Enhanced Antimicrobial Potency and Therapeutic Index of a Defined C12 Quaternary Ammonium Compound

Skin and soft tissue infections (SSTIs) remain a major clinical burden and are complicated by antimicrobial resistance, polymicrobial infections, and biofilm-mediated therapeutic tolerance. Topical antimicrobials can provide localized activity while reducing systemic exposure, but currently available agents are limited by narrow spectra, cytotoxicity, poor antibiofilm activity, and resistance development. We evaluated C12-alkyl(ethylbenzyl)dimethyl ammonium chloride (EQ12), a next-generation quaternary ammonium compound, in vitro as a topical antimicrobial candidate for leading bacterial causes of SSTIs. Broth microdilution assays were used to determine minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) against methicillin-sensitive Staphylococcus aureus (MSSA), methicillin-resistant S. aureus (MRSA), Acinetobacter baumannii, Klebsiella pneumoniae, and Pseudomonas aeruginosa. Species-selective plating was used to assess bactericidal activity in staphylococcal-P. aeruginosa co-cultures. Crystal violet assays quantified biofilm inhibition against MRSA, P. aeruginosa, and A. baumannii, and serial passage experiments assessed resistance emergence over 14 passages. EQ12 inhibited all species tested, with MICs of 2.3 to 40 micrograms/mL and MBCs of 4.7 to 40 micrograms/mL. EQ12 outperformed benzalkonium chloride and povidone-iodine against Gram-negative isolates and retained bactericidal activity in mixed cultures, although staphylococcal susceptibility decreased in the presence of P. aeruginosa. EQ12 inhibited biofilm formation at sub-MIC to near-MIC concentrations and did not induce resistance in MRSA, P. aeruginosa, or A. baumannii, unlike comparator topical antibiotics. These findings support further development of EQ12 as a topical antimicrobial for SSTI-associated pathogens.