Occurrence of multidrug-resistant Salmonella species with resistance genes in recreational surface waters in southern Illinois.

The increasing antimicrobial resistance (AMR) in the environment poses a significant threat to public health. In the United States, AMR Salmonella species are associated with over 200,000 human infections annually. Salmonella spp. are frequently detected in aquatic environments, raising concerns about AMR in natural recreational waters. This study investigated the occurrence of antibiotic-resistant Salmonella spp. in Horseshoe Lake and Richland Creek in southern Illinois, USA. Antimicrobial susceptibility of 47 Salmonella isolates was determined using the Kirby-Bauer and broth microdilution assays. Multidrug-resistant (MDR) isolates were screened for resistance genes, and species identity was confirmed by 16S rRNA gene sequencing. Salmonella isolates showed resistance to ampicillin (70 %), azithromycin (57 %), tetracycline (34 %), gentamicin (9 %), ciprofloxacin (6 %), and nalidixic acid (6 %). Ten isolates were MDR, all exceeding the ampicillin resistance breakpoint, with seven surpassing the tetracycline threshold. Integron or plasmid-borne resistance genes (blaTEM, AmpC, and sul2) were detected in 100 %, 70 %, and 10 % of the MDR strains, respectively. Additional resistance determinants (qnrB, qnrA, and qnrS) were detected in 70 %, 40 %, and 20 % of the MDR strains, respectively. All 10 MDR strains were identified as Salmonella spp.; seven Salmonella enterica and three Salmonella bongori. Multiple antibiotic resistance indices were highest at Richland Creek near urban areas and a wastewater treatment plant, reflecting land use influence on AMR. These findings highlight the need for integrated AMR control under a One Health approach to protect global health.