Carbapenem resistance in members of the order Enterobacterales is a growing public health problem that is associated with high mortality in developing and industrialized countries. Moreover, in the field of veterinary medicine, the occurrence of New Delhi metallo-β-lactamase-producing Escherichia coli isolates in animals, especially food-producing animals, has become a growing concern in recent years.

The emergence and increase in prevalence of resistance to cephalosporins amongst isolates of Salmonella from food animals imposes a public health threat. The aim of the present study was to investigate the prevalence and characteristics of CTX-M-producing Salmonella isolates from raw meat and food animals. 27 of 152 (17.76%) Salmonella isolates were ESBL-positive including 21/70 (30%) from food animals and 6/82 (7.32%) from raw meat. CTX-M-55 was the most prevalent ESBL type observed (12/27, 44.44%). 7 of 12 CTX-M-55-positive Salmonella isolates were Salmonella Indiana, 2 were Salmonella Typhimurium, 2 were Salmonella Chester, and the remaining isolate was not typeable. Eight CTX-M-55-positive Salmonella isolates were highly resistant to fluoroquinolones (MIC_CIP = 64 ug/mL) and co-harbored aac(6’)-Ib-cr and oqxAB. Most of the CTX-M-55 positive isolates (11/12) carried bla_CTX-M-55 genes on the chromosome, with the remaining isolate carrying this gene on a transferable 280 kb IncHI2 plasmid. A chromosomal bla_CTX-M-55 gene from one isolate transferred onto a 250 kb IncHI2 plasmid which was subsequently conjugated into recipient strain J53. PFGE and MLST profiles showed a wide range of strain types were carrying bla_CTX-M-55. Our study demonstrates the emergence and prevalence of foodborne Salmonella harboring a chromosomally located bla_CTX-M-55 in China. The co-existence of PMQR genes with bla_CTX-M-55 in Salmonella isolates suggests co-selection and dissemination of resistance to both fluoroquinolones and cephalosporins in Salmonella via the food chain in China represents a public health concern.

Development of fluoroquinolone resistance can involve several mechanisms that include chromosomal mutations in genes (gyrAB and parCE) encoding the target bacterial topoisomerase enzymes, increased expression of the AcrAB-TolC efflux system, and acquisition of transmissible quinolone-resistance genes. In this study, 176 Salmonella isolates from animals with a broad range of ciprofloxacin MICs were collected to analyze the contribution of these different mechanisms to different phenotypes. All isolates were classified according to their ciprofloxacin susceptibility pattern into five groups as follows: highly resistant (HR), resistant (R), intermediate (I), reduced susceptibility (RS), and susceptible (S). We found that the ParC T57S substitution was common in strains exhibiting lowest MICs of ciprofloxacin while increased MICs depended on the type of GyrA mutation. The ParC T57S substitution appeared to incur little cost to bacterial fitness on its own. The presence of PMQR genes represented an route for resistance development in the absence of target-site mutations. Switching of the plasmid-mediated quinolone resistance (PMQR) gene location from a plasmid to the chromosome was observed and resulted in decreased ciprofloxacin susceptibility; this also correlated with increased fitness and a stable resistance phenotype. The overexpression of AcrAB-TolC played an important role in isolates with small decreases in susceptibility and expression was upregulated by MarA more often than by RamA. This study increases our understanding of the relative importance of several resistance mechanisms in the development of fluoroquinolone resistance in Salmonella from the food chain.

To obtain a broad molecular epidemiological characterization of plasmid-mediated AmpC β-lactamase CMY-2 in Escherichia coli isolates from food animals in China. A total of 1083 E. coli isolates from feces, viscera, blood, drinking water, and sub-surface soil were examined for the presence of CMY-2 β-lactamases. CMY-2-producing isolates were characterized as follows: the bla_CMY-2 genotype was determined using PCR and sequencing, characterization of the bla_CMY-2 genetic environment, plasmid sizing using S1 nuclease pulsed-field gel electrophoresis (PFGE), PCR-based replicon typing, phylogenetic grouping, XbaI-PFGE, and multi-locus sequence typing (MLST). All 31 CMY-2 producers were only detected in feces, and presented with multidrug resistant phenotypes. All CMY-2 strains also co-harbored genes conferring resistance to other antimicrobials, including extended spectrum β-lactamases genes (bla_CTX-M-14 or bla_CTX-M-55), plasmid-mediated quinolone resistance determinants (qnr, oqxA, and aac-(6′)-Ib-cr), floR and rmtB. The co-transferring of bla_CMY-2 with qnrS1 and floR (alone and together) was mainly driven by the Inc A/C type plasmid, with sizes of 160 or 200 kb. Gene cassette arrays inserted in the class 1 or class 2 integron were amplified among 12 CMY-2 producers. CMY-2 producers belonged to avirulent groups B1 (n = 12) and A (n = 11), and virulent group D (n = 8). There was a good correlation between phylogenetic groups and sequence types (ST). Twenty-four STs were identified, of which the ST complexes (STC) 101/B1 (n = 6), STC10/A (n = 5), and STC155/B1 (n = 3) were dominant. CMY-2 is the dominant AmpC β-lactamase in food animals and is associated with a transferable replicon IncA/C plasmid in the STC101, STC10, and STC155 strains.

Bacterial resistance to the third-generation cephalosporin antibiotics has become a major concern for public health. This study was aimed to determine the characteristics and distribution of bla_CTX-M-14, which encodes an extended-spectrum β-lactamase, in Escherichia coli isolated from Guangdong Province, China. A total of 979 E. coli isolates isolated from healthy or diseased food-producing animals including swine and avian were examined for bla_CTX-M-14 and then the bla_{CTX-M-14 -}positive isolates were detected by other resistance determinants [extended-spectrum β-lactamase genes, plasmid-mediated quinolone resistance, rmtB, and floR] and analyzed by phylogenetic grouping analysis, PCR-based plasmid replicon typing, multilocus sequence typing, and plasmid analysis. The genetic environments of bla_CTX-M-14 were also determined by PCR. The results showed that fourteen CTX-M-14-producing E. coli were identified, belonging to groups A (7/14), B1 (4/14), and D (3/14). The most predominant resistance gene was bla_TEM (n = 8), followed by floR (n = 7), oqxA (n = 3), aac(6′)-1b-cr (n = 2), and rmtB (n = 1). Plasmids carrying bla_CTX-M-14 were classified to IncK, IncHI2, IncHI1, IncN, IncFIB, IncF or IncI1, ranged from about 30 to 200 kb, and with insertion sequence of ISEcp1, IS26, or ORF513 located upstream and IS903 downstream of bla_CTX-M-14. The result of multilocus sequence typing showed that 14 isolates had 11 STs, and the 11 STs belonged to five groups. Many of the identified sequence types are reported to be common in E. coli isolates associated with extraintestinal infections in humans, suggesting possible transmission of bla_CTX-M-14 between animals and humans. The difference in the flanking sequences of bla_CTX-M-14 between the 2009 isolates and the early ones suggests that the resistance gene context continues to evolve in E. coli of food producing animals.