Widespread dissemination of ESBL-producing Salmonella enterica serovar Infantis exhibiting intermediate fluoroquinolone resistance and harboring blaCTX-M-65-positive pESI-like megaplasmids in Chile

Background Multidrug-resistant (MDR) Salmonella Infantis has disseminated worldwide, mainly linked to the consumption of poultry products. Evidence shows dissemination of this pathogen in Chile; however, studies are primarily limited to phenotypic data or involve few isolates. As human cases of Salmonella Infantis infections have substantially increased in recent years, a better understanding of its molecular epidemiology and antimicrobial-resistance profiles are required to inform effective surveillance and control measures. Methods We sequenced 396 Salmonella Infantis genomes and analyzed them with all publicly available genomes of this pathogen from Chile (440 genomes in total), representing isolates from environmental, food, animal, and human sources obtained from 2009 to 2022. Based on bioinformatic and phenotypic methods, we assessed the population structure, dissemination among different niches, and AMR profiles of Salmonella Infantis in the country. Findings The genomic and phylogenetic analyses showed that Salmonella Infantis from Chile comprised several clusters of highly related isolates dominated by sequence type 32. The HC20\_343 cluster grouped an important proportion of all isolates. The latter was the only cluster associated with pESI-like megaplasmids, and up to 12 acquired AMR genes/mutations predicted to result in an MDR phenotype. Accordingly, antimicrobial-susceptibility testing revealed a strong concordance between the AMR genetic determinants and their matching phenotypic expression, indicating that a significant proportion of HC20\_343 isolates produce extended- spectrum β-lactamases and have intermediate fluoroquinolone resistance. HC20_343 Salmonella Infantis were spread among environmental, animal, food, and human niches, showing a close relationship between isolates from different years and sources, and a low intra-source genomic diversity. Interpretation Our findings show a widespread dissemination of MDR Salmonella Infantis from the HC20\_343 cluster in Chile. The high proportion of isolates with resistance to first-line antibiotics and the evidence of active transmission between the environment, animals, food, and humans highlight the urgency of improved surveillance and control measures in the country. As HC20\_343 isolates predominate in the Americas, our results suggest a high prevalence of ESBL- producing Salmonella Infantis with intermediate fluoroquinolone resistance in the continent. Funding Agencia de Investigación y Desarrollo de Chile (ANID) through FONDECYT de Postdoctorado Folio 3230796 and Folio 3210317, FONDECYT Regular Folio 1231082, and ANID – Millennium Science Initiative Program – ICN2021_044. Evidence before the study In the last decade, emergent multidrug-resistant Salmonella Infantis has spread worldwide, primarily linked to poultry product consumption. However, in most countries from the Americas Region, such as Chile, the extent of the dissemination of emergent Salmonella Infantis and its molecular epidemiology remains unknown. In May and September 2023, an online search was conducted using the Google engine and the PMC database with the terms “ Salmonella ,” “Infantis,” and “Chile,” with no language restrictions. We assessed the results to select those presenting antimicrobial resistance, epidemiologic, or genomic data directly associated with isolates from Chile (13 studies). The selected studies showed that the prevalence of Salmonella Infantis in poultry-meat production systems, its resistance to different antibiotics, and the number of human cases of infection caused by this serovar have increased since 2014-2016. However, these reports were limited to phenotypic data or involved the genomic analysis of a few isolates (<50) obtained from the same source. No study has assessed the genomic epidemiology of the Salmonella Infantis population at the country level. Added value of this study Here, we present the first large-scale genomic epidemiology analysis of Salmonella Infantis in Chile, including isolates from environmental, food, animal, and human sources obtained from 2009 to 2022. We found that Salmonella Infantis in Chile is divided into several clusters of highly related isolates and that only a single cluster, the HC20\_343, was associated with multiple antimicrobial-resistance determinants and pESI-like megaplasmids. We also report that isolates from this cluster are widespread among most sources, including irrigation water, poultry, food, and human cases. Detection of AMR determinants coupled with antimicrobial- susceptibility testing indicated that most HC20\_343 isolates are ESBL-producers and have intermediate resistance to ciprofloxacin. Population structure analysis of this foodborne pathogen evidenced an active transmission of MDR Salmonella Infantis between different niches. This study reveals the widespread dissemination of MDR Salmonella Infantis in Chile. Implications of all the available evidence The evidence indicates that emerging Salmonella Infantis from the HC20\_343 cluster is spreading among various niches, including irrigation water, poultry, and food, causing human infections in Chile. Its resistance to first-line antibiotics used for treating salmonellosis in individuals with a higher risk of severe or invasive infections is concerning. Currently, most surveillance and control efforts to reduce salmonellosis in Chile are focused on the poultry industry, and the study of outbreaks does not include whole-genome sequence analyses. Our findings highlight the urgent necessity to improve the surveillance and control measures to include agricultural waters to prevent contamination of produce and the further dissemination of resistance genes in the environment. As the HC20\_343 cluster is highly prevalent in the Americas, further research involving large-scale genomic population analyses would shed light on the extent of the dissemination and transmission routes of emergent Salmonella Infantis in the continent and may contribute to informing surveillance and control policies. ### Competing Interest Statement The authors have declared no competing interest.