Molecular Insights into the MdtABC Efflux Pump Genes in Clinical Salmonella typhi Isolates from Typhoid Patients

Typhoid fever is a potentially fatal systemic infection caused by Salmonella typhi (S. typhi), a Gram-negative, rod-shaped, facultative anaerobe. This study aimed to characterize the efflux pump genes, molecular mutations, and antibiotic susceptibility patterns in clinical isolates.

A total of 2950 blood samples were collected from suspected typhoid patients. Of these, 380 (12.88%) bacterial isolates were recovered, of which 236 (62.10%) were Gram-negative. S. typhi was detected in 95 isolates (25% of all bacterial isolates), corresponding to an overall prevalence of 3.22%. Identification was performed using API 20-E strips and partial 16S rRNA gene sequencing.

Resistance profiling classified the isolates into multidrug-resistant (MDR, 13.68%) and extensively drug-resistant (XDR, 86.31%) categories. High resistance was observed to ampicillin (98.94%) and chloramphenicol (93.68%), whereas no resistance was detected to azithromycin or meropenem. Minimum inhibitory concentration (MIC) testing using E-Strips showed the highest MIC values for trimethoprimsulfamethoxazole (MIC₅₀: 0.125 μg/ml, MIC₉₀: 0.50 μg/ml) and the lowest for ciprofloxacin (MIC₅₀: 0.008 μg/ml, MIC₉₀: 0.023 μg/ml). Among the efflux pump genes, mdtB (32.63%) was the most prevalent, followed by mdtC (30.52%) and mdtA (29.47%). Multiple mutations were identified and evaluated using the I-Mutant 2.0 server. Protein structures were modeled using trRosetta and Discovery Studio. Phylogenetic analysis using MEGA-11 revealed evolutionary relationships with other Salmonella enterica serovars.

A serious public health issue that increases mortality and morbidity rates worldwide is antibiotic resistance. Among the mechanisms underlying antibiotic resistance in bacteria are degradation of antibiotics and/or modification of enzymes, changing drug targets to alter the affinity of antibiotics, altering bacterial surfaces by changing the expression of external membrane proteins, and active bacterial efflux of drugs. The drug's binding affinity can be changed by mutations in efflux pump genes that change the shape of the substrate- binding pocket. These structural alterations may improve the pump's capacity to identify and eliminate antibiotics, reducing intracellular drug levels and increasing resistance to several drugs.

This study highlights the alarming rise of multidrug resistance in S. typhi isolates and identifies key genetic mutations that may influence efflux pump function, potentially contributing to enhanced antibiotic resistance.