Emergence of plasmid-mediated colistin resistance -3.5 gene in and isolated from healthy individual in Thailand.

spp. are Gram-negative bacteria commonly found in environments and intestinal tracts of humans and animals. They are generally susceptible to third-generation cephalosporins, carbapenems and colistin. However, several antibiotic resistant genes have been increasingly reported in spp., which leads to the postulation that spp. could potentially be a reservoir for spreading of antimicrobial resistant genes. In this study, we characterized two colistin-resistant spp. isolated from the feces of a healthy individual in Thailand. Based on MALDI-TOF and ribosomal multilocus sequence typing, both strains were identified as and . Genomic analysis and S1-nuclease pulsed field gel electrophoresis/DNA hybridization revealed that and harbored gene on pSY_CS01 and pSY_CA01 plasmids, respectively. Both plasmids belonged to IncFII(pCoo) replicon type, contained the same genetic context (TnISΔTnAsIS) and exhibited high transferring frequencies ranging from 1.03×10 - 4.6×10 CFU/recipient cell J53. Colistin-MICs of transconjugants increased ≥ 16-fold suggesting that on these plasmids can be expressed in other species. However, beside , other major antimicrobial resistant determinants in multidrug resistant Enterobacterales were not found in these two isolates. These findings indicate that gene continued to evolve in the absence of antibiotics selective pressure. Our results also support the hypothesis that could be a reservoir for spreading of antimicrobial resistant genes. To the best of our knowledge, this is the first report that discovered human-derived spp. that harbored but no other major antimicrobial resistant determinants. Also, this is the first report that described the presence of gene in and in .