Highly virulent colistin-susceptible Salmonella Havana ST1524 carrying the mcr-9.1 gene in food

Salmonella enterica subsp. enterica serovar Havana is a potential pathogenic serotype that can cause human foodborne illness. Therefore, we have conducted a microbiological and genomic surveillance study of Salmonella Havana from food in Brazil. Strikingly, we identified a colistin-susceptible Salmonella Havana strain (NC_STy4) carrying the mcr-9 gene, in salted chicken breast. Genomic analysis revealed that Salmonella Havana strain belonged to the international sequence type (ST) ST1524, circulating at the human-animal-environmental interface. Although the resistome analysis confirmed the presence of mcr-9 and aac(6')-Iaa genes, as well as a parC (Thr57→Ser) mutation, this strain exhibited a pansuceptible phenotype. Noteworthy, Salmonella Havana NC_STy4 exhibited a highly virulent behavior, killing 100 % of Galleria mellonella larvae at 13 h post-infection. In this regard, invasins (invA, ibeB), non-fimbrial adherence (misL), fimbrial adherence (fimH), magnesium uptake (mgtB and mgtC), regulation (phoP, phoQ, and pipB), secretion system (ssaHJKLNOQRU), and translocated effector (sopB/sigD and sopE2) virulence genes were predicted. IncHI2 and IncHI2A plasmids were also identified. While these findings underscore the urgent need for surveillance and monitoring foodborne pathogens carrying a wide virulome, colistin-susceptible Salmonella isolates may be silent reservoirs of the mcr genes.