Effect of different drying conditions for the viability of Candida albicans present on carriers

Problems in substantial under recovery of Pseudomonas aeruginosa and Candida albicans from carriers have been demonstrated for laboratories performing phase 2, step 2 efficacy tests on disinfectants relative to levels required by the EN 13697 standard. It was thus necessary to determine recoveries of these microorganisms following procedural losses incurred during drying and to optimise drying conditions such that recoveries then complied with the standard.

The aim of the study was to establish optimal drying conditions for the recovery of Candida albicans ATCC 10231 from carriers.

The evaluation was performed according to the EN 13697:2001 standard procedure. A test suspension of Candida albicans and interfering substance were inoculated onto the surface of carriers (2 cm diameter stainless steel discs) and then dried under different conditions consisting of: a 37 degrees C incubation with and without an incubator fan as well as at 23 degrees C (room temperature) in a laminar air flow cabinet. Carriers were dried until the surfaces appeared visibly dry and the number of surviving organisms then recovered from the surface were quantified. The following were calculated for colony forming units (cfu); N (log10 cfu in a 0.05 ml test suspension), NC (the control log10 cfu in neutralizing medium), Nts (cfu numbers remaining on the surface) and the N-NC difference which should not exceed 2 log10 when microorganism recoveries are adequate and without any toxicity effects of the neutralising medium. Experiments was conducted using validating procedure (NC) which is performed with distilled water.

Drying at 37 degrees C adversely affected the survival of Candida albicans and prevented the levels of microbial recovery from carriers to reach those specified by the EN 13697 standard. However, drying at around room temperatures of 23 degrees C reduced Candida albicans mortality and increased recoveries from the carrier to levels compliant with the standard, where the N-NC differences were not greater than 2 log10.

The viability of Candida albicans ATCC 10231 is sufficiently improved when carriers are dried at 23 degrees C, even if the drying time exceeds 60 minutes. The density of the initial test suspension (N) should also be increased.