P-144. Antimicrobial resistance to ceftriaxone and ciprofloxacin is increasingly observed in Salmonella Infantis isolates from clinical and retail meat sources in the United States

Abstract Background Nontyphoidal Salmonella (NTS), a leading cause of bacterial foodborne infections in the United States, is typically acquired from contaminated meat. Antimicrobial resistance in NTS varies by serotype. S. Infantis is one of the top ten most frequently isolated NTS and resistance to first-line antibiotics is emerging in this serotype. The National Antimicrobial Resistance Monitoring System (NARMS) tracks Salmonella from humans and retail meats nationwide.Figure. Percentages of isolates resistant to amoxicillin-clavulanate (AMC), ampicillin, ceftriaxone or with decreased susceptibility to ciprofloxacin (DSC) in patient (blue) and meat (orange) samples. Methods We analyzed whole genome sequences and antimicrobial susceptibility profiles for each S. Infantis isolate from two NARMS programs: 1. CDC--clinical isolates submitted by public health laboratories; 2. FDA--food isolates identified by collaborating laboratories through prospective testing for Salmonella in retail meats. Results Among 620 S. Infantis isolates from patients during 2012-2023, 157 (25.3%) had decreased susceptibility to ciprofloxacin (DSC) and 107 (17.3%) were ceftriaxone-resistant. Resistance to ampicillin was found in 118 (19.0%) isolates, to amoxicillin-clavulanate in 16 (2.6%), and to azithromycin in two (0.3%). Genetic determinants: 93.5% (100/107) of ceftriaxone-resistant isolates carried extended-spectrum β-lactamase (ESBL) genes [blaCTX-M-65, n=86] and [blaCMY−2, n=14]; 99.3% (156/157) of isolates with DSC had gyrA mutations. Among 963 S. Infantis isolates from retail meat sources during 2002-2021, 836 (86.9%) demonstrated DSC and 401 (41.6%) were ceftriaxone-resistant. Resistance to ampicillin was found in 403 (41.9%) isolates and to amoxicillin-clavulanate in 14 (1.5%). Genetic determinants: 99% (397/401) of ceftriaxone-resistant isolates carried ESBLs [blaCTX-M-65, n=384] and [blaCMY−2, n=13]; 99.8% (834/836) of isolates with DSC had gyrA mutations. While detected in < 1% of S. Infantis in 2012, most recently ESBLs blaCTX-M-65 were found in 16.9% (11/65 in 2021) of isolates from clinical samples and 47.7% (91/213 in 2023) from retail meats. Conclusion Genetic analysis shows that S. Infantis isolates from patients, and more frequently from retail meat, harbor ESBL and/or gyrA mutations. Resistance to first-line therapies (ceftriaxone and/or ciprofloxacin) in this particular Salmonella serotype is emerging as a therapeutic challenge. Disclosures All Authors: No reported disclosures