Food Workers as a Reservoir of Extended-Spectrum-Cephalosporin-Resistant Salmonella Strains in Japan

Abstract

Dissemination of extended-spectrum-cephalosporin (ESC)-resistant Salmonella, especially extended-spectrum-β-lactamase (ESBL)-producing Salmonella, is a concern worldwide. Here, we assessed Salmonella carriage by food workers in Japan to clarify the prevalence of ESC-resistant Salmonella harboring blaCTX-M. We then characterized the genetic features, such as transposable elements, of blaCTX-M-harboring plasmids using whole-genome sequencing. A total of 145,220 stool samples were collected from food workers, including cooks and servers from several restaurants, as well as food factory workers, from January to October 2017. Isolated salmonellae were subjected to antimicrobial susceptibility testing (disk diffusion method), and whole-genome sequencing was performed for Salmonella strains harboring blaCTX-M. Overall, 164 Salmonella isolates (0.113%) were recovered from 164 samples, from which we estimated that at least 0.113% (95% confidence interval [CI]: 0.096 to 0.132%) of food workers may carry Salmonella. Based on this estimation, 3,473 (95% CI = 2,962 to 4,047) individuals among the 3,075,330 Japanese food workers are likely to carry Salmonella. Of the 158 culturable isolates, seven showed resistance to ESCs: three isolates harbored blaCMY-2 and produced AmpC β-lactamase, while four ESBL-producing isolates harbored blaCTX-M-14 (n = 1, Salmonella enterica serovar Senftenberg) or blaCTX-M-15 (n = 3, S. enterica serovar Haardt). blaCTX-M-15 was chromosomally located in the S. Haardt isolates, which also contained ISEcp1, while the S. Senftenberg isolate contained an IncFIA(HI1)/IncHI1A/IncHI1B(R27) hybrid plasmid carrying blaCTX-M-14 along with ISEcp1. This study indicates that food workers may be a reservoir of ESBL-producing Salmonella and associated genes. Thus, these workers may contribute to the spread of blaCTX-M via plasmids or mobile genetic elements such as ISEcp1. IMPORTANCE Antimicrobial-resistant Salmonella bacteria arise in farm environments through imprudent use of antimicrobials. Subsequently, these antimicrobial-resistant strains, such as extended-spectrum-β-lactamase (ESBL)-producing Salmonella, may be transmitted to humans via food animal-derived products. Here, we examined Salmonella carriage among food handlers in Japan. Overall, 164 of 145,220 fecal samples (0.113%) were positive for Salmonella. Among the 158 tested isolates, four were identified as ESBL-producing isolates carrying ESBL determinants blaCTX-M-15 or blaCTX-M-14. In all cases, the genes coexisted with ISEcp1, regardless of whether they were located on the chromosome or on a plasmid. Our findings suggest that food workers may be a reservoir of ESBL-producing strains and could contribute to the spread of resistance genes from farm-derived Salmonella to other bacterial species present in the human gut.

Epidemiology and Characterization of CTX-M-55-Type Extended-Spectrum β-Lactamase-Producing Salmonella enterica Serovar Enteritidis Isolated from Patients in Shanghai, China

“…ESBL-SE are commonly considered donors for the transmission of antibiotic resistance through mobile genetic elements among different hosts [ 20 ]. Worryingly, CTX-M-type ESBL-SE are increasingly being detected from various sample types in multiple countries, such as China, the United States, England, Wales, and Japan [ 24 ]. As bla CTX-M-55 -positive ESBL-SE show high-level resistance to extended-spectrum cephalosporins and often cross-tolerate other antibiotics [ 15 ], more attention should be focused on the prevalence of bla CTX-M-55 -carrying ESBL-SE.…”

Transmission of extended-spectrum cephalosporin-resistant <i>Salmonella</i> harboring a <i>bla</i>_CMY-2-carrying IncA/C₂ plasmid chromosomally integrated by IS<i>Ecp1</i> or IS<i>26</i> in layer breeding chains in Japan

“…Antimicrobial susceptibility tests were performed using the disk diffusion method in accordance with the Clinical and Laboratory Standards Institute (CLSI) guidelines [4] previous reports [28,29]. We also performed boronic acid tests using CPDX and cefoxitin disks as per our previous reports [28,29]. Escherichia coli strain ATCC 25922 was used as the quality control strain in all the assays in accordance with CLSI guidelines [5].…”

“…Therefore, investigating the prevalence of antimicrobial-resistant Salmonella, especially ESC-resistant Salmonella, in the LB-EP chains, including the upper levels of the layer breeding chains, is important. ESC-resistant Salmonella is generated when bla genes encoding AmpC β-lactamase and extended-spectrum β-lactamase (ESBL) are acquired by Salmonella [8,[27][28][29]. bla genes translocate between plasmids and chromosomes containing insertion sequences (ISs) among Enterobacteriaceae, including Salmonella [8,23,27,29].…”

“…In fact, blaCTX-M-14 was translocated by ISEcp1 from a plasmid into another plasmid in Salmonella enterica subsp. enterica serovar Senftenberg (S. Senftenberg), whereas blaCTX-M-15 was translocated by ISEcp1 from a plasmid into S. Haardt chromosomes [29]. Obtaining further knowledge about IS26 is also important because IS26 may generate multiple antimicrobialresistant Salmonella via gene integration [9,23,27].…”

“…ESC-resistant Salmonella is generated when bla genes encoding AmpC β-lactamase and extended-spectrum β-lactamase (ESBL) are acquired by Salmonella [ 8 , 27 , 28 , 29 ]. bla genes translocate between plasmids and chromosomes containing insertion sequences (ISs) among Enterobacteriaceae, including Salmonella [ 8 , 23 , 27 , 29 ]. In fact, bla CTX-M-14 was translocated by IS Ecp1 from a plasmid into another plasmid in Salmonella enterica subsp.…”