Emergence of Multidrug-Resistant Salmonella enterica in Lagos: Insights from MALDI–TOF MS and β-Lactamase Profiling

Non-typhoidal Salmonella (NTS) is a major cause of foodborne illness and invasive infections, particularly in low-resource settings. This study aimed to identify and characterise Salmonella enterica isolates from human, animal, and environmental sources in Lagos, Nigeria, using Matrix-Assisted Laser Desorption Ionisation–Time of Flight Mass Spectrometry (MALDI–TOF MS), and to evaluate their antimicrobial resistance profiles and β-lactamase production. A total of 2,522 samples were collected and processed using culture-based methods, biochemical profiling (Microbact 24E), and MALDI–TOF MS. Antimicrobial susceptibility testing was performed via the Kirby–Bauer disc diffusion method. AmpC β- lactamase and extended-spectrum β-lactamase (ESβL) production were assessed using disc antagonism and double disc synergy tests. Salmonella was detected in 1.9% of samples, with the highest prevalence in environmental effluents (12%), followed by animal sources (4.2%) and human clinical samples (0.95%). Forty-eight isolates were recovered, predominantly Salmonella Newbrunswick, alongside Bredeney, Choleraesuis, Abortusequi, Montevideo, and untyped Salmonella spp. All isolates were resistant to ampicillin, cefoxitin, amikacin, gentamicin, tetracycline, and chloramphenicol. Imipenem showed the highest susceptibility (93.75%), while ciprofloxacin and nitrofurantoin had moderate activity. AmpC β- lactamase production was confirmed in 22.9% of isolates, mainly among Choleraesuis and Newbrunswick; no ESβL producers were detected. The emergence of multidrug-resistant Salmonella and AmpC producers across clinical and environmental settings highlights the need for enhanced surveillance, antimicrobial stewardship, and rapid diagnostic tools such as MALDI–TOF MS to guide effective treatment and containment strategies.