Co-production of Tet(X) and MCR-1, two resistance enzymes by a single plasmid

Structural insights into the advancements of mobile colistin resistance enzymes

The plasmid-encoded mobile colistin resistance enzyme (MCR) is challenging the clinical efficacy of colistin as a last-resort antibiotic against multidrug-resistant bacteria. This transferase catalyzes the addition of positively charged phosphoethanolamine to lipid A, and its catalytic domain in the periplasm has been elucidated. To date, there are many works on the catalytic domain and function of this enzyme class. However, the roles of unreported soluble or inter-membrane domains remain undefined, which might cause an inaccurate or even incorrect understanding of substrate recognition and binding. In this review, MCR-1 is first compared and analyzed from the perspective of the full-length alpha-fold MCR-1. Specifically, some disputed issues, especially in its architecture and catalytic mechanism are discussed independently. Meanwhile, the structure-based insights into MCRs variants, their evolutions, and the balance between colistin-resistance and survival costs, are also critically analyzed. Importantly, by comparing it with the full-length MCR-1, several potential pockets for drug design have been re-identified. Finally, recent advancements in inhibitors targeting MCR-1 are also in-depth summarized. These details offer a new perspective on MCRs and serve as a valuable foundation for drug development.

Spreading Ability of Tet(X)-Harboring Plasmid and Effect of Tetracyclines as a Selective Pressure

Tigecycline is a last-resort antimicrobial in humans. Tetracyclines are the most widely used antimicrobials in livestock. Mobile tigecycline resistance genes [tet(X)] are disseminated worldwide, and tetracycline use may have promoted the selection of tet(X) genes. Thus, the selective pressure on tet(X) genes and their plasmids in livestock must be elucidated. We performed a retrospective study to clarify the prevalence of tigecycline-resistant Escherichia coli from pigs in Thailand. Screening for tigecycline resistance was performed on 107 E. coli strains from 25 samples, and tet(X)-carrying plasmids were characterized. tet(X) genes were cloned and expressed in E. coli. Bacterial growth rate in the presence of tetracycline as a result of the presence of tet(X) genes was also evaluated. Thirty-two tet(X4)-harboring tigecycline-resistant E. coli strains were detected in 10/25 samples (40%). The tet(X4) genes were carried on various Inc-type plasmids and flanked by ISCR2. The tet(X)-carrying plasmids were transferred to E. coli and Klebsiella pneumoniae. Acquisition of tet(X) genes and their plasmids improved bacterial growth in the presence of tetracycline. In summary, tetracycline use exerts selective pressure on tet(X) genes and their various backbone plasmids; therefore, a reduced amount of tetracycline use is important to limit the spreading of tet(X) genes.

The tigecycline resistance mechanisms in Gram-negative bacilli

Tigecycline, hailed as a pivotal agent in combating multidrug-resistant bacterial infections, confronts obstacles posed by the emergence of resistance mechanisms in Gram-negative bacilli. This study explores the complex mechanisms of tigecycline resistance in Gram-negative bacilli, with a particular focus on the role of efflux pumps and drug modification in resistance. By summarizing these mechanisms, our objective is to provide a comprehensive understanding of tigecycline resistance in Gram-negative bacilli, thereby illuminating the evolving landscape of antimicrobial resistance. This review contributes to the elucidation of current existing tigecycline resistance mechanisms and provides insights into the development of effective strategies to manage the control of antimicrobial resistance in the clinical setting, as well as potential new targets for the treatment of tigecycline-resistant bacterial infections.

Emergence and Characterization of the High-Level Tigecycline Resistance Gene tet(X4) in Salmonella enterica Serovar Rissen from Food in China

The plasmid-mediated tet(X4) gene has exhibited a high-level resistance to tigecycline (TGC), which has raised concerns globally regarding antibiotic resistance. Although the widespread tet(X4) has been found widely in Escherichia coli, it is scarcely found in other Enterobacteriaceae. This study aimed to characterize a ST469 Salmonella enterica serovar Rissen (S. Rissen) isolate harboring tet(X4) from pork, which was identified and characterized via antimicrobial susceptibility testing, conjugation assays, plasmid curing testing, whole-genome sequencing, and bioinformatic analysis. Ten ST469 S. Rissen isolates of 223 Salmonella spp. isolates were isolated from food samples in China during 2021-2023. One of 10 S. Rissen isolates, SM2301, carrying tet(X4) conferred high-level resistance to TGC (minimum inhibitory concentration > 8 µg/mL). The tet(X4) could be conjugated into different recipients, including E. coli, S. enteritidis, and K. pneumoniae isolates. Plasmid curing confirmed that tet(X4) was plasmid-mediated. Genetic analysis revealed that the tet(X4) in the SM2301 isolate was located in the IncFIA(HI1)-IncHI1A-IncHI1B(R27) hybrid plasmid, and the structure of tet(X4) was abh-tet(X4)-ISCR2. To the best of our knowledge, this is the first report of a tet(X4)-positive food-derived S. Rissen isolate. The extending bacterial species of tet(X4)-bearing plasmids suggested the increasing transmission risk of the mobile TGC resistance gene tet(X4) beyond E. coli. This study highlights the emerging and evolution risk of novel resistance genes across various bacterial species. Therefore, further surveillance is warranted to monitor the prevalence of tet(X4) in Salmonella spp. and other bacterial species.

The prevalent dynamic and genetic characterization of mcr-1 encoding multidrug resistant Escherichia coli strains recovered from poultry in Hebei, China

OBJECTIVES

Colistin is known as the last resort antibiotic to treat the infections caused by multidrug resistant foodborne pathogens. The emergence and widespread dissemination of plasmid-mediated colistin resistance gene mcr-1 in the Escherichia coli (E. coli) incurs potential threat to public health. Here, we investigated the epidemiology, transmission dynamics, and genetic characterization of mcr-1 harbouring E. coli isolates from poultry originated in Hebei Province, China.

METHODS

A total of 297 faecal samples were collected from the two large poultry farms in Hebei Province, China. The samples were processed for E. coli identification by matrix-assisted laser desorption ionization-time of flight mass spectrometry and 16S rDNA sequencing. Then, the mcr-1 gene harbouring E. coli strains were identified by polymerase chain reaction and subjected to antimicrobial susceptibility testing by broth microdilution assay. The genomic characterization of the isolates was done by whole genome sequencing using the various bioinformatics tools, and multi-locus sequence typing was done by sequence analysis of the seven housekeeping genes. The conjugation experiment was done to check the transferability of mcr-1 along with the plasmid stability testing.

RESULTS

A total of six mcr-1 E. coli isolates with minimum inhibitory concentration of 4 μg/mL were identified from 297 samples (2.02%). The mcr-1 harbouring E. coli were identified as multidrug resistant and belonged to ST101 (n = 4) and ST410 (n = 2). The genetic environment of mcr-1 presented its position on IncHI2 plasmid in 4 isolates and p0111 in 2 isolates, which is a rarely reported plasmid type for mcr-1. Moreover, both type of plasmids was transferable to recipient J53, and mcr-1 was flanked by 3 mobile elements ISApl1, Tn3, and IS26 forming a novel backbone Tn3-IS26-mcr-1- pap2-ISApl1 on the p0111 plasmid. The phylogenetic analysis shared a common lineage with mcr-1 harbouring isolates from the environment, humans, and animals, which indicate its horizontal spread among the diverse sources, species, and hosts.

CONCLUSION

This study recommends the one health approach for future surveillance across multiple sources and bacterial species to adopt relevant measures and reduce global resistance crises.

Identification of novel Tet(X6)-Tet(X2) recombinant variant in Elizabethkingia meningoseptica from a bullfrog farm and downstream river in China

Introduction

The dissemination of strains producing tetracyclines monooxygenase Tet(X) from breeding farms to the natural environment poses a potential threat to public health.

Methods

Antimicrobial susceptibility testing and WGS were performed to identify resistance phenotypes and genotypes. Cloning experiments, sequence alignment, and homology modeling were used to characterize the function and formation mechanisms of the recombinant variant. The mobilization potential of Tet(X) was assessed by collinearity analysis, conjugation experiments, and phylogenetic analysis.

Results

Three tet(X)-producing Elizabethkingia meningoseptica strains were isolated from bullfrog breeding ponds, the sewage outlet, and downstream river in Zhejiang Province, China. These strains carry a novel Tet(X) variant, differing from Tet(X6) by seven residues, and possess the ability to degrade tetracyclines. Interestingly, the novel Tet(X) is a recombinant variant formed by homologous recombination of Tet(X6) and the C-terminal of Tet(X2). Further analysis revealed that Tet(X6) formed several Tet(X) variants, including Tet(X5), through homologous recombination. The novel tet(X) gene is located on a circularizable integrative and conjugative element (ICEEmeChn3), with ISwz1 participating in the recombination of its multi-drug resistance region, potentially facilitating the mobilization and recombination of tet(X) in early hosts. These three strains were clonally transmitted and shared a close genetic relationship (SNP < 62) with a clinically-sourced strain isolated from the same province.

Discussion

To our knowledge, this is the first report of homologous recombination between Tet(X) variants with differing activities. These clonal strains provide evidence of the transmission of tet(X)-positive strains from aquaculture sewage to the natural environment, highlighting the need to strengthen the monitoring and management of this emerging farming model.

Molecular mechanisms of tigecycline-resistance among Enterobacterales

The global emergence of antimicrobial resistance to multiple antibiotics has recently become a significant concern. Gram-negative bacteria, known for their ability to acquire mobile genetic elements such as plasmids, represent one of the most hazardous microorganisms. This phenomenon poses a serious threat to public health. Notably, the significance of tigecycline, a member of the antibiotic group glycylcyclines and derivative of tetracyclines has increased. Tigecycline is one of the last-resort antimicrobial drugs used to treat complicated infections caused by multidrug-resistant (MDR) bacteria, extensively drug-resistant (XDR) bacteria or even pan-drug-resistant (PDR) bacteria. The primary mechanisms of tigecycline resistance include efflux pumps' overexpression, tet genes and outer membrane porins. Efflux pumps are crucial in conferring multi-drug resistance by expelling antibiotics (such as tigecycline by direct expelling) and decreasing their concentration to sub-toxic levels. This review discusses the problem of tigecycline resistance, and provides important information for understanding the existing molecular mechanisms of tigecycline resistance in Enterobacterales. The emergence and spread of pathogens resistant to last-resort therapeutic options stands as a major global healthcare concern, especially when microorganisms are already resistant to carbapenems and/or colistin.

Convergence of plasmid-mediated Colistin and Tigecycline resistance in Klebsiella pneumoniae

Objective

The co-occurrence of colistin and tigecycline resistance genes in Klebsiella pneumoniae poses a serious public health problem. This study aimed to characterize a K. pneumoniae strain, K82, co-harboring a colistin resistance gene (CoRG) and tigecycline resistance gene (TRG), and, importantly, investigate the genetic characteristics of the plasmid with CoRG or TRG in GenBank.

Methods

K. pneumoniae strain K82 was subjected to antimicrobial susceptibility testing, conjugation assay, and whole-genome sequencing (WGS). In addition, comparative genomic analysis of CoRG or TRG-harboring plasmids from K82 and GenBank was conducted. K. pneumoniae strain K82 was resistant to all the tested antimicrobials including colistin and tigecycline, except for carbapenems.

Results

WGS and bioinformatic analysis showed that K82 belonged to the ST656 sequence type and carried multiple drug resistance genes, including mcr-1 and tmexCD1-toprJ1, which located on IncFIA/IncHI2/IncHI2A/IncN/IncR-type plasmid pK82-mcr-1 and IncFIB/IncFII-type plasmid pK82-tmexCD-toprJ, respectively. The pK82-mcr-1 plasmid was capable of conjugation. Analysis of the CoRG/TRG-harboring plasmid showed that mcr-8 and tmexCD1-toprJ1 were the most common CoRG and TRG of Klebsiella spp., respectively. These TRG/CoRG-harboring plasmids could be divided into two categories based on mash distance. Moreover, we found an IncFIB/IncHI1B-type plasmid, pSYCC1_tmex_287k, co-harboring mcr-1 and tmexCD1-toprJ1. To the best of our knowledge, this is the first report on the co-occurrence of mcr-1 and tmexCD1-toprJ1 on a single plasmid.

Conclusion

Our research expands the known diversity of CoRG and TRG-harboring plasmids in K. pneumoniae. Effective surveillance should be implemented to assess the prevalence of co-harboring CoRG and TRG in a single K. pneumoniae isolate or even a single plasmid.

Chasing the landscape for intrahospital transmission and evolution of hypervirulent carbapenem-resistant Klebsiella pneumoniae

The spread of hypervirulent carbapenem-resistant Klebsiella pneumoniae (Hv-CRKP) is a global health concern. Here, we report the intrahospital colonization and spread of Hv-CRKP isolates in a tertiary hospital from 2017 to 2022. Analyses of 90 nonredundant CRKP isolates from 72 patients indicated that Hv-CRKP transferability relies on the dominant ST11-K64 clone. Whole-genome sequencing of 11 representative isolates gave 31 complete plasmid sequences, including 12 KPC-2 resistance carriers and 10 RmpA virulence vehicles. Apart from the binary vehicles, we detected two types of fusion plasmids, favoring the cotransfer of RmpA virulence and KPC-2 resistance. The detection of ancestry/relic plasmids enabled us to establish genetic mechanisms by which rare fusion plasmids form. Unexpectedly, we found a total of five rmpA promoter variants (P9T-P13T) exhibiting distinct activities and varying markedly in their geographic distributions. CRISPR/Cas9 manipulation confirmed that an active PT11-rmpA regulator is a biomarker for the "high-risk" ST11-K64/CRKP clone. These findings suggest clonal spread and clinical evolution of the prevalent ST11-K64/Hv-CRKP clones. Apart from improved public awareness of Hv-CRKP convergence, our findings might benefit the development of surveillance (and/or intervention) strategies for the dominant ST11-K64 lineage of the Hv-CRKP population in healthcare sectors.

Metagenomic analysis of antimicrobial resistance in ducks, workers, and the environment in duck farms, southern China

Duck farms are one of the important reservoirs of antimicrobial resistance genes (ARGs) that spread to humans and the environment. However, few studies have focused on the characteristics of antimicrobial profiles in duck farms. Here we explored the distribution characteristics and potential transmission mechanisms of ARGs in ducks, farm workers, and the environment in duck farms by a metagenomic approach. The results showed that the highest abundance and diversity of ARGs were found in duck manure. The abundance and diversity of ARGs in workers and environmental samples were higher than those in the control group. tet(X) and its variants were prevalent in duck farms, with tet(X10) being the most abundant. The genetic structure "tet(X)-like + α/β hydrolase" was found in ducks, workers, and the environment, implying that tet(X) and its variants have been widely spread in duck farms. Network analysis indicated that ISVsa3 and IS5075 might play an important role in the coexistence of ARGs and metal resistance genes (MRGs). The Mantel tests showed that mobile genetic elements (MGEs) were significantly correlated with ARG profiles. The results suggest that duck manure may be a potential hotspot source of ARGs, including tet(X) variants that spread to the surrounding environment and workers via MGEs. These results help us optimize the antimicrobials strategy and deepen our understanding of ARG spread in duck farms.

Carriage and Transmission of mcr-1 in Salmonella Typhimurium and Its Monophasic 1,4,[5],12:i:- Variants from Diarrheal Outpatients: a 10-Year Genomic Epidemiology in Guangdong, Southern China

The banning of colistin as a feed additive for food-producing animals in mainland China in 2017 caused the decline in the prevalence of Escherichia coli-mobilized colistin resistance (mcr-1) in China. Salmonella Typhimurium and its monophasic 1,4,[5],12:i:- variants are also the main species associated with the spread of mcr-1; however, the evidence of the prevalence and transmission of mcr-1 among Salmonella is lacking. Herein, the 5,354 Salmonella isolates recovered from fecal samples of diarrheal patients in Guangdong, Southern China, from 2009 to 2019 were screened for colistin resistance and mcr-1, and mcr-1-positive isolates were characterized based on whole-genome sequencing (WGS) data. Relatively high prevalence rates of colistin resistance and mcr-1 (4.05%/4.50%) were identified, and more importantly, the prevalence trends of colistin-resistant and mcr-1-positive Salmonella isolates had a similar dynamic profile, i.e., both were first detected in 2012 and rapidly increased during 2013 to 2016, followed by a sharp decrease since 2017. WGS and phylogenetic analysis indicate that, whether before or after the ban, the persistence and cross-hospital transmission of mcr-1 are primarily determined by IncHI2 plasmids with similar backbones and sequence type 34 (ST34) Salmonella in specific clades that are associated with a high prevalence of IncHI2 plasmids and clinically important antimicrobial resistance genes, including bla_CTX-M-14-fosA3-oqxAB-floR genotypes. Our work reveals the difference in the prevalence rate of mcr-1 in clinical Salmonella before and after the Chinese colistin ban, whereas mcr-1 transmission was closely linked to multidrug-resistant IncHI2 plasmid and ST34 Salmonella across diverse hospitals over 10 years. Continued surveillance is required to explore the factors related to a sharp decrease in mcr-1 after the recent ban and determine whether the ban has affected the carriage of mcr-1 in Salmonella circulating in the health care system. IMPORTANCE Colistin is one of the last-line antibiotics for the clinical treatment of Enterobacteriaceae. However, the emergence of the mobilized colistin resistance (mcr-1) gene has spread throughout the entire human health system and largely threatens the usage of colistin in the clinical setting. In this study, we investigated the existence of mcr-1 in clinical Salmonella from a 10-year continuous surveillance and genomic study. Overall, the colistin resistance rate and mcr-1 carriage of Salmonella in tertiary hospitals in Guangdong (2009 to 2019) were relatively high and, importantly, rapidly increased from 2013 to 2016 and significantly decreased after the Chinese colistin withdrawal. However, before or after the ban, the MDR IncHI2 plasmid with a similar backbone and ST34 Salmonella were the main vectors involved in the spread of mcr-1. Interestingly, these Chinese mcr-1-carrying Salmonella obtain phylogenetically and phylogeographically distinct patterns compared with those from other continents and are frequently associated with clinically important ARGs including the extended-spectrum β-lactamases. Our data confirmed that the national stewardship intervention seems to be successful in blocking antibiotic resistance determinants and that continued surveillance of colistin resistance in clinical settings, farm animals, and related products is necessary.

Dissemination and prevalence of plasmid-mediated high-level tigecycline resistance gene tet (X4)

With the large-scale use of antibiotics, antibiotic resistant bacteria (ARB) continue to rise, and antibiotic resistance genes (ARGs) are regarded as emerging environmental pollutants. The new tetracycline-class antibiotic, tigecycline is the last resort for treating multidrug-resistant (MDR) bacteria. Plasmid-mediated horizontal transfer enables the sharing of genetic information among different bacteria. The tigecycline resistance gene tet(X) threatens the efficacy of tigecycline, and the adjacent ISCR2 or IS26 are often detected upstream and downstream of the tet(X) gene, which may play a crucial driving role in the transmission of the tet(X) gene. Since the first discovery of the plasmid-mediated high-level tigecycline resistance gene tet(X4) in China in 2019, the tet(X) genes, especially tet(X4), have been reported within various reservoirs worldwide, such as ducks, geese, migratory birds, chickens, pigs, cattle, aquatic animals, agricultural field, meat, and humans. Further, our current researches also mentioned viruses as novel environmental reservoirs of antibiotic resistance, which will probably become a focus of studying the transmission of ARGs. Overall, this article mainly aims to discuss the current status of plasmid-mediated transmission of different tet(X) genes, in particular tet(X4), as environmental pollutants, which will risk to public health for the "One Health" concept.

Metagenomic insights into the antibiotic resistomes of typical Chinese dairy farm environments

Antibiotic resistance genes (ARGs) in the environment pose a threat to human and animal health. Dairy cows are important livestock in China; however, a comprehensive understanding of antibiotic resistance in their production environment has not been well clarified. In this study, we used metagenomic methods to analyze the resistomes, microbiomes, and potential ARG bacterial hosts in typical dairy farm environments (including feces, wastewater, and soil). The ARGs resistant to tetracyclines, MLS, β-lactams, aminoglycoside, and multidrug was dominant in the dairy farm ecosystem. The abundance and diversity of total ARGs in dairy feces and wastewater were significantly higher than in soil (P < 0.05). The same environmental samples from different dairy have similar resistomes and microbiomes. A high detection rate of tet(X) in wastewater and feces (100% and 71.4%, respectively), high abundance (range from 5.74 to 68.99 copies/Gb), and the finding of tet(X5) challenged the clinical application of the last antibiotics resort of tigecycline. Network analysis identified Bacteroides as the dominant genus in feces and wastewater, which harbored the greatest abundance of their respective total ARG coverage and shared ARGs. These results improved our understanding of ARG profiles and their bacterial hosts in dairy farm environments and provided a basis for further surveillance.

Classification and molecular characteristics of tet(X)-carrying plasmids in Acinetobacter species

The rapid dissemination of plasmid-mediated tet(X) genes in Acinetobacter species has compromised the clinical effectiveness of tigecycline, one of the last-resort antibiotics. However, the classification strategy and homology group of tet(X)-positive Acinetobacter spp. plasmids remain largely unknown. In this study, we classified them by genome-based replicon typing, followed by analyses of structural characteristics, transferability and in vivo effect. A total of 34 plasmids distributed in at least nine Acinetobacter species were collected, including three tet(X3)-positive plasmids and one tet(X6)-positive plasmid from our genome sequencing results. Among them, there were 28 plasmids carrying Rep_3 superfamily replicase genes and classified into six homology groups, consisting of GR31 (82.1%), GR26 (3.6%), GR41 (3.6%), GR59 (3.6%), and novel groups GR60 (3.6%) and GR61 (3.6%). Our tet(X3)-positive plasmids pYH16040-1, pYH16056-1, and pYH12068-1 belonged to the dominant GR31 group, whereas the tet(X6)-positive plasmid pYH12068-2 was unclassified. Structurally, all tet(X)-positive GR31 plasmids shared similar plasmid replication (repB), stability (parA and parB) and accessory modules [tet(X) and sul2], and 97.6% of plasmid-mediated tet(X) genes in Acinetobacter species were adjacent to ISCR2. Conjugation and susceptibility testing revealed pYH16040-1, pYH16056-1, and pYH12068-2, carrying plasmid transfer modules, were able to mediate the mobilization of multiple antibiotic resistance. Under the treatment of tigecycline, the mortality rate of Galleria mellonella infected by pYH16040-1-mediated tet(X3)-positive Acinetobacter spp. isolate significantly increased when compared with its plasmid-cured strain (p < 0.0001). The spread of such plasmids is of great clinical concern, more effects are needed and will facilitate the future analysis of tet(X)-positive Acinetobacter spp. plasmids.

Mobile Tigecycline Resistance: An Emerging Health Catastrophe Requiring Urgent One Health Global Intervention

Mobile tigecycline resistance (MTR) threatens the clinical efficacy of the salvage antibiotic, tigecycline (TIG) used in treating deadly infections in humans caused by superbugs (multidrug-, extensively drug-, and pandrug-resistant bacteria), including carbapenem- and colistin-resistant bacteria. Currently, non-mobile tet(X) and mobile plasmid-mediated transmissible tet(X) and resistance-nodulation-division (RND) efflux pump tmexCD-toprJ genes, conferring high-level TIG (HLT) resistance have been detected in humans, animals, and environmental ecosystems. Given the increasing rate of development and spread of plasmid-mediated resistance against the two last-resort antibiotics, colistin (COL) and TIG, there is a need to alert the global community on the emergence and spread of plasmid-mediated HLT resistance and the need for nations, especially developing countries, to increase their antimicrobial stewardship. Justifiably, MTR spread projects One Health ramifications and portends a monumental threat to global public and animal health, which could lead to outrageous health and economic impact due to limited options for therapy. To delve more into this very important subject matter, this current work will discuss why MTR is an emerging health catastrophe requiring urgent One Health global intervention, which has been constructed as follows: (a) antimicrobial activity of TIG; (b) mechanism of TIG resistance; (c) distribution, reservoirs, and traits of MTR gene-harboring isolates; (d) causes of MTR development; (e) possible MTR gene transfer mode and One Health implication; and (f) MTR spread and mitigating strategies.

Convergence of MCR-8.2 and Chromosome-Mediated Resistance to Colistin and Tigecycline in an NDM-5-Producing ST656 Klebsiella pneumoniae Isolate From a Lung Transplant Patient in China

We characterized the first NDM-5 and MCR-8.2 co-harboring ST656 Klebsiella pneumoniae clinical isolate, combining with chromosomal gene-mediated resistance to colistin and tigecycline. The K. pneumoniae KP32558 was isolated from the bronchoalveolar lavage fluid from a lung transplant patient. Complete genome sequences were obtained through Illumina HiSeq sequencing and nanopore sequencing. The acquired resistance genes and mutations in chromosome-encoded genes associated with colistin and tigecycline resistance were analyzed. Comparative genomic analysis was conducted between mcr-8.2-carrying plasmids. The K. pneumoniae KP32558 was identified as a pan-drug resistant bacteria, belonging to ST656, and harbored plasmid-encoded blaNDM-5 and mcr-8.2 genes. The blaNDM-5 gene was located on an IncX3 type plasmid. The mcr-8.2 gene was located on a conjugative plasmid pKP32558-2-mcr8, which had a common ancestor with another two mcr-8.2-carrying plasmids pMCR8_020135 and pMCR8_095845. The MIC of KP32558 for colistin was 256 mg/L. The mcr-8.2 gene and mutations in the two-component system, pmrA and crrB, and the regulator mgrB, had a synergistic effect on the high-level colistin resistance. The truncation in the acrR gene, related to tigecycline resistance, was also identified. K. pneumoniae has evolved a variety of complex resistance mechanisms to the last-resort antimicrobials, close surveillance is urgently needed to monitor the prevalence of this clone.

Large-Scale Analysis of Fitness Cost of tet(X4)-Positive Plasmids in Escherichia coli

Tigecycline is one of important antimicrobial agents for the treatment of infections caused by multidrug-resistant (MDR) Gram-negative bacteria. However, the emergence and prevalence of plasmid-mediated tigecycline resistance gene tet(X4) are threatening human and animal health. Fitness cost elicited by resistance plasmids is a key factor affecting the maintenance and transmission of antibiotic resistance genes (ARGs) in the host. A comparative analysis of the fitness cost of different types of tet(X4)-positive plasmids is helpful to understand and predict the prevalence of dominant plasmids. In this study, we performed a large-scale analysis of fitness cost of tet(X4)-positive plasmids origin from clinical isolates. These plasmids were successfully electroporated into a reference strain Escherichia coli TOP10, and a series of transformants carrying the tet(X) gene were obtained. The effects of tet(X4)-positive plasmids on the growth rate, plasmid stability, relative fitness, biofilm formation, and virulence in a Galleria mellonella model were evaluated. Consequently, we found that these plasmids resulted in varying degrees of fitness cost on TOP10, including delayed bacterial growth and attenuated virulence. Out of these plasmids, tet(X4)-harboring IncFII plasmids showed the lowest fitness cost on the host. Furthermore, by means of experimental evolution in the presence of commonly used drugs in clinic, the fitness cost of tet(X4)-positive plasmids was substantially alleviated, accompanied by increased plasmid stability. Collectively, our data reveal the differential fitness cost caused by different types of tet(X4)-positive plasmids and suggest that the wide use of tetracycline antibiotics may promote the evolution of plasmids.

Rapid Detection of MCR-Mediated Colistin Resistance in Escherichia coli

Colistin is one of the last-resort antibiotics for infections caused by multidrug-resistant Gram-negative bacteria. However, the wide spread of novel plasmid-carrying colistin resistance genes mcr-1 and its variants substantially compromise colistin's therapeutic effectiveness and pose a severe danger to public health. To detect colistin-resistant microorganisms induced by mcr genes, rapid and reliable antibiotic susceptibility testing (AST) is imminently needed. In this study, we identified an RNA-based AST (RBAST) to discriminate between colistin-susceptible and mcr-1-mediated colistin-resistant bacteria. After short-time colistin treatment, RBAST can detect differentially expressed RNA biomarkers in bacteria. Those candidate mRNA biomarkers were successfully verified within colistin exposure temporal shifts, concentration shifts, and other mcr-1 variants. Furthermore, a group of clinical strains were effectively distinguished by using the RBAST approach during the 3-h test duration with over 93% accuracy. Taken together, our findings imply that certain mRNA transcripts produced in response to colistin treatment might be useful indicators for the development of fast AST for mcr-positive bacteria.

IMPORTANCE The emergence and prevalence of mcr-1 and its variants in humans, animals, and the environment pose a global public health threat. There is a pressing urgency to develop rapid and accurate methods to identify MCR-positive colistin-resistant bacteria in the clinical samples, providing a basis for subsequent effective antibiotic treatment. Using the specific mRNA signatures, we develop an RNA-based antibiotic susceptibility testing (RBAST) for effectively distinguishing colistin-susceptible and mcr-1-mediated colistin-resistant strains. Meanwhile, the detection efficiency of these RNA biomarkers was evidenced in other mcr variants-carrying strains. By comparing with the traditional AST method, the RBAST method was verified to successfully characterize a set of clinical isolates during 3 h assay time with over 93% accuracy. Our study provides a feasible method for the rapid detection of colistin-resistant strains in clinical practice.

Coexistence of tet(X4), mcr-1, and blaNDM-5 in ST6775 Escherichia coli Isolates of Animal Origin in China

Emergence of pathogens harboring multiple resistance genes incurs great concerns. Cooccurrence of mobile resistance genes conferring resistance to tigecycline, colistin, and carbapenems in Escherichia coli has not been investigated. This study aimed to characterize three E. coli isolates coharboring tet(X4), mcr-1, and bla_NDM-5. Isolates coharboring tet(X4), mcr-1, and bla_NDM-5 were identified and characterized by PCR, Sanger sequencing, antimicrobial susceptibility testing, conjugation assays, Illumina sequencing, nanopore sequencing, and bioinformatic analysis. Three E. coli isolates carrying tet(X4), mcr-1, and bla_NDM-5 were identified from pigeons in China. They were resistant to almost all antimicrobials except enrofloxacin. tet(X4) and bla_NDM-5 could be conjugated into E. coli C600, but mcr-1 was nontransferable in three isolates. Three isolates belonged to sequence type 6775 (ST6775), and clonal dissemination of isolates carrying tet(X4), mcr-1, and bla_NDM-5 existed in the pigeon farm. Genetic analysis revealed that mcr-1 mediated by the Tn6330 was located on the chromosome, tet(X4) was located on the IncFII plasmid, and bla_NDM-5 was located on the IncX3 plasmid. We first characterized the E. coli isolates carrying tet(X4), mcr-1, and bla_NDM-5 simultaneously. Relevant measures should be taken to decrease the prevalence of pathogens carrying tet(X4), mcr-1, and bla_NDM-5. IMPORTANCE Tigecycline and colistin are regarded as vital antimicrobials to treat multidrug-resistant (MDR) bacterial infections, such as that caused by carbapenemase-producing Enterobacteriaceae (CPE). Cooccurrence of mobile resistance genes conferring resistance to last-resort antimicrobials in E. coli remains unknown. Here, we characterized E. coli strains coharboring tet(X4), mcr-1, and bla_NDM-5 phenotypically and genetically. Resistance genes tet(X4), mcr-1, and bla_NDM-5 were located on transposons or plasmids that were mobile genetic elements related to the capture, accumulation, and dissemination of such important resistance genes. The emergence of E. coli isolates carrying tet(X4), mcr-1, and bla_NDM-5 highlights the importance of monitoring the coexistence of novel mobile resistance genes in different settings with a One Health approach. Risk of transmission of such MDR pathogens from animals to humans should be evaluated comprehensively.

Widespread prevalence and molecular epidemiology of tet(X4) and mcr-1 harboring Escherichia coli isolated from chickens in Pakistan

The emergence and spread of plasmid-mediated tigecycline resistance gene tet(X4) and colistin resistance gene mcr-1 in Escherichia coli (E. coli) pose a potential threat to public health, due to the importance of colistin and tigecycline for treating serious clinical infections. However, the characterization of bacteria coharboring both genes was few reported. Here, we described the molecular epidemiology of tet(X4) and mcr-1 harboring E. coli strains of chicken origin in Pakistan, with methods including PCR, antimicrobial susceptibility testing, DNA transfer assays, plasmid replicon typing, whole-genome sequencing and bioinformatics analysis. The tet(X4) gene was identified in 36 isolates exhibiting high levels of tigecycline resistance (MICs, 16-128 mg/L). Worryingly, 24 of the 36 tet(X4)-bearing isolates were confirmed as colistin resistance, positive for plasmid-borne mcr-1. We observed the prevalence of tet(X4)-bearing IncFII plasmid with mcr-1-bearing IncI2 plasmid in 12 E. coli isolates, with a high co-transfer frequency except for one strain PK8233, in which tet(X4)- and mcr-1-bearing plasmids were non-transferable. Coexistence of tet(X4)-bearing IncFII plasmid with mcr-1-carrying multidrug-resistant (MDR) IncHI2 plasmid was also identified in 10 E. coli isolates, and a relatively low co-transfer frequency was obtained except PK8575, in which mcr-1 was non-transferable. The transferability of pPK8275-tetX in PK8275 and pPK8233-tetX in PK8233, that could transfer from E. coli J53 to C600 by conjugation, was interfered by certain factors in PK8275 and PK8233. This may provide new insights to prevent and control the spread of antibiotic resistance genes. Two strains were reported to co-carry tet(X4)-positive IncQ1 plasmid and mcr-1-positive IncI2 plasmid. Convergence of tet(X4) and mcr-1 genes in E. coli by conjugative or mobilizable plasmids may lead to potentially widespread transmission of such resistance genes, which may incur antibiotic-resistance crisis globally.

Emerging Opportunity and Destiny of mcr-1- and tet(X4)-Coharboring Plasmids in Escherichia coli

The emergence of the plasmid-mediated colistin resistance gene mcr-1 and the plasmid-mediated tigecycline resistance gene tet(X4) represents a significant threat to public health. Although mcr-1 and tet(X4) have been reported to coexist in the same isolate, there are no reports on the emergence of plasmids coharboring mcr-1 and tet(X4). In this study, we aimed to investigate the opportunities for the emergence of mcr-1- and tet(X4)-coharboring plasmids and their destiny in Escherichia coli. Two plasmids carrying both mcr-1 and tet(X4) were constructed through conjugation assays and confirmed by S1 nuclease pulsed-field gel electrophoresis (S1-PFGE) and Nanopore long-read sequencing. Seven evolved plasmids carrying mcr-1 and tet(X4) from one of the two plasmids were acquired after continuous evolutionary processes. The fitness effects of mcr-1- and tet(X4)-coharboring plasmids were studied by stability experiments, competition experiments, and growth curve measurements. A plasmid carrying mcr-1 and tet(X4) and conferring no fitness cost to its host strain E. coli C600 emerged after evolution during serial passages of bacteria. We proved that it can be anticipated that mcr-1 and tet(X4) could appear in a single plasmid, and the possibility of occurrence in field strains should be monitored constantly. The originally formed cointegrate plasmids coharboring mcr-1 and tet(X4) could evolve into a plasmid with lower fitness costs. This will undoubtedly accelerate the transmission of mcr-1 and tet(X4) globally. The findings highlighted the great possibility of novel hybrid plasmids positive for mcr-1 and tet(X4), and the risk is worthy of increasing attention and public concern globally.

IMPORTANCE Tigecycline and colistin are used as last-resort therapies to treat infections caused by multidrug-resistant (MDR) Gram-negative bacteria. However, the emergence of the plasmid-mediated tigecycline resistance gene tet(X4) and the plasmid-mediated colistin resistance gene mcr-1 represents a significant threat to human health. A plasmid coharboring mcr-1 and tet(X4) has not emerged so far, but the potential risk should not be ignored. Plasmids coharboring such vital resistance genes will greatly accelerate the progression of pan-drug resistance among pathogens globally. Therefore, evaluation of the emerging opportunity for the mcr-1- and tet(X4)-coharboring plasmids and their destiny in E. coli is of great significance. We provide important insight into the contributions of intI1, IS26, a truncated ISCR2 (ΔISCR2), and IS4321R during the generation of cointegrate plasmids carrying mcr-1 and tet(X4) and highlight the importance of antimicrobials in the evolution and diversity of mcr-1- and tet(X4)-coharboring plasmids. We show that monitoring of the occurrence of mcr-1-carrying MDR plasmids and tet(X4)-bearing MDR plasmids in the same strain should be strengthened to avoid the formation of mcr-1- and tet(X4)-coharboring plasmids.

More diversified antibiotic resistance genes in chickens and workers of the live poultry markets

BACKGROUND

Poultry farms and LPMs are a reservoir of antimicrobial resistant bacteria and resistance genes from feces. The LPM is an important interface between humans, farm animals, and environments in a typical urban environment, and it is considered a reservoir for ARGs and viruses. However, the antibiotic resistomes shared between chicken farms and LPMs, and that of LPM workers and people who have no contact with the LPMs remains unknown.

METHODS

We characterized the resistome and bacterial microbiome of farm chickens and LPMs and LPM workers and control subjects. The mobile ARGs identified in chickens and the distribution of the mcr-family genes in publicly bacterial genomes and chicken gut metagenomes was analyzed, respectively. In addition, the prevalence of mcr-1 in LPMs following the ban on colistin-positive additives in China was explored.

RESULTS

By profiling the microbiomes and resistomes in chicken farms, LPMs, LPM workers, and LPM environments, we found that the bacterial community composition and resistomes were significantly different between the farms and the LPMs, and the LPM samples possessed more diversified ARGs (59 types) than the farms. Some mobile ARGs, such as mcr-1 and tet(X3), identified in chicken farms, LPMs, LPM workers, and LPM environments were also harbored by human clinical pathogens. Moreover, we found that the resistomes were significantly different between the LPM workers and those who have no contact with the LPMs, and more diversified ARGs (188 types) were observed in the LPM workers. It is also worth noting that mcr-10 was identified in both human (5.2%, 96/1,859) and chicken (1.5%, 14/910) gut microbiomes. Although mcr-1 prevalence decreased significantly in the LPMs across the eight provinces in China, from 190/333 (57.1%) samples in September 2016-March 2017 to 208/544 (38.2%) samples in August 2018-May 2019, it is widespread and continuous in the LPMs.

CONCLUSION

Live poultry trade has a significant effect on the diversity of ARGs in LPM workers, chickens, and environments in China, driven by human selection with the live poultry trade. Our findings highlight the live poultry trade as ARG disseminators into LPMs, which serve as an interface of LPM environments even LPM workers, and that could urge Government to have better control of LPMs in China. Further studies on the factors that promote antibiotic resistance exchange between LPM environments, human commensals, and pathogens, are warranted.

The poultry pathogen Riemerella anatipestifer appears as a reservoir for Tet(X) tigecycline resistance

The transferability of bacterial resistance to tigecycline, the 'last-resort' antibiotic, is an emerging challenge of global health concern. The plasmid-borne tet(X) that encodes a flavin-dependent monooxygenase represents a new mechanism for tigecycline resistance. Natural source for an ongoing family of Tet(X) resistance determinants is poorly understood. Here, we report the discovery of 26 new variants [tet(X18) to tet(X44)] from the poultry pathogen Riemerella anatipestifer, which expands extensively the current Tet(X) family. R. anatipestifer appears as a natural reservoir for tet(X), of which the chromosome harbours varied copies of tet(X) progenitors. Despite that an inactive ancestor rarely occurs, the action and mechanism of Tet(X2/4)-P, a putative Tet(X) progenitor, was comprehensively characterized, giving an intermediate level of tigecycline resistance. The potential pattern of Tet(X) dissemination from ducks to other animals and humans was raised, in the viewpoint of ecological niches. Therefore, this finding defines a large pool of natural sources for Tet(X) tigecycline resistance, heightening the need of efficient approaches to manage the inter-species transmission of tet(X) resistance determinants.