Three Fe(III)-reducing and nitrogen-fixing bacteria, Anaeromyxobacter terrae sp. nov., Anaeromyxobacter oryzisoli sp. nov. and Anaeromyxobacter soli sp. nov., isolated from paddy soil.

Three microaerophilic bacterial strains, designated SG22T, SG63T and SG29T were isolated from paddy soils in PR China. Cells of these strains were Gram-staining-negative and long rod-shaped. SG22T, SG63T and SG29T showed the highest 16S rRNA gene sequence similarities with the members of the genus Anaeromyxobacter. The results of phylogenetic and phylogenomic analysis also indicated that these strains clustered with members of the genus Anaeromyxobacter. The main respiratory menaquinone of SG22T, SG63T and SG29T was MK-8 and the major fatty acids were iso-C15 : 0, iso-C17 : 0 and C16 : 0. SG22T, SG29T and SG63T not only possessed iron reduction ability but also harboured genes (nifHDK) encoding nitrogenase. The genomic DNA G+C contents of SG22T, SG63T and SG29T ranged from 73.3 to 73.5 %. The average nucleotide identity (ANI) and digital DNA-DNA hybridisation (dDDH) values between SG22T, SG63T and SG29T and the closely related species of the genus Anaeromyxobacter were lower than the cut-off values (dDDH 70 % and ANI 95-96 %) for prokaryotic species delineation. On the basis of these results, strains SG22T, SG63T and SG29T represent three novel species within the genus Anaeromyxobacter, for which the names Anaeromyxobacter terrae sp. nov., Anaeromyxobacter oryzisoli sp. nov. and Anaeromyxobacter soli sp. nov., are proposed. The type strains are SG22T (= GDMCC 1.3185T = JCM 35581T), SG63T (= GDMCC 1.2914T = JCM 35124T) and SG29T (= GDMCC 1.2911T = JCM 35123T).

Mesophilic and thermophilic viruses are associated with nutrient cycling during hyperthermophilic composting.

While decomposition of organic matter by bacteria plays a major role in nutrient cycling in terrestrial ecosystems, the significance of viruses remains poorly understood. Here we combined metagenomics and metatranscriptomics with temporal sampling to study the significance of mesophilic and thermophilic bacteria and their viruses on nutrient cycling during industrial-scale hyperthermophilic composting (HTC). Our results show that virus-bacteria density dynamics and activity are tightly coupled, where viruses specific to mesophilic and thermophilic bacteria track their host densities, triggering microbial community succession via top-down control during HTC. Moreover, viruses specific to mesophilic bacteria encoded and expressed several auxiliary metabolic genes (AMGs) linked to carbon cycling, impacting nutrient turnover alongside bacteria. Nutrient turnover correlated positively with virus-host ratio, indicative of a positive relationship between ecosystem functioning, viral abundances, and viral activity. These effects were predominantly driven by DNA viruses as most detected RNA viruses were associated with eukaryotes and not associated with nutrient cycling during the thermophilic phase of composting. Our findings suggest that DNA viruses could drive nutrient cycling during HTC by recycling bacterial biomass through cell lysis and by expressing key AMGs. Viruses could hence potentially be used as indicators of microbial ecosystem functioning to optimize productivity of biotechnological and agricultural systems.

The evolution of colistin resistance increases bacterial resistance to host antimicrobial peptides and virulence.

Antimicrobial peptides (AMPs) offer a promising solution to the antibiotic resistance crisis. However, an unresolved serious concern is that the evolution of resistance to therapeutic AMPs may generate cross-resistance to host AMPs, compromising a cornerstone of the innate immune response. We systematically tested this hypothesis using globally disseminated mobile colistin resistance (MCR) that has been selected by the use of colistin in agriculture and medicine. Here, we show that MCR provides a selective advantage to Escherichia coli in the presence of key AMPs from humans and agricultural animals by increasing AMP resistance. Moreover, MCR promotes bacterial growth in human serum and increases virulence in a Galleria mellonella infection model. Our study shows how the anthropogenic use of AMPs can drive the accidental evolution of resistance to the innate immune system of humans and animals. These findings have major implications for the design and use of therapeutic AMPs and suggest that MCR may be difficult to eradicate, even if colistin use is withdrawn.

Citrobacter portucalensis Sb-2 contains a metalloid resistance determinant transmitted by Citrobacter phage Chris1.

Bacterial adaptation to extreme environments is often mediated by horizontal gene transfer (HGT) via genetic mobile elements. Nevertheless, phage-mediated HGT conferring bacterial arsenic resistance determinants has rarely been investigated. In this study, a highly arsenite and antimonite resistant bacterium, Citrobacter portucalensis strain Sb-2, was isolated, and genome analysis showed that several putative arsenite and antimonite resistance determinants were flanked or embedded in prophages. Furthermore, an active bacteriophage carrying one of the ars clusters (arsRDABC arsR-yraQ/arsP) was obtained and sequenced. These genes encoding putative arsenic resistance determinants were induced by arsenic and antimony as demonstrated by RT-qPCR, and one gene arsP/yraQ of the ars cluster was shown to give resistance to MAs(III) and Rox(III), thereby showing function. Here, we were able to directly show that these phage-mediated arsenic and antimony resistances play a significant role in adapting to As- and Sb-contaminated environments. In addition, we demonstrate that this phage is responsible for conferring arsenic and antimony resistances to C. portucalensis strain Sb-2.

Distinctive community assembly enhances the adaptation to extreme environments during hyperthermophilic composting.

Hyperthermophilic composting (hTC) is a promising technique for solid waste treatment due to its distinctive microbiomes. However, the assembly process of the hTC microbial community remains unclear. We investigated the assembly process of hTC and explored the underlying drivers influencing community assembly in this work by employing conventional thermophilic composting (cTC) as a comparison group. Our results showed that the two composting treatments have different community assembly processes. Especially for the initial and thermophilic phases, hTC is affected by homogeneous dispersal (48%) and homogeneous selection (44%), respectively, while cTC is controlled by undominant (38%) and homogeneous selection (92%), respectively. Furthermore, random forest models and network results suggested that different factors govern the community assembly in these two composting methods. Specifically, the hTC community increases the stability of the thermophilic community via enhancing the interactions of low-abundance taxa with other operational taxonomic units (OTUs) in community assembly. Our results suggested that the distinctive nature of hTC community assembly may be responsible for its adaptation to extreme environments.

Pre-existing chromosomal polymorphisms in pathogenic E. coli potentiate the evolution of resistance to a last-resort antibiotic.

Bacterial pathogens show high levels of chromosomal genetic diversity, but the influence of this diversity on the evolution of antibiotic resistance by plasmid acquisition remains unclear. Here, we address this problem in the context of colistin, a 'last line of defence' antibiotic. Using experimental evolution, we show that a plasmid carrying the MCR-1 colistin resistance gene dramatically increases the ability of Escherichia coli to evolve high-level colistin resistance by acquiring mutations in lpxC, an essential chromosomal gene involved in lipopolysaccharide biosynthesis. Crucially, lpxC mutations increase colistin resistance in the presence of the MCR-1 gene, but decrease the resistance of wild-type cells, revealing positive sign epistasis for antibiotic resistance between the chromosomal mutations and a mobile resistance gene. Analysis of public genomic datasets shows that lpxC polymorphisms are common in pathogenic E. coli, including those carrying MCR-1, highlighting the clinical relevance of this interaction. Importantly, lpxC diversity is high in pathogenic E. coli from regions with no history of MCR-1 acquisition, suggesting that pre-existing lpxC polymorphisms potentiated the evolution of high-level colistin resistance by MCR-1 acquisition. More broadly, these findings highlight the importance of standing genetic variation and plasmid/chromosomal interactions in the evolutionary dynamics of antibiotic resistance.

Isolation and Characterization of the Lytic Pseudoxanthomonas kaohsiungensi Phage PW916.

The emergence of multidrug-resistant bacterial pathogens poses a serious global health threat. While patient infections by the opportunistic human pathogen Pseudoxanthomonas spp. have been increasingly reported worldwide, no phage associated with this bacterial genus has yet been isolated and reported. In this study, we isolated and characterized the novel phage PW916 to subsequently be used to lyse the multidrug-resistant Pseudoxanthomonas kaohsiungensi which was isolated from soil samples obtained from Chongqing, China. We studied the morphological features, thermal stability, pH stability, optimal multiplicity of infection, and genomic sequence of phage PW916. Transmission electron microscopy revealed the morphology of PW916 and indicated it to belong to the Siphoviridae family, with the morphological characteristics of a rounded head and a long noncontractile tail. The optimal multiplicity of infection of PW916 was 0.1. Moreover, PW916 was found to be stable under a wide range of temperatures (4-60 °C), pH (4-11) as well as treatment with 1% (v/w) chloroform. The genome of PW916 was determined to be a circular double-stranded structure with a length of 47,760 bp, containing 64 open reading frames that encoded functional and structural proteins, while no antibiotic resistance nor virulence factor genes were detected. The genomic sequencing and phylogenetic tree analysis showed that PW916 was a novel phage belonging to the Siphoviridae family that was closely related to the Stenotrophomonas phage. This is the first study to identify a novel phage infecting the multidrug-resistant P. kaohsiungensi and the findings provide insight into the potential application of PW916 in future phage therapies.

Herbicide promotes the conjugative transfer of multi-resistance genes by facilitating cellular contact and plasmid transfer.

The global dissemination of antibiotic resistance genes (ARGs), especially via plasmid-mediated horizontal transfer, is becoming a pervasive health threat. While our previous study found that herbicides can accelerate the horizontal gene transfer (HGT) of ARGs in soil bacteria, the underlying mechanisms by which herbicides promote the HGT of ARGs across and within bacterial genera are still unclear. Here, the underlying mechanism associated with herbicide-promoted HGT was analyzed by detecting intracellular reactive oxygen species (ROS) production, extracellular polymeric substance composition, cell membrane integrity and proton motive force combined with genome-wide RNA sequencing. Exposure to herbicides induced a series of the above bacterial responses to promote HGT except for the ROS response, including compact cell-to-cell contact by enhancing pilus-encoded gene expression and decreasing cell surface charge, increasing cell membrane permeability, and enhancing the proton motive force, providing additional power for DNA uptake. This study provides a mechanistic understanding of the risk of bacterial resistance spread promoted by herbicides, which elucidates a new perspective on nonantibiotic agrochemical acceleration of the HGT of ARGs.

Airborne and indigenous microbiomes co-drive the rebound of antibiotic resistome during compost storage.

Composting is widely used to reduce the abundance of antibiotic resistance genes (ARGs) in solid waste. While ARG dynamics have been extensively investigated during composting, the fate and abundance of residual ARGs during the storage remain unexplored. Here, we tested experimentally how ARG and mobile genetic element (MGE) abundances change during compost storage using metagenomics, quantitative PCR and direct culturing. We found that 43.8% of ARGs and 39.9% of MGEs quickly recovered already during the first week of storage. This rebound effect was mainly driven by the regrowth of indigenous, antibiotic-resistant bacteria that survived the composting. Bacterial transmission from the surrounding air had a much smaller effect, being most evident as MGE rebound during the later stages of storage. While hyperthermophilic composting was more efficient at reducing the relative abundance of ARGs and MGEs, relatively greater ARG rebound was observed during the storage of hyperthermophilic compost, exceeding the initial levels of untreated sewage sludge. Our study reveals that residual ARGs and MGEs left in the treated compost can quickly rebound during the storage via airborne introduction and regrowth of surviving bacteria, highlighting the need to develop better storage strategies to prevent the rebound of ARGs and MGEs after composting.

Herbicide Selection Promotes Antibiotic Resistance in Soil Microbiomes.

Herbicides are one of the most widely used chemicals in agriculture. While they are known to be harmful to nontarget organisms, the effects of herbicides on the composition and functioning of soil microbial communities remain unclear. Here we show that application of three widely used herbicides-glyphosate, glufosinate, and dicamba-increase the prevalence of antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) in soil microbiomes without clear changes in the abundance, diversity and composition of bacterial communities. Mechanistically, these results could be explained by a positive selection for more tolerant genotypes that acquired several mutations in previously well-characterized herbicide and ARGs. Moreover, herbicide exposure increased cell membrane permeability and conjugation frequency of multidrug resistance plasmids, promoting ARG movement between bacteria. A similar pattern was found in agricultural soils across 11 provinces in China, where herbicide application, and the levels of glyphosate residues in soils, were associated with increased ARG and MGE abundances relative to herbicide-free control sites. Together, our results show that herbicide application can enrich ARGs and MGEs by changing the genetic composition of soil microbiomes, potentially contributing to the global antimicrobial resistance problem in agricultural environments.

Balancing mcr-1 expression and bacterial survival is a delicate equilibrium between essential cellular defence mechanisms.

MCR-1 is a lipid A modifying enzyme that confers resistance to the antibiotic colistin. Here, we analyse the impact of MCR-1 expression on E. coli morphology, fitness, competitiveness, immune stimulation and virulence. Increased expression of mcr-1 results in decreased growth rate, cell viability, competitive ability and significant degradation in cell membrane and cytoplasmic structures, compared to expression of catalytically inactive MCR-1 (E246A) or MCR-1 soluble component. Lipopolysaccharide (LPS) extracted from mcr-1 strains induces lower production of IL-6 and TNF, when compared to control LPS. Compared to their parent strains, high-level colistin resistance mutants (HLCRMs) show reduced fitness (relative fitness is 0.41-0.78) and highly attenuated virulence in a Galleria mellonella infection model. Furthermore, HLCRMs are more susceptible to most antibiotics than their respective parent strains. Our results show that the bacterium is challenged to find a delicate equilibrium between expression of MCR-1-mediated colistin resistance and minimalizing toxicity and thus ensuring cell survival.

Design, synthesis, and structure-activity relationship studies of novel pleuromutilin derivatives having a piperazine ring.

A series of novel pleuromutilin derivatives possessing piperazine moieties were synthesized under mild conditions. The in vitro antibacterial activities of these derivatives against Staphylococcus aureus and Escherichia coli were tested by the agar dilution method. Structure-activity relationship studies resulted in compounds 11b, 13b, and 14a with the most potent in vitro antibacterial activity among the series (minimal inhibitory concentration = 0.0625-0.125 µg/mL). The binding of compounds 11b, 13b, and 14a to the E. coli ribosome was investigated by molecular modeling, and it was found that there is a reasonable correlation between the binding free energy and the antibacterial activity.

IncF plasmid diversity in multi-drug resistant Escherichia coli strains from animals in China.

The purpose of this study was to characterize a collection of 103 multidrug resistance IncF plasmids recovered from Escherichia coli of food producing and companion animals between 2003 and 2012. A total of 103 incF plasmids were characterized using an established PCR-based IncF replicon sequence typing (RST) system to identify FII, FIA, and FIB (FAB) groups. Plasmids were also analyzed using-restriction fragment length polymorphism (RFLP). Antibiotic Resistance determinants bla CTX-M , plasmid-mediated quinolone resistance (PMQR) genes and rmtB and plasmid addiction systems (PAS) were identified by PCR screening. A total of 20 different RSTs from 103 IncF plasmids were identified. The groups F2 and F33 with the RST formulae A-: B- were the most frequently encountered types (63.1%). The antibiotic resistance genes (ARGs) bla CTX-M , rmtB, and oqxB were carried by 82, 37, and 34 IncF plasmids, respectively. Most of these plasmids carried more than one resistance gene (59.2%, 61/103). The IncF plasmids also had a high frequency of addiction systems (mean 2.54) and two antisense RNA-regulated systems (hok-sok and srnBC) and a protein antitoxin-regulated system (pemKI) were the most prevalent. Not surprisingly, RFLP profiles among the IncF plasmids were diverse even though some shared identical IncF-RSTs. This is the first extensive study of IncF plasmid-positive E. coli isolates from animals in China. Our results demonstrate that IncF is the most prevalent plasmid family in E. coli plasmids and they commonly carry multiple resistance determinants that render them resistant to different antibiotic classes simultaneously. IncF plasmids also harbor addiction systems, promoting their stability and maintenance in the bacterial host, under changing environmental conditions.

Characterization of chromosomal qnrB and ampC alleles in Citrobacter freundii isolates from different origins.

The association of ESBLs (extended-spectrum beta-lactamases)/pAmpCs (plasmid-mediated AmpC ß-lactamases) with PMQR (plasmid mediated quinolone resistance) in gram-negative bacteria has been of great concern. The present study was performed to characterize the diversity, gene location, genetic context, and evolution of ampC and qnrB alleles in isolates of Citrobacter freundii. Fifteen isolates of C. freundii were identified from a total of 788 isolates of Enterobacteriaceae derived from humans, animals, animal food products, and the environment between 2010 and 2012. Co-existence of qnrB/ΔqnrB with ampC was detected in all C. freundii isolates. Both ampC and qnrB genes were found to be located on the chromosome, but were distantly separated on the chromosome. Seven and six novel alleles were discovered for the 10 ampC and qnrB variants detected in this study, respectively. Phylogenetic analysis showed that the new alleles differed a little from the variants of ampC/qnrB previously described in this genus. The genetic context surrounding ampC genes was AmpR-AmpC-Blc-SugE. However, five different genetic contexts surrounding qnrB/ΔqnrB genes were observed, but they occurred in all cases between the pspF and sapA genes. Additionally, cloning experiments showed that the regions containing different qnrB alleles, even with different genetic contexts, contributed to the reduction of quinolone susceptibility. Our results showed that the chromosomal ampC and qnrB alleles are closely related to C. freundii. However, unlike ampC, qnrB alleles seemed to be related to the genetic contexts surrounding them. The evolution of these two genes in C. freundii isolates might be through different pathways.

Dissemination of the chromosomally encoded CMY-2 cephalosporinase gene in Escherichia coli isolated from animals.

In this study, 619 individual Escherichia coli isolates from food-producing and companion animals were analysed to determine the prevalence of the cephalosporinase gene blaCMY-2. In total, 18 CMY-2-producers (2.9%) were detected and exhibited multidrug-resistant phenotypes. One of the CMY-2-producers was found to possess a novel blaCMY-2-like allele, blaCMY-130. The isolates belonged to distinct pulsotypes, suggesting that the blaCMY-2 gene was not disseminated by clonal expansion of blaCMY-2-positive strains. The blaCMY-2 genes were located on IncA/C-, IncHI2- or IncX-type plasmids in 9 (50%) of the 18 E. coli isolates. However, in the other nine isolates I-CeuI-PFGE and hybridisation analyses revealed that the blaCMY-2 gene was chromosomally located. A CMY gene-containing region composed of five open reading frames (ORFs) (ISEcp1-blaCMY-2-blc-sugE-ΔencR) was observed in plasmids from eight strains. A CMY gene-containing region composed of ten ORFs was observed in all of the nine chromosomally encoded blaCMY-2 genes, including a putative IS66-like element inserted in this conserved CMY genetic region in three strains. This conserved CMY genetic region was also found to be inserted into the oriVγ (putative gamma origin), part of the IncX plasmid backbone, by a complete transposition unit flanked by 5-bp DRs (direct repeat sequence) in pS62T. These results demonstrate the high prevalence of the chromosomally encoded blaCMY-2 gene in E. coli. This is the first study reporting a chromosomally encoded blaCMY-2 gene in E. coli. Chromosomally encoded blaCMY-2 might be a source of some plasmid-mediated blaCMY-2 genes and this probably facilitates the spread of cephalosporin-resistant strains.

Complete nucleotide sequence of cfr-carrying IncX4 plasmid pSD11 from Escherichia coli.

We report the complete nucleotide sequence of a plasmid carrying the multiresistance gene cfr. This plasmid was isolated from an Escherichia coli strain of swine origin in 2011. This 37,672-bp plasmid, pSD11, had an IncX4 backbone similar to those of the IncX4 plasmids obtained from the United States and Australia, in which the cfr gene was flanked by two copies of IS26 and a truncated Tn1331 was inserted.

Characterization of plasmids carrying oqxAB in bla(CTX-M)-negative Escherichia coli isolates from food-producing animals.

To study the characteristics of plasmids harboring oqxAB among bla(CTX-M)-negative Escherichia coli isolates and search for oqxAB-harboring plasmids similar to plasmids carrying oqxAB-bla(CTX-M) reported previously, conjugation experiment was performed for 115 randomly selected oqxAB-positive but bla(CTX-M)-negative E. coli isolates from diseased animals in Guangdong, China. S1 nuclease pulsed-field gel electrophoresis (PFGE) and southern blotting experiments were performed to investigate the location of oqxAB and other resistance genes. The EcoRI digestion profiles of the plasmids with oqxAB were also analyzed. The clonal relatedness of donor isolates was investigated by PFGE. In this study, 32 oqxAB transconjugants were successfully obtained and most transconjugants showed multidrug resistances. Eleven replicon combination types were found in these transconjugants. floR and oqxAB were found on the same plasmids in all nine transconjugants resistant to florfenicol. The sequences between floR and oqxAB were identical in most transconjugants and the two genes were both linked with tnp in insertion sequences. Nine F18:A-:B1 plasmids with only oqxAB shared identical EcoRI digestion profiles and the profiles were also identical with that of a plasmid carrying oqxAB-bla(CTX-M) found previously. Co-transfer of plasmids carrying oqxAB and fosA3, respectively, was also observed in one isolate. This study demonstrates the dissemination of oqxAB among bla(CTX-M)-negative E. coli isolates was mainly mediated by identical F18:A-:B1 plasmids. A novel arrangement of regions between floR and oqxAB might play an important role in the dissemination of floR-oqxAB. This is the first description of the genetic environment of the relationship between oqxAB and floR in E. coli.

Comparison of plasmids coharboring 16s rrna methylase and extended-spectrum ß-lactamase genes among Escherichia coli isolates from pets and poultry.

A total of 247 Escherichia coli isolates (148 from diseased or dead poultry and 99 from diseased pets in the People's Republic of China) were screened for extended-spectrum ß-lactamase (ESBL) determinants by PCR and sequencing. Then, 16S rRNA methylase genes were detected among ESBL-producing isolates. Clonal relatedness of the E. coli isolates was examined by pulsed-field gel electrophoresis. Conjugation experiments were performed to investigate the association of 16S rRNA methylases and ESBLs, and plasmid contents were also characterized. Among 247 E. coli isolates, 74 (29.96%) isolates were positive for blaCTX-M genes, 42 from pets (12 from cats and 30 from dogs) and 32 from poultry (12 from chickens and 20 from ducks). The most common CTX-M type in isolates from pets was blaCTX-M-14, whereas blaCTX-M-27 was the most common for poultry. rmtB was dectected in 39 of the 74 blaCTX-M-positive isolates, 18 from pets and 21 from poultry. One strain from a pet was found to harbor blaCTX-M-14, blaCTX-M-15, and rmtB. blaCTX-M and rmtB were found to be colocated on the same transferable plasmid in 16 isolates. These genes were on the same or similar plasmids (eight F2:A-:B- and two IncN) in isolates from ducks, whereas they were colocated on the similar F2:A-:B- or similar F33:A-:B- plasmids in isolates of pets origin. In conclusion, similar F2:A-:B- plasmids and similar F33:A-:B- plasmids are responsible for the dissemination of both rmtB and blaCTX-Mgenes in E. coli isolates from poultry and pets, respectively.

Dissemination and characterization of plasmids carrying oqxAB-bla CTX-M genes in Escherichia coli isolates from food-producing animals.

BACKGROUND: The association of PMQR and ESBLs in negative-bacteria isolates has been of great concern. The present study was performed to investigate the prevalence of co-transferability of oqxAB and bla CTX-M genes among the 696 Escherichia coli (E. coli) isolates from food-producing animals in South China, and to characterize these plasmids. METHODS: The ESBL-encoding genes (bla(CTX-M), bla(TEM) and bla(SHV)), and PMQR (qnrA, qnrB, qnrS, qnrC, qnrD, aac(6')-Ib-cr, qepA, and oqxAB) of these 696 isolates were determined by PCR and sequenced directionally. Conjugation, S1 nuclease pulsed-field gel electrophoresis (PFGE) and Southern blotting experiments were performed to investigate the co-transferability and location of oqxAB and bla(CTX-M). The EcoRI digestion profiles of the plasmids with oqxAB-bla(CTX-M) were also analyzed. The clonal relatedness was investigated by PFGE. RESULTS: Of the 696 isolates, 429 harbored at least one PMQR gene, with oqxAB (328) being the most common type; 191 carried bla(CTX-M), with bla(CTX-M-14) the most common. We observed a significant higher prevalence of bla(CTX-M) among the oqxAB-positive isolates (38.7%) than that (17.4%) in the oqxAB-negative isolates. Co-transferability of oqxAB and bla(CTX-M) was found in 18 of the 127 isolates carrying oqxAB-bla(CTX-M). These two genes were located on the same plasmid in all the 18 isolates, with floR being on these plasmids in 13 isolates. The co-dissemination of these genes was mainly mediated by F33:A-: B- and HI2 plasmids with highly similar EcoRI digestion profiles. Diverse PFGE patterns indicated the high prevalence of oqxAB was not caused by clonal dissemination. CONCLUSION: bla(CTX-M) was highly prevalent among the oqxAB-positive isolates. The co-dissemination of oqxAB-bla(CTX-M) genes in E. coli isolates from food-producing animals is mediated mainly by similar F33:A-: B- and HI2 plasmids. This is the first report of the co-existence of oqxAB, bla(CTX-M), and floR on the same plasmids in E. coli.