Comparative genomics of the monophasic variant of Salmonella Typhimurium: analysis of Colombian genomes and their relationship with international lineages

ABSTRACT The monophasic variant of Salmonella enterica serovar Typhimurium (STVM) represents a growing threat to global public health owing to its wide dissemination, capacity to adapt to multiple hosts, and antimicrobial resistance. In this study, 98 STVM isolates recovered in Colombia (57 from humans and 41 from pig farms and abattoirs) were genomically characterized between 2015 and 2022 and compared with 102 representative genomes of international lineages by whole-genome sequencing (WGS) and phylogenomic analysis. Phylogenomic analysis revealed the existence of two well-defined endemic lineages in Colombia (Clusters 1 and 2), arising from independent introduction events and subsequent local stabilization. Both lineages comprise isolates of human and swine origin without clear phylogenetic separation by host species, suggesting active zoonotic cocirculation and closely integrated interspecies transmission dynamics. Marked differences were observed in the accessory genome, including the differential presence of prophages (e.g., Gifsy-2, Fels-2, SW9), virulence plasmids, and resistance profiles. The Colombian lineages exhibited a high frequency of the pSTV plasmid (85%, n = 84/98) and a substantial burden of resistance determinants to quinolones (such as qnrB19 , 74.5%; gyrA S83F mutation, 19.4%), phenicols ( floR ), tetracyclines ( tetA , tetB ), β-lactams ( bla TEM-1B), and heavy metals. In contrast, the Colombian genomes clustered with the European ST34 lineage lacked pSTV but retained resistance and heavy metal operons. These findings demonstrate that international and endemic lineages coexist in Colombia with independent evolutionary trajectories, underscoring the need to strengthen genomic surveillance under the “One Health” approach to anticipate emerging threats and develop integrated control strategies. IMPORTANCE The monophasic variant of Salmonella Typhimurium (STVM) has emerged as a predominant serovar in both humans and swine internationally. In Colombia, a fundamental question driving this study was whether local isolates belonged to international lineages or represented endemic strains. This study provides the first comprehensive genomic characterization demonstrating that two Colombian endemic lineages circulate simultaneously between humans and pigs without phylogenetic separation by host species, confirming active zoonotic transmission. The results demonstrate the coexistence of both lineages, each with distinctive repertoires of mobile genetic elements and specific antimicrobial resistance profiles. Understanding these transmission dynamics and evolutionary patterns is crucial for public health, as it demonstrates how zoonotic pathogens can establish locally adapted lineages with distinct resistance patterns. The genomic evidence of sustained interspecies circulation highlights the critical need for integrated surveillance strategies under the “One Health” framework. This will enable anticipating emerging threats, tracing transmission routes, and developing targeted interventions in food production systems.