Antibiotic concentrations in raw hospital wastewater surpass minimal selective and minimum inhibitory concentrations of resistant Acinetobacter baylyi strains

Antibiotics are essential for modern medicine, they are employed frequentlyin hospitals and, therefore, present in hospital wastewater. Even in concen-trations, that are lower than the minimum inhibitory concentrations (MICs) ofsusceptible bacteria, antibiotics may exert an influence and select resistantbacteria, if they exceed the MSCs (minimal selective concentrations) ofresistant strains. Here, we compare the MSCs of fluorescently labelledAci-netobacter  baylyistrains harboring spontaneous resistance mutations or aresistance plasmid with antibiotic concentrations determined in hospitalwastewater. Low MSCs in theμg/L range were measured for the quinoloneciprofloxacin (17μg/L) and for the carbapenem meropenem (30μg/L). A24 h continuous analysis of hospital wastewater showed daily fluctuationsof the concentrations of these antibiotics with distinctive peaks at 7–8p.m. and 5–6 a.m. The meropenem concentrations were always above theMSC and MIC values ofA.  baylyi. In addition, the ciprofloxacin concentra-tions were in the range of the lowest MSC for about half the time. Theseresults explain the abundance of strains with meropenem and ciprofloxacinresistance in hospital wastewater and drains.