Co-existence of mphA, oqxAB and blaCTX-M-65 on the IncHI2 Plasmid in highly drug-resistant Salmonella enterica serovar Indiana ST17 isolated from retail foods and humans in China

Salmonellosis is an important global foodborne disease, and Salmonella isolates that are resistant to a wide variety of antibiotics have become a major public health concern. The aim of this study was to investigate the distribution and transmission of mphA gene in azithromycin-resistant Salmonella isolates and characterize the plasmid bearing mphA. In this study, the azithromycin resistance was shown in 15 Salmonella enterica serovar Indiana (S. Indiana) isolates from chicken, duck and humans with concurrent resistance to ciprofloxacin and ceftriaxone, two of which were resistant to all antibiotics tested except colistin and imipenem. MLST and PFGE results suggested that these 15 isolates were closely related in phylogeny. The azithromycin resistance gene (mphA), extended-spectrum beta-lactamase (ESBL)-encoding genes (bla(CTX-M-65), blaCTX-M-14 and bla(CTX-M-15)) and plasmid-mediated quinolone resistance (PMQR) genes [oqxAB, qepA, qnrA and aac(6')-Ib-cr] with mutations in the quinolone resistance determining region (QRDR) (gyrA and parC) were identified in these S. Indiana isolates. Plasmid conjugation and transformation were performed in these 15 isolates. The co-existence of the mphA gene with oqxAB-bla(CTX-M-65) was observed on the IncHI2 plasmid with a size of similar to 240 kbp in 7 transformants, and these 7 transformants exhibited the resistance to azithromycin (MIC from 32 mu g/mL to 64 mu g/mL) and ceftriaxone (MIC= 256 mu g/mL). The complete sequence of an IncHI2 type plasmid, p13520, was determined. An approximately 80-kbp of multidrug resistance region (MRR) comprising 29 antimicrobial resistance genes (ARGs) including mphA, oqxAB, blaCTX-M-65, fosA3 and rmtB interspersed with different insertion sequences (ISs) (IS26, ISAba1, IS91, IS1006 and IS903B) was located on the plasmid. The typical IS26-mphA-mrx-mphR-IS6100 transposable structure was identified in the IncHI2 plasmid of nine transformants via genome sequencing and PCR mapping, which suggested that azithromycin resistance in S. Indiana was probably due to the dissemination of the IncHI2 plasmid carrying the mphA-mrx-mphR cluster. This study highlighted the emergence of S. Indiana isolates with concurrent resistance to azithromycin, ciprofloxacin and ceftriaxone in humans and retail foods in China. This is likely due to the co-existence of mphA, oqxAB and bla(CTX-M-65) carried by the IncHI2 plasmid. These findings emphasized the importance of ongoing surveillance for the prevalence and transmission of the p13520like plasmid in S. Indiana, so as to better understand the potential threat to public health.