The effects of peracetic acid-based (PAA) disinfectant with, and without, additional drying on Candida albicans, Candida parapsilosis, Pseudomonas aeruginosa and Stenotrophomonas maltophilia, isolated from contaminated flexible endoscopes, in single- and dual-species biofilms were studied. Biofilms were prepared in sterile tissue culture polystyrene 96-well microtitre plates and were quantified using the tetrazolium salt (MTT) reduction assay and by counting colony-forming yeasts and bacteria from 10-fold serial biofilm dilutions on agar plates. An in vitro biofilm model was applied to mimic the biofilm formation inside the endoscope channels and to imitate the disinfection and drying procedures used for reprocessing of flexible endoscopes. The PAA-based disinfectant was effective against bacteria and yeasts in the planktonic and biofilm states directly after treatment, but allowed regrowth of all biofilms if the drying procedure was skipped. No biofilm regrowth occurred in wells after a drying procedure in all single- and dual-species biofilms. Routine cleaning procedures do not remove biofilm reliably from endoscope channels if the accurate drying procedure is not applied. This may explain the failure of decontamination during endoscope reprocessing.
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