ortho-Phthalaldehyde (OPA) has become the preferred choice over glutaraldehyde for use as a high-level disinfectant for hospital instrument processing. Its superior antimicrobial performance is not well understood. To explain the exceptional microbicidal activity, a multi-step mechanism combining medium or solvent-induced molecular switching between the lipophilic dialdehyde OPA and the amphiphilic non-aldehyde form, 1,3-phthalandiol, is proposed based on chemical and spectral studies. In this model, OPA is a hydrophobe (the dialdehyde in “open” position) and 1,3- phthalandiol (in “locked” position), is a hydrophile. The amount of each which is present depends on the medium (or solvent) being employed. OPA exists as the dialdehyde in lipophilic media (or solvents) and becomes 1,3-phthalandiol in hydrophilic media (or solvents). These two forms can switch back and forth depending on the medium or solvent being used. The following mechanistic aspects of this model are discussed: (1) the medium-induced molecular switching between OPA and 1,3-phthalandiol and cell-wall penetration via this mechanism; (2) an OPA equilibrium moving in-andout of the bacterial cell aided by a gradient driving force in combination with the molecular switching mechanism which assists significant penetration of OPA into the bacteria cells; (3) the formation of significant amounts of amphiphilic 1,3- phthalandiol from OPA explains the moderate water solubility of OPA, low volatility, and suggests that a different biocidal mechanism operates versus that of glutaraldehyde, and (4) the SAM (self-assembled monolayer) hypothesis, which explains the first-step-attack of OPA on bacteria cell-walls via 1,3-phthalandiol. These observations may explain the superior bactericidal efficacy of OPA against glutaraldehyde-resistant mycobacteria.
Keywords: disinfectant, glutaraldehyde (gta), nmr, uv absorption, phthalandiol, hydration, glutaraldehyde oligomers, self-assembled monolayer (sam)
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