therefore, this review focused on Enterococcus spp. and E. coli from the broiler industry to better understand how antibiotic-resistant strains have emerged, which antibiotic-resistant genes are most common, what clonal lineages are shared between broilers and humans, and their impact through a One Health perspective.

Emergence and Genomic Characterization of the First Reported optrA-Carrying Linezolid-Resistant Enterococci Isolated from Retail Broiler Meat in the United Arab Emirates

The foodborne transfer of resistant genes from enterococci to humans and their tolerance to several commonly used antimicrobials are of growing concern worldwide. Linezolid is a last-line drug for managing complicated illnesses resulting from multidrug-resistant Gram-positive bacteria. The optrA gene has been reported in enterococci as one of the acquired linezolid resistance mechanisms. The present study uses whole-genome sequencing analysis to characterize the first reported isolates of linezolid-resistant E. faecium (n = 6) and E. faecalis (n = 10) harboring the optrA gene isolated from samples of supermarket broiler meat (n = 165) in the United Arab Emirates (UAE). The sequenced genomes were used to appraise the study isolates’ genetic relatedness, antimicrobial resistance determinants, and virulence traits. All 16 isolates carrying the optrA gene demonstrated multidrug-resistance profiles. Genome-based relatedness classified the isolates into five clusters that were independent of the isolate sources. The most frequently known genotype among the isolates was the sequence type ST476 among E. faecalis (50% (5/10)). The study isolates revealed five novel sequence types. Antimicrobial resistance genes (ranging from 5 to 13) were found among all isolates that conferred resistance against 6 to 11 different classes of antimicrobials. Sixteen different virulence genes were found distributed across the optrA-carrying E. faecalis isolates. The virulence genes in E. faecalis included genes encoding invasion, cell adhesion, sex pheromones, aggregation, toxins production, the formation of biofilms, immunity, antiphagocytic activity, proteases, and the production of cytolysin. This study presented the first description and in-depth genomic characterization of the optrA-gene-carrying linezolid-resistant enterococci from retail broiler meat in the UAE and the Middle East. Our results call for further monitoring of the emergence of linezolid resistance at the retail and farm levels. These findings elaborate on the importance of adopting a One Health surveillance approach involving enterococci as a prospective bacterial indicator for antimicrobial resistance spread at the human–food interface.

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

Sentinel Surveillance Reveals Emerging Daptomycin-Resistant ST736 Enterococcus faecium and Multiple Mechanisms of Linezolid Resistance in Enterococci in the United States

Enterococcus faecalis and faecium with resistance to daptomycin and/or linezolid are emerging globally. We present the genomic characterization of daptomycin- and linezolid-resistant E. faecalis and E. faecium surveillance isolates from the United States, 2013–2016. Daptomycin resistance was low among E. faecalis (2/364, 0.5%) and E. faecium (17/344, 5%). The majority (71%, 12/17) of daptomycin-resistant E. faecium isolates belonged to the emerging ST736 clone and contained mutations in liaFSR and cls previously associated with resistance. However, 1/2 E. faecalis and 3/17 E. faecium did not contain these mutations previously associated with daptomycin resistance. Linezolid resistance was rare among E. faecalis (1/364, 0.3%) and E. faecium (2/344, 0.6%). These two E. faecium isolates, one of which was also resistant to daptomycin and vancomycin, contained the 23S rRNA nucleotide mutation (G2576T) associated with linezolid resistance. Long-read sequencing revealed the linezolid-resistant E. faecalis isolate contained chromosomal- and plasmid-encoded copies of optrA. The chromosomal optrA was located on the recently described Tn6674 multiresistance transposon. The second copy of optrA was encoded on an ∼65 kb mosaic plasmid, with component regions sharing high sequence identity to optrA-encoding multiresistance plasmids of animal origin. The optrA-encoding plasmid contained open reading frames predicted to encode proteins associated with a pheromone-responsive plasmid transfer system, and filter mating experiments confirmed the plasmid was conjugative. Continued surveillance of enterococci is necessary to assess the prevalence and trends of daptomycin and linezolid resistance in the United States, characterize resistance mechanisms and how they transfer, and monitor for emerging sequence types associated with resistance.

Presence of optrA-mediated linezolid resistance in multiple lineages and plasmids of Enterococcus faecalis revealed by long read sequencing

Transferable linezolid resistance due to optrA, poxtA, cfr and cfr-like genes is increasingly detected in enterococci associated with animals and humans globally. We aimed to characterize the genetic environment of optrA in linezolid-resistant Enterococcus faecalis isolates from Scotland. Six linezolid-resistant E. faecalis isolated from urogenital samples were confirmed to carry the optrA gene by PCR. Short read (Illumina) sequencing showed the isolates were genetically distinct (>13900 core SNPs) and belonged to different MLST sequence types. Plasmid contents were examined using hybrid assembly of short and long read (Oxford Nanopore MinION) sequencing technologies. The optrA gene was located on distinct plasmids in each isolate, suggesting that transfer of a single plasmid did not contribute to optrA dissemination in this collection. pTM6294-2, BX5936-1 and pWE0438-1 were similar to optrA-positive plasmids from China and Japan, while the remaining three plasmids had limited similarity to other published examples. We identified the novel Tn6993 transposon in pWE0254-1 carrying linezolid (optrA), macrolide (ermB) and spectinomycin [ANT(9)-Ia] resistance genes. OptrA amino acid sequences differed by 0–20 residues. We report multiple variants of optrA on distinct plasmids in diverse strains of E. faecalis. It is important to identify the selection pressures driving the emergence and maintenance of resistance against linezolid to retain the clinical utility of this antibiotic.

Prevalence and Characteristics of Phenicol-Oxazolidinone Resistance Genes in Enterococcus Faecalis and Enterococcus Faecium Isolated from Food-Producing Animals and Meat in Korea

The use of phenicol antibiotics in animals has increased. In recent years, it has been reported that the transferable gene mediates phenicol-oxazolidinone resistance. This study analyzed the prevalence and characteristics of phenicol-oxazolidinone resistance genes in Enterococcus faecalis and Enterococcus faecium isolated from food-producing animals and meat in Korea in 2018. Furthermore, for the first time, we reported the genome sequence of E. faecalis strain, which possesses the phenicol-oxazolidinone resistance gene on both the chromosome and plasmid. Among the 327 isolates, optrA, poxtA, and fexA genes were found in 15 (4.6%), 8 (2.5%), and 17 isolates (5.2%), respectively. Twenty E. faecalis strains carrying resistance genes belonged to eight sequence types (STs), and transferability was found in 17 isolates. The genome sequences revealed that resistant genes were present in the chromosome or plasmid, or both. In strains EFS17 and EFS108, optrA was located downstream of the ermA and ant(9)-1 genes. The strains EFS36 and EFS108 harboring poxtA-encoding plasmid cocarried fexA and cfr(D). These islands also contained IS1216E or the transposon Tn554, enabling the horizontal transfer of the phenicol-oxazolidinone resistance with other antimicrobial-resistant genes. Our results suggest that it is necessary to promote the prudent use of antibiotics through continuous monitoring and reevaluation.

Linezolid- and Multidrug-Resistant Enterococci in Raw Commercial Dog Food, Europe, 2019-2020

We describe enterococci in raw-frozen dog food commercialized in Europe as a source of genes encoding resistance to the antibiotic drug linezolid and of strains and plasmids enriched in antibiotic-resistance and virulence genes in hospitalized patients. Whole-genome sequencing was fundamental to linking isolates from dog food to human cases across Europe.

Genetic context of optrA and poxtA in florfenicol-resistant enterococci isolated from flowing surface water in Switzerland

Linezolid is an important last-resort antibiotic for the treatment of multi-drug resistant enterococci. The aim of this study was to further characterize the genetic context of optrA and poxtA in ten florfenicol-resistant enterococci isolated from flowing surface water. In most genomes, optrA and poxtA were embedded in transposition units integrated into plasmids or into the chromosomal radC. For the first time a chromosomally integrated optrA in an Enterococcus raffinosus isolate is described.

The Role of Whole Genome Sequencing in the Surveillance of Antimicrobial Resistant Enterococcus spp.: A Scoping Review

Enterococcus spp. have arisen as important nosocomial pathogens and are ubiquitous in the gastrointestinal tracts of animals and the environment. They carry many intrinsic and acquired antimicrobial resistance genes. Because of this, surveillance of Enterococcus spp. has become important with whole genome sequencing emerging as the preferred method for the characterization of enterococci. A scoping review was designed to determine how the use of whole genome sequencing in the surveillance of Enterococcus spp. adds to our knowledge of antimicrobial resistance in Enterococcus spp. Scoping review design was guided by the PRISMA extension and checklist and JBI Reviewer's Guide for scoping reviews. A total of 72 articles were included in the review. Of the 72 articles included, 48.6% did not state an association with a surveillance program and 87.5% of articles identified Enterococcus faecium. The majority of articles included isolates from human clinical or screening samples. Significant findings from the articles included novel sequence types, the increasing prevalence of vancomycin-resistant enterococci in hospitals, and the importance of surveillance or screening for enterococci. The ability of enterococci to adapt and persist within a wide range of environments was also a key finding. These studies emphasize the importance of ongoing surveillance of enterococci from a One Health perspective. More studies are needed to compare the whole genome sequences of human enterococcal isolates to those from food animals, food products, the environment, and companion animals.

Genomic and phenotypic diversity of Enterococcus faecalis isolated from endophthalmitis

Enterococcus faecalis are hospital-associated opportunistic pathogens and also causative agents of post-operative endophthalmitis. Patients with enterococcal endophthalmitis often have poor visual outcomes, despite appropriate antibiotic therapy. Here we investigated the genomic and phenotypic characteristics of E. faecalis isolates collected from 13 patients treated at the University of Pittsburgh Medical Center Eye Center over 19 years. Comparative genomic analysis indicated that patients were infected with E. faecalis belonging to diverse multi-locus sequence types (STs) and resembled E. faecalis sampled from clinical, commensal, and environmental sources. We identified known E. faecalis virulence factors and antibiotic resistance genes in each genome, including genes conferring resistance to aminoglycosides, erythromycin, and tetracyclines. We assessed all isolates for their cytolysin production, biofilm formation, and antibiotic susceptibility, and observed phenotypic differences between isolates. Fluoroquinolone and cephalosporin susceptibilities were particularly variable between isolates, as were biofilm formation and cytolysin production. In addition, we found evidence of E. faecalis adaptation during recurrent endophthalmitis by identifying genetic variants that arose in sequential isolates sampled over eight months from the same patient. We identified a mutation in the DNA mismatch repair gene mutS that was associated with an increased rate of spontaneous mutation in the final isolate from the patient. Overall this study documents the genomic and phenotypic variability among E. faecalis causing endophthalmitis, as well as possible adaptive mechanisms underlying bacterial persistence during recurrent ocular infection.

Linezolid Resistance Genes in Enterococci Isolated from Sediment and Zooplankton in Two Italian Coastal Areas

Linezolid is a last-resort antibiotic for the treatment of severe infections caused by multidrug-resistant Gram-positive organisms; although linezolid resistance remains uncommon, the number of linezolid-resistant enterococci has increased in recent years due to worldwide spread of acquired resistance genes (cfr, optrA, and poxtA) in clinical, animal, and environmental settings. In this study, we investigated the occurrence of linezolid-resistant enterococci in marine samples from two coastal areas in Italy. Isolates grown on florfenicol-supplemented Slanetz-Bartley agar plates were investigated for their carriage of optrA, poxtA, and cfr genes; optrA was found in one Enterococcus faecalis isolate, poxtA was found in three Enterococcus faecium isolates and two Enterococcus hirae isolates, and cfr was not found. Two of the three poxtA-carrying E. faecium isolates and the two E. hirae isolates showed related pulsed-field gel electrophoresis (PFGE) profiles. Two E. faecium isolates belonged to the new sequence type 1710, which clustered in clonal complex 94, encompassing nosocomial strains. S1 PFGE/hybridization assays showed a double (chromosome and plasmid) location of poxtA and a plasmid location of optrA Whole-genome sequencing revealed that poxtA was contained in a Tn6657-like element carried by two plasmids (pEfm-EF3 and pEh-GE2) of similar size, found in different species, and that poxtA was flanked by two copies of IS1216 in both plasmids. In mating experiments, all but one strain (E. faecalis EN3) were able to transfer the poxtA gene to E. faecium 64/3. The occurrence of linezolid resistance genes in enterococci from marine samples is of great concern and highlights the need to improve practices aimed at limiting the transmission of linezolid-resistant strains to humans from environmental reservoirs.IMPORTANCE Linezolid is one of the few antimicrobials available to treat severe infections due to drug-resistant Gram-positive bacteria; therefore, the emergence of linezolid-resistant enterococci carrying transferable resistance determinants is of great concern for public health. Linezolid resistance genes (cfr, optrA, and poxtA), often plasmid located, can be transmitted via horizontal gene transfer and have the potential to spread globally. This study highlights the detection of enterococci carrying linezolid resistance genes from sediment and zooplankton samples from two coastal urban areas in Italy. The presence of clinically relevant resistant bacteria, such as linezolid-resistant enterococci, in marine environments could reflect their spillover from human and/or animal reservoirs and could indicate that coastal seawaters also might represent a source of these resistance genes.

Genomic Insights Into Last-Line Antimicrobial Resistance in Multidrug-Resistant Staphylococcus and Vancomycin-Resistant Enterococcus

Multidrug-resistant Staphylococcus and vancomycin-resistant Enterococcus (VRE) are important human pathogens that are resistant to most clinical antibiotics. Treatment options are limited and often require the use of 'last-line' antimicrobials such as linezolid, daptomycin, and in the case of Staphylococcus, also vancomycin. The emergence of resistance to these last-line antimicrobial agents is therefore of considerable clinical concern. This mini-review provides an overview of resistance to last-line antimicrobial agents in Staphylococcus and VRE, with a particular focus on how genomics has provided critical insights into the emergence of resistant clones, the molecular mechanisms of resistance, and the importance of mobile genetic elements in the global spread of resistance to linezolid.

Spread of Linezolid-Resistant Enterococcus spp. in Human Clinical Isolates in the Czech Republic

The aim of this study was to map and investigate linezolid resistance mechanisms in linezolid-resistant enterococci in the Czech Republic from 2009 to 2019. Altogether, 1442 isolates of Enterococcus faecium and Enterococcus faecalis were examined in the National Reference Laboratory for Antibiotics. Among them, 8% of isolates (n = 115) were resistant to linezolid (E. faecium/n = 106, E. faecalis/n = 9). Only three strains of E. faecium were resistant to tigecycline, 72.6% of isolates were resistant to vancomycin. One isolate of E. faecium harbored the cfr gene. The majority (87%, n = 11) of E. faecium strains were resistant to linezolid because of the mutation G2576T in the domain V of the 23S rRNA. This mutation was detected also in two strains of E. faecalis. The presence of the optrA gene was the dominant mechanism of linezolid resistance in E. faecalis isolates. None of enterococci contained cfrB, poxtA genes, or any amino acid mutation in genes encoding ribosomal proteins. No mechanism of resistance was identified in 4 out of 106 E. faecium linezolid resistant isolates in this study. Seventeen sequence types (STs) including four novel STs were identified in this work. Clonal complex CC17 was found in all E. faecium isolates.

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.