Role of Plasmids in Antibiotic Resistance and Biofilm Formation Among Clinical Bacterial Isolates

Background: Plasmids are extra-chromosomal circular DNA molecules that often carry virulence and resistance genes, transmitted horizontally among bacteria through conjugation or transformation. Their role makes them central to the global spread of resistance. Methods: In this study, 32 Salmonella typhi isolates were obtained from blood samples of typhoid fever patients. Biofilm formation was assessed by the tissue culture plate (TCP) method, antibiotic susceptibility by disk diffusion (CLSI 2021), and plasmid profiles by electrophoresis. Results: Results showed that all isolates (100%) were strong biofilm producers, enhancing survival against host defences and antibiotic action. Antimicrobial testing revealed high resistance to multiple drugs: cefotaxime and ciprofloxacin (100%), ampicillin (93.7%), piperacillin, and aztreonam (96.8%). In contrast, isolates were fully sensitive to carbapenems such as meropenem (100%). Thus, all were classified as multidrug-resistant (MDR). Plasmid analysis indicated the presence of 3–5 distinct plasmid bands of varying molecular sizes across isolates, suggesting genetic diversity. Conclusions. This diversity likely contributes to the carriage of genes associated with resistance and biofilm formation. The findings highlight a strong correlation between plasmid content, biofilm-forming ability, and MDR in S. typhi. Such traits complicate treatment, given the bacteria’s enhanced survival and resistance capacity. The study underscores the need for detailed plasmid sequencing to identify specific resistance and virulence genes. Understanding this genetic basis may provide insights for novel therapeutic strategies aimed at controlling the spread of resistant S. typhi infections and mitigating public health risks.