Ortho-phthalaldehyde is a Reliable Chemical Sterilant

Ortho-phthalaldehyde (OPA) received clearance by FDA in October 1999. OPA solution is a clear, pale-blue liquid (pH, 7.5), which typically contains 0.55% OPA. OPA has demonstrated excellent microbiocidal activity in in vitro studies[2,3]. For example, it has shown superior mycobactericidal activity (5-log10 reduction in 5 minutes) compared with glutaraldehyde. The mean time required to effect a 6-log10 reduction for M. bovis using 0.21% OPA was 6 minutes, compared with 32 minutes using 1.5% glutaraldehyde (Table 2)[4]. When tested against a wide range of microorganisms, including glutaraldehyde-resistant mycobacteria and Bacillus subtilis spores[5], OPA showed good activity against the mycobacteria tested, including the glutaraldehyde-resistant strains, but 0.5% OPA was not sporicidal within 270 minutes of exposure. Increasing the pH from its unadjusted level (about 6.5) to pH 8 improved sporicidal activity.

OPA has several potential advantages compared with glutaraldehyde. It requires no activation, is not a known irritant to the eyes and nasal passages, has excellent stability over a wide range of pH (pH 3-9), does not require exposure monitoring, and has a barely perceptible odor.

Ortho-phthalaldehyde (OPA) was tested against a range of organisms including glutaraldehyde-resistant mycobacteria, Bacillus subtilis spores and coat-defective spores. Glutaraldehyde (GTA) and peracetic acid (PAA) were tested for comparative purposes. Both suspension and carrier tests were performed using a range of concentrations and exposure times. All three biocides were very effective (> or = 5 log reduction) against Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa in suspension tests. OPA and GTA (PAA was not tested) were also very effective against Staph. aureus and Ps. aeruginosa in carrier tests. OPA showed good activity against the mycobacteria tested including the two GTA-resistant strains, but 0.5% w/v OPA was found not to be sporicidal. However, limited activity was found with higher concentrations and pH values. Coat-defective spores were more susceptible to OPA, suggesting that the coat may be responsible for this resistance. The findings of this study suggest that OPA is effective against GTA-resistant mycobacteria and that it is a viable alternative to GTA for high level disinfection.

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