The Effect of Formulation on Microbicide Potency and Mitigation of the Development of Bacterial Insusceptibility.

Risk assessments into the potential for microbicides to select for reduced bacterial susceptibility have been based largely on data generated through the exposure of bacteria to microbicides in aqueous solution. Since microbicides are normally formulated with multiple excipients, we have investigated the effect of formulation on antimicrobial activity and the induction of bacterial insusceptibility. The susceptibilities of 8 species of bacteria (7 genera) were determined before and after repeated exposure (14 passages) using a previously validated gradient plating system, to the microbicides benzalkonium chloride, benzisothiozolinone, chlorhexidine, didecyldimethyl ammonium chloride, DMDM-hydantoin, polyhexamethylene biguanide, thymol and triclosan in aqueous solution (non-formulated) and in formulation with excipients often deployed in consumer products. Susceptibilities were also assessed following an additional 14 passages without microbicide to determine the stability of any susceptibility changes. Minimum inhibitory concentrations (MIC) and minimum bactericidal concentrations (MBC) were on average 11-fold lower for formulated vs. non-formulated microbicides. After exposure to the antimicrobial compounds, of 72 combinations of microbicide and bacterium, there were 19 ≥4- fold (mean 8-fold) increases in MIC for non-formulated and 8 ≥4-fold (mean 2-fold) increases in MIC for formulated microbicides. Furthermore, there were 20 ≥4-fold increases in MBC (mean 8-fold) for non-formulated and 10 ≥4-fold (mean 2-fold) increases in MBC for formulated microbicides. Susceptibility decreases fully or partially reverted back to pre- exposure values for 49% of MICs and 72% of MBCs after further passage. In summary, formulated microbicides exhibited greater antibacterial potency than unformulated actives and susceptibility decreases following repeated exposure were lower in frequency and extent.