Aims: To investigate the prevalence of -lactamase genes in domestic washing machines and dishwashers, and the decontamination efficacy of laundering.

Methods and Results: For the first investigation, swab samples from washing machines (n = 29) and dishwashers (n = 24) were analysed by real-time quantitative PCR to detect genes encoding beta-lactamases. To test the impact of laundering on resistant bacteria, cotton test swatches were artificially contaminated with susceptible and resistant strains of Pseudomonas aeruginosa, Klebsiella pneumoniae and Staphylococcus aureus within a second investigation. They were washed in a domestic washing machine with or without activated oxygen bleach (AOB)-containing detergent at 20-50 degrees C. beta-Lactamase genes (most commonly of the AmpC- and OXA-type) were detected in 79% of the washing machines and in 96% of the dishwashers and Pseudomonadaceae dominated the microbiota. The level of bacterial reduction after laundering was >= 80% for all Ps.aeruginosa and Kl.pneumoniae strains, while it was only 37-61% for the methicillin-resistant Staph.aureus outbreak strain. In general, the reduction was tendentially higher for susceptible bacteria than for the resistant outbreak strains, especially for Staph.aureus.

Conclusions: beta-Lactamase genes seem to be frequently present in domestic appliances and may pose a potential risk for cross-contamination and horizontal transfer of genes encoding resistance against clinically important beta-lactams. In general, higher temperatures and the use of AOB can improve the reduction of antibiotic-resistant bacteria, including Staph.aureus which appears to be less susceptible to the decontamination effect of laundering.

Significance and Impact of this Study: Data on the presence of antibiotic-resistant bacteria in the domestic environment are limited. This study suggests that -lactamase genes in washing machines and dishwashers are frequent, and that antibiotic-resistant strains are generally more resistant to the used washing conditions.