Antimicrobial susceptibility and plasmid profiles of Salmonella Typhi in Sagamu, southwest Nigeria

Background: Salmonella enterica serovar Typhi is the causative agent of typhoid fever, a major public health concern in Nigeria and other low-resource settings. The emergence of antimicrobial resistance (AMR), often plasmid-mediated, poses a serious challenge to treatment. Local surveillance data on resistance and plasmid carriage are critical for guiding effective interventions. This study investigated the antimicrobial susceptibility and plasmid profiles of clinical Salmonella Typhi isolates from stool samples in Sagamu, southwest Nigeria.Methodology: A cross-sectional study of 100 patients with clinical features of typhoid fever and reactive for S. Typhi antibody by the Widal agglutination test, was conducted across health facilities in Sagamu, including Olabisi Onabanjo University Teaching Hospital (OOUTH), private hospitals, and selected diagnostic laboratories. Stool samples were collected from each participant and cultured on selenite F broth enrichment, followed by subculture on MacConkey and Salmonella-Shigella agar plates. Salmonella Typhi was identified by colony morphology, Gram stain reaction, biochemical test scheme of triple sugar iron, citrate, and urease tests, and confirmed by serotyping with agglutination of sample using specific antisera. Antimicrobial susceptibility testing was performed using the Kirby-Bauer disk diffusion method. Plasmid DNA was extracted by the alkaline lysis method, and electrophoresis was used for plasmid size determination. Mating experiments with Escherichia coli K-12 were conducted to assess plasmid transferability. Statistical analysis of data was done using SPSS (version 25), with significance set at p<0.05.Results: Sixty-six isolates were confirmed as S. Typhi. Of these, 28.8% (n=19) were multidrug-resistant (MDR), although all isolates were susceptible to ciprofloxacin and ofloxacin. Plasmids with sizes ranging from 4.6 to 5.7kb were detected in 16 of the 19 MDR isolates, 9 (56.3%) of which successfully transferred their plasmids to E. coli K-12. A significant association was found between plasmid carriage and MDR (OR=447.86, p<0.0001).Conclusion: The findings of this study suggest that plasmid-mediated resistance plays a major role in the emergence of MDR S. Typhi strains in the region, posing a challenge to the effective treatment of typhoid fever. Continuous surveillance and prudent use of antibiotics are necessary to combat the rising threat of AMR in S. Typhi population.