University of Guilan Caspian Journal of Environmental Sciences 1735-3033 24 2 2026 04 01 Antimicrobial activity of natural compounds against multidrug-resistant Staphylococcus aureus and Pseudomonas aeruginosa: Effects on viability and resistance expression 343 351 9619 10.22124/cjes.2026.9619 EN Gulmira Kyzdarbekova Sh. Ualikhanov Kokshetau University, Pedagogical Institute, Department of Biology and teaching methods, Kokshetau, Kazakhstan Anna Kornilova M. Kozybaev North Kazakhstan State University, Faculty of Natural sciences, Department of Biology, Petropavlovsk, Kazakhstan Saule Bazarbaeva M. Kozybaev North Kazakhstan State University, Faculty of Natural sciences, Department of Biology, Petropavlovsk, Kazakhstan Elmira Shamshualieva Sh. Ualikhanov Kokshetau University, Pedagogical Institute, Department of Biology and teaching methods, Kokshetau, Kazakhstan Marina Kuznetsova M. Kozybaev North Kazakhstan State University, Faculty of Natural sciences, Department of Biology, Petropavlovsk, Kazakhstan Aiganym Kazhibayeva Sh. Ualikhanov Kokshetau University, Pedagogical Institute, Department of Biology and teaching methods, Kokshetau, Kazakhstan Marat Tynykulov L.N. Gumilyov Eurasian National University, Departments Biotechnology and Microbiology, Astana, Kazakhstan Botagoz Sharipova Sh. Ualikhanov Kokshetau University, Pedagogical Institute, Department of Biology and teaching methods, Kokshetau, Kazakhstan Journal Article 2026 05 19 Multidrug‑resistant Staphylococcus aureus and Pseudomonas aeruginosa have become a serious problem in Kazakh hospitals, and new treatment options are urgently needed. We investigated whether five locally collected plant extracts could inhibit clinical MDR isolates and suppress the expression of key resistance genes. The work was performed between January and July 2025 in Almaty, Kazakhstan, using 15 S. aureus (10 MRSA) and 15 P. aeruginosa (7 carbapenem‑resistant) isolates. Minimum inhibitory concentrations (MICs) were determined by broth microdilution. Subinhibitory (½ MIC) exposure was followed by quantitative real‑time PCR to measure mecA (MRSA), mexB and ampC (P. aeruginosa) transcripts. Thymus serpyllum (wild thyme) showed the strongest activity, with MIC₅₀ of 16 µg mL-1 against MRSA and 32 µg mL-1 against carbapenem‑resistant P. aeruginosa. No cross‑resistance was observed: MICs did not differ significantly between MRSA and methicillin‑susceptible strains (p = 0.34). At half the MIC, T. serpyllum reduced mecA expression by 59% (mean fold change 0.41, 95% CI 0.36–0.46, p < 0.001). For P. aeruginosa, mexB and ampC transcripts fell to 0.61‑fold (95% CI 0.54–0.68) and 0.71‑fold (0.64–0.78), respectively (both p < 0.001). Suppression was more pronounced in carbapenem‑resistant isolates than in susceptible ones. None of the tested extracts caused upregulation of resistance genes. Glycyrrhiza uralensis was inactive against P. aeruginosa (MIC > 512 µg mL-1). These results demonstrate that T. serpyllum is not only a direct inhibitor of MDR pathogens but also a potent modifier of resistance gene expression. Further studies should identify the active compounds and test combinations with conventional antibiotics.

https://cjes.guilan.ac.ir/article_9619_a34424282c12be84bb2657c829293736.pdf