The influence of laboratory adaptation on test strains, such as Pseudomonas aeruginosa, in the evaluation of the antimicrobial efficacy of ortho-phthalaldehyde R. Herruzo-Cabrera * , M.J. Vizcaino-Alcaide, M.J. Ferna ´ndez-Acen˜ero Department of Preventive Medicine, School of Medicine, Universidad Autonoma of Madrid and Hospital “La Paz”, Madrid, Spain Received 9 October 2002; accepted 14 January 2004 Available online 2 June 2004 KEYWORDS Bactericidal effect; Laboratory adaptation; P. aeruginosa; OPA Summary The microbiocidal efficacy of 0.55% ortho-pthalaldehyde (OPA) was evaluated in a rough carrier test, using more than 200 strains of bacteria and yeasts from patients and reference ATCC strains. This test was then compared with the European carrier test (prEN14561) using Pseudomonas aeruginosa. We also sought to determine whether recently isolated P. aeruginosa had the same susceptibility to OPA, after laboratory adaptation. It was shown that P. aeruginosa was less susceptible to OPA (being reduced by a factor of 10 3.8 ) than the other strains (reduced by a factor of 10 4 ). The surface test used, produced a lesser reduction of P. aeruginosa than the European test. For recently isolated strains ðN ¼ 66Þ; the rough model demonstrated that the number of survivors increased both quantitatively and qualitatively from day one to day seven. It was concluded that disinfectant efficacy should be confirmed with recently isolated organisms. Q 2004 The Hospital Infection Society. Published by Elsevier Ltd. All rights reserved. Introduction Glutaraldehyde (GTA) has been employed as a high- level disinfectant for approximately 30 years. 1,2 It is considered a potential mutagen and its use in hospitals can cause occupational asthma and it has been linked to other side effects, such as eye and skin irritation, allergic reactions, respiratory disorders, headache and nausea. 1,3 – 5 These risks, and the appearance of GTA-resistant microbial strains, have necessitated the search for efficient alternatives to this product. 6–9 A feasible alternative for high-level disinfection is ortho-phthalaldehyde (OPA), which shows a bactericidal effect at a concentration of 0.55% and is active with no need for prior activation, remaining stable for at least 14 days. 10 The mycobactericidal action of OPA can reduce the test strain by a factor of 10 5 in 5 min. 11 – 13 OPA has some advantages over GTA: (1) it does not need to 0195-6701/$ - see front matter Q 2004 The Hospital Infection Society. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jhin.2004.01.031 Journal of Hospital Infection (2004) 57, 217–222 www.elsevierhealth.com/journals/jhin *Corresponding author. Address: Medicina Preventiva UAM, C/Arzobispo Morcillo 4, 28029 Madrid, Spain. E-mail address: rafael.herruzo@uam.es