Prevalence of β‐lactamase genes in domestic washing machines and dishwashers and the impact of laundering processes on antibiotic‐resistant bacteria

AIMS: To investigate the prevalence of β‐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 β‐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°C. β‐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: β‐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 β‐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 β‐lactamase genes in washing machines and dishwashers are frequent, and that antibiotic‐resistant strains are generally more resistant to the used washing conditions.