Washing machines should be further investigated as possible sites for horizontal gene transfer (ESBL genes) and cross-contamination with clinically important Gram-negative strains. Particularly in the health care sector, the knowledge of possible (re-)contamination of laundry (patients’ clothes and staff uniforms) with multidrug-resistant Gram-negative bacteria could help to prevent and to control nosocomial infections. This report describes an outbreak with a single strain of a multidrug-resistant bacterium (Klebsiella oxytoca sequence type 201) in a neonatal intensive care unit that was terminated only when the washing machine was removed. In addition, the study implies that changes in washing machine design and processing are required to prevent accumulation of residual water where microbial growth can occur and contaminate clothes.

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Antibiotics are essential for modern medicine, they are employed frequently in hospitals and, therefore, present in hospital wastewater. Even in concentrations, that are lower than the minimum inhibitory concentrations (MICs) of susceptible bacteria, antibiotics may exert an influence and select resistant bacteria, if they exceed the MSCs (minimal selective concentrations) of resistant strains. Here, we compare the MSCs of fluorescently labelled Acinetobacter baylyi strains harboring spontaneous resistance mutations or a resistance plasmid with antibiotic concentrations determined in hospital wastewater. Low MSCs in the μg/L range were measured for the quinolone ciprofloxacin (17 μg/L) and for the carbapenem meropenem (30 μg/L). A 24 h continuous analysis of hospital wastewater showed daily fluctuations of the concentrations of these antibiotics with distinctive peaks at 7-8 p. m. and 5-6 a.m. The meropenem concentrations were always above the MSC and MIC values of A. baylyi. In addition, the ciprofloxacin concentrations were in the range of the lowest MSC for about half the time. These results explain the abundance of strains with meropenem and ciprofloxacin resistance in hospital wastewater and drains.Originality-Significance Statement: In previous studies, we had isolated a great variety of multi-resistant bacteria (simultaneously resistant to the critically important antibiotics 3 rd gen. cephalosporins, carbapenems, piperacillin/tazobactam, and ciprofloxacin) from the drains and the wastewater of a maximum care hospital. These bacteria reach the environment, since they are still detected in the effluent of the wastewater treatment plant and they cause infections that are very difficult to treat. So far, it has been assumed that good hygienic measures should be able to avoid nosocomial infections of patients with such bacteria. However, the key aspect of this manuscript is that fluctuating high antibiotic concentrations of meropenem, ciprofloxacin and piperacillin are present in hospital wastewater and that these antibiotic concentrations reach or surpass the minimal selective concentrations of the model organism Acinetobacter baylyi. In conclusion, the colonization of hospital drains and wastewater pipes with multi-resistant bacteria is probably driven by the high antibiotic concentrations in the wastewater. These results are very important, since the antibiotic concentrations and the resulting selective pressure will complicate attempts to such remove the bacteria from the drains and wastewater pipes by cleaning or disinfection measures.