The Dual Impact of Some Essential Oils and Metals on Biofilm Inhibition and Virulence Gene Expression in S. aureus, K. pneumoniae, and P. aeruginosa

The emergence of multidrug-resistant (MDR) pathogens, such as Staphylococcus aureus, Klebsiella pneumoniae, and Pseudomonas aeruginosa, presents a major global health challenge, necessitating the development of novel therapeutic strategies. This study investigates the dual antimicrobial and antivirulence potential of selected essential oils (Allium cepa, Boswellia carterii, Curcuma longa, and Nigella sativa) and transition metals (cobalt and molybdenum), individually and in synergistic combinations, against clinical MDR isolates. Antibacterial activity was assessed using the disk diffusion method, while quantitative real-time PCR (qRT-PCR) was employed to evaluate the transcriptional modulation of key virulence genes: icaA and clfA in S. aureus; rmpA, uge, and wcaG in K. pneumoniae; and pslA and toxA in P. aeruginosa, with 16S and 23S rRNA serving as internal controls. Among all treatments, the Co + A. cepa formulation exhibited the most pronounced inhibitory effect, with inhibition zones of 13–17mm and significant alterations in virulence gene expression. The combination caused downregulation of adhesion- and toxin-related genes (clfA and toxA) and upregulation of biofilm-associated and capsule-maintenance genes (icaA, rmpA, uge, and pslA), indicating a shift from acute virulence to a stress-adaptive biofilm phenotype. These findings highlight a dual-action mechanism by which metal–essential oil complexes suppress bacterial pathogenicity while maintaining moderate antibacterial activity. The Co + A. cepa complex emerges as a promising antivirulence candidate capable of attenuating pathogenic potential in MDR bacteria. Further in vivo evaluation and formulation optimization are warranted to advance its clinical applicability as a next-generation antimicrobial approach.