Biofilm formation and associated biomechanical traits co-segregate with multidrug resistance in typhoidal Salmonella

Typhoidal Salmonella continues to pose a severe public health threat, with its management increasingly complicated by the rise of antimicrobial resistance. This study investigated 50 clinical isolates of Salmonella Typhi (S. Typhi) and S. Paratyphi to delineate the association between antibiotic resistance, biofilm formation, and nanoscale mechanical traits. Our results revealed that 22% of isolates were multidrug-resistant (MDR), displaying the classical resistance pattern against ampicillin, chloramphenicol, and trimethoprim-sulfamethoxazole. Among these resistant isolates, 54% formed biofilms, and this trait was strongly associated with multidrug resistance; 100% of MDR isolates were biofilm-positive (p = 0.001). Atomic force microscopy (AFM) revealed a distinct "hard-shell" bio-mechanical phenotype in biofilm-positive isolates, exhibiting significantly higher stiffness (31.3 ± 9.8 vs. 8.2 ± 2.3 kPa), adhesion force (17.8 ± 4.6 vs. 5.4 ± 1.4 nN), and surface roughness (11.6 ± 3.2 vs. 3.6 ± 1.0 nm) (p < 0.001 for all). This mechanical reinforcement was accompanied by a 2.7-fold increase in cell surface hydrophobicity (80.4 ± 8.9% vs. 30.3 ± 11.9%) and a 13.5-fold enhancement in desiccation survival (40.4 ± 10.7% vs. 3.0 ± 2.9%). Correlation analysis revealed these traits are highly interdependent (ρ = 0.78--0.89, p < 0.001), forming a cohesive "hard-shell" persistence phenotype. In summary, multidrug-resistant Salmonella possesses a unified trait that enhances its structural strength, ability to adhere, and environmental survival.