Possible mechanisms for the relative efficacies of ortho-phthalaldehyde and glutaraldehyde against glutaraldehyde-resistant Mycobacterium chelonae

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Abstract
Aims: This investigation compared glutaraldehyde (GTA)‐sensitive and ‐resistant strains of Mycobacterium chelonae and examined the effects of pretreatment of GTA‐sensitive and ‐resistant strains of Myco. chelonae with chemical agents that interfere with cell wall synthesis. Methods and Results: When exposed to 2% (v/v) GTA at 25°C, GTA‐resistant strains of Myco. chelonae dried on to glass carriers were not inactivated to any significant extent. By contrast, GTA‐sensitive strains of Myco. chelonae and a strain of Myco. terrae suffered a > 6 log reduction in viability in 5 min. However, ortho‐phthalaldehyde (OPA; 0·5% w/v) achieved a corresponding inactivation against two GTA‐resistant strains within 5–10 and 10–20 min, respectively. Electron microscopy, using a non‐aldehyde fixation process and also negative staining, failed to detect any extensive changes in GTA‐sensitive and ‐resistant cultures exposed to GTA or OPA. Thin‐layer chromatography was unsuccessful in detecting differences between GTA‐resistant and ‐sensitive strains of Myco. chelonae. However, pretreatment of GTA‐resistant cells with mycobacterial cell wall synthesis inhibitors increased their subsequent susceptibility further to OPA but not to GTA. Conclusions: Ortho‐phthalaldehyde is an effective new biocidal agent that, at its in‐use concentration, is rapidly bactericidal to non‐sporulating bacteria, including GTA‐sensitive and ‐resistant mycobacteria. Significance and Impact of the Study: Pretreatment of GTA‐resistant cells with mycobacterial cell wall synthesis inhibitors increased their subsequent susceptibility to OPA but not to GTA.