Tackling Drug Resistance and Virulence in Proteus mirabilis Infections: Evaluating Rosemary Extract as a Complementary Therapeutic Agent

Background and Aims The rising antimicrobial resistance in Proteus mirabilis , a primary contributor to catheter‐associated urinary tract infections, underscores the urgent need to explore alternative therapeutic approaches. In this study, we evaluated the antibacterial and antivirulence potential of Rosmarinus officinalis L. extract (ROE) against P. mirabilis and concurrently assessed clinical isolates for the presence of key resistance and virulence genes. Methods Forty multi–drug‐resistant (MDR) P. mirabilis isolates were analyzed using multiplex PCR to detect carbapenemase genes ( b l a NDM , b l a OXA family, b l a SIM−1 , b l a VIM−1 ) and uniplex PCR to identify virulence determinants ( mrp A, pmf A, rsb A, rsm A, lux S). ROE was extracted and analyzed using high‐performance liquid chromatography (HPLC). The antibacterial, antibiofilm, and antiswarming effects of the extract were assessed. Results All isolates carried all five virulence genes. Carbapenemase gene profiling revealed b l a VIM−1 in 20% of isolates, while b l a OXA−23 , b l a OXA−24 , and b l a OXA−58 were detected in 10%, 2.5%, and 2.5% of isolates, respectively. Neither b l a SIM−1 nor b l a NDM was identified. HPLC analysis of ROE identified rosmarinic acid as the predominant compound. Although the agar well diffusion assay indicated no bactericidal activity, the broth microdilution method determined a minimum inhibitory concentration (MIC) of 8 mg/mL for both reference and MDR strains. At sub‐MIC levels, ROE significantly inhibited swarming motility (up to 86% at 250 μg/mL) and biofilm formation (up to 78% in the reference strain and 76% in MDR isolates at 15.6 μg/mL) in a dose‐dependent manner. Notably, biofilm eradication activity was observed only against preformed biofilms of the reference strain, achieving up to 53% reduction. MTT cytotoxicity assays confirmed that ROE in low concentrations (1000‐15.6 μg/mL) was nontoxic to Vero cells. Conclusion These results highlight the concerning prevalence of carbapenemase genes among clinical P. mirabilis isolates and identify ROE as a promising, nontoxic candidate for targeting motility and biofilm formation. These findings suggest ROE may offer a viable strategy for managing drug‐resistant P. mirabilis infections.