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An assay is described that has allowed rapid determination of the dosage and contact times in the presence of organic soiling, as demanded by the EN 14564 standard, by using an optimized variant of the green fluorescent protein (Gfp). The [gfp.sub.m.sup.2+]-expressing Mycobacterium terrace is established as a new reporter strain for evaluating mycobactericidal activities of disinfectants with a superior sensitivity and in a shorter time.

Quantitative carrier tests were carried out with monodispersed cells of Mycobacterium terrae using ortho-phthaldialdehyde (OPA) and coco fatty aminoxethylate as biocides. In contrast to OPA, with coco fatty aminoxethylate the reductions obtained with freshly homogenized mycobacteria were significantly higher than those obtained with mycobacteria kept in the refrigerator for four days.