Cultivation-based quantification and identification of bacteria at two hygienic key sides of domestic washing machines

Detergent drawer and door seal represent important sites for microbial life in domestic washing machines. Interestingly, quantitative data on microbial contamination of these sites is scarce. Here, 10 domestic washing machines were swap-sampled for subsequent bacterial cultivation at four different sampling sites, each: detergent drawer, detergent drawer chamber as well as top and bottom part of the rubber door seal. The average bacterial load over all washing machines and sites was 2.1 {+/-} 1.0 x 104 CFU cm-2 (average {+/-} standard error of the mean (SEM)). The top part of the door seal showed the lowest contamination (11.1 {+/-} 9.2 x 101 CFU cm-2), probably due to less humidity. Out of 212 isolates, 178 (84%) were identified on genus level and 118 (56%) on species level using MALDI biotyping, resulting in 29 genera and 40 identified species across all machines. The predominant bacterial genera were Staphylococcus and Micrococcus, which were found at all sites. 21 out of 40 species were classified as opportunistic pathogens, emphasizing the need for regular cleaning of the investigated sites.