A study by North Carolina State University (NC State) researchers has demonstrated a concerning presence of antimicrobial resistance (AMR) genes among Salmonella isolated from retail poultry meat.

AMR is a major threat to global public health, causing a growing number of difficult-to-treat infections in humans, and recognized by the World Health Organization (WHO) as one of the top ten health challenges facing humanity in the 21^st century. At present, AMR illnesses cause more than 700,000 deaths annually and are expected to outnumber cancer fatalities by 2050. The excessive use of antibiotics in farmed animals is a leading cause of rising AMR among bacteria, such as Salmonella.

The blaCTX-M-65 gene, an AMR gene with traits that facilitate easy transfer between bacteria, has been increasingly reported in Salmonella in food animals. Therefore, the researchers sought to understand the prevalence of and characterize the blaCTX-M-65 gene in Salmonella enterica isolated from poultry meat at retail.

To do so, the NC State researchers analyzed U.S. National Antimicrobial Resistance Monitoring System (NARMS) data for poultry products sampled from North Carolina retail stores between 2020 and 2024, including 132 S. enterica isolates representing 25 serovars, with S. Infantis (34 isolates) and S. Kentucky being the most prevalent. The isolates were then subjected to antibiotic susceptibility testing.

Of the 132 Salmonella isolates, 14 were multidrug resistant (MDR), which whole genome sequencing (WGS) analysis revealed as belonging to three serovars: S. Infantis (11 isolates), S. Seftenberg (one isolate), and S. I -:r:1,5 (two isolates). All 14 of the isolates carried the blaCTX-M-65 gene, and 13 isolates harbored a quinolone resistance-determining mutation. Genes encoding resistance to aminoglycosides, sulfonamides, and tetracyclines were also identified.

The presence of genes that could provide potential environmental survival advantages were also noted, raising concern about Salmonella persistence in food production environments.

Furthermore, the presence of Sequence Type 32 (ST32) among S. Infantis isolates, an ST frequently associated with this serovar, supports the notion that certain lineages of S. Infantis are adept at acquiring and disseminating AMR genes via mobile genetic elements, such as plasmids. ST32 was also found in the two S. I -:r:1,5 isolates, which may indicate a shared evolutionary pathway between the serovars regarding the acquisition of AMR traits. Previous studies have shown a high prevalence of MDR associated with ST32, suggesting a history of successful gene transfer between bacteria.

Additionally, in the sole S. Seftenberg isolate, blaCTX-M-65 was assigned to ST14, pointing to the adaptability of AMR genes across different Salmonella serovars. “This highlights the importance of continuous surveillance, especially in understudied or less prevalent serovars, to prevent the silent spread of critical resistance genes such as blaCTX-M-65,” said the researchers.

Overall, the detection of blaCTX-M-65 across multiple serovars suggests that its spread is not restricted to a single lineage or serovar; rather, it is a widespread phenomenon. The researchers underline the importance of routine surveillance for MDR Salmonella enterica serovars, particularly those harboring blaCTX-M-65, and call for comprehensive strategies, including genomic monitoring and responsible antimicrobial use, to curb the spread of AMR in food production settings.

The study was authored by Daniel F. M. Monte, Ph.D.; Erin Harrell, M.S.; Lyndy Harden; and Siddhartha Thakur, D.V.M., Ph.D. of the Department of Population Health and Pathobiology in NC State’s College of Veterinary Medicine. The full study can be read in Scientific Reports.