Microbiological flora on the interior of automatic-type washing machines were examined in 24 homes. Mold contamination as well as general bacteria and yeast were higher in the upper part of the spin tub than in other parts. Of molds contaminating washing machine interiors, Exophiala and Scolecobasidium, which have dark hyphae, were predominant, followed by Torula and Cladosporium. Dirt found in the upper part of the spin tub was darker than in other parts, similar to the distribution of mold. Dirt suspended in washing water seemed to derive chiefly from the upper part of the spin tub. Suspended dirt was thought to be fragments from the old hyphae of mold grown on the interior of washing machines. Yeast contamination was comparatively higher in new washing machines while mold contamination was higher in old machines. Microbiological “succession” was also found to be present in washing machines. Cleaning of washing machines is generally ineffective for killing spores and eliminating hyphae of molds, ways of cleaning washing machines are also discussed.
The distribution and composition of dirt on the interior of washing machines were examined. Dirt accumulated most at the bottom of spin tubs with large aluminum die-cast flanges and accumulated in constant weight in other parts. Dirt consisted of mold, textile fiber and zeolite, added to detergent as a water softener. Zeolite was a main component of dirt adhering to the interior of washing machines. Mold hyphae were present more in the upper part of the interior, and large amounts of textile fibers were contained in dirt from other parts of the interior. Weight of dirt on the spin tub increased with increasing of length of use, which differed from the quantity of surfactants on the interior. Environmental factors affecting mold contamination in automatic-type washing machines were also studied. As regards factors related to nutrients, no correlation was found between recycling of bath water and mold contamination in washing machines. However, the quantity of surfactant adhering to the washing machine interior was found to promote mold contamination. Ways of controlling mold contamination in washing machines are also discussed.