title: Effects of Formulation on Microbicide Potency and Mitigation of the Development of Bacterial Insusceptibility.
abstract: Risk assessments of 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. We tested 8 species of bacteria (7 genera) before and after repeated exposure (14 passages), using a previously validated gradient plating system, for their susceptibilities to the microbicides benzalkonium chloride, benzisothiozolinone, chlorhexidine, didecyldimethyl ammonium chloride, DMDM-hydantoin, polyhexamethylene biguanide, thymol, and triclosan in aqueous solution (nonformulated) 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. MICs and minimum bactericidal concentrations (MBC) were on average 11-fold lower for formulated microbicides than for nonformulated 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 nonformulated 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 nonformulated and 10≥4-fold (mean, 2-fold) increases in MBC for formulated microbicides. Susceptibility decreases fully or partially reverted back to preexposure 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 after repeated exposure were lower in frequency and extent.
authors.affiliation: Manchester Pharmacy School, The University of Manchester, Manchester, United Kingdom.
authors.forename: Nicola L
authors.initials: NL
authors.lastname: Cowley
authors.affiliation: Manchester Pharmacy School, The University of Manchester, Manchester, United Kingdom.
authors.forename: Sarah
authors.initials: S
authors.lastname: Forbes
authors.affiliation: Unilever Safety and Environmental Assurance Centre, Colworth Science Park, Bedford, United Kingdom.
authors.forename: Alejandro
authors.initials: A
authors.lastname: Amézquita
authors.affiliation: Unilever Safety and Environmental Assurance Centre, Colworth Science Park, Bedford, United Kingdom.
authors.forename: Peter
authors.initials: P
authors.lastname: McClure
authors.affiliation: Manchester Pharmacy School, The University of Manchester, Manchester, United Kingdom.
authors.forename: Gavin J
authors.initials: GJ
authors.lastname: Humphreys
authors.affiliation: Manchester Pharmacy School, The University of Manchester, Manchester, United Kingdom andrew.mcbain@manchester.ac.uk.
authors.forename: Andrew J
authors.identifier: http://orcid.org/0000-0002-5255-5425
authors.initials: AJ
authors.lastname: McBain
mesh_terms: D000900:Anti-Bacterial Agents / Q000737:chemistry* / Q000494:pharmacology*; D000890:Anti-Infective Agents / Q000737:chemistry* / Q000494:pharmacology*; D001419:Bacteria / Q000187:drug effects*; D001548:Benzalkonium Compounds / Q000737:chemistry; D001645:Biguanides / Q000737:chemistry; D002710:Chlorhexidine / Q000737:chemistry; D008826:Microbial Sensitivity Tests; D014260:Triclosan / Q000737:chemistry
chemical_list: D000900:Anti-Bacterial Agents; D000890:Anti-Infective Agents; D001548:Benzalkonium Compounds; D001645:Biguanides; C031233:polihexanide; D014260:Triclosan; D002710:Chlorhexidine