Hospital wastewaters are a source of bacteria with acquired resistance against antibiotics with a level of at least a factor of
2 to 10 times higher than in domestic wastewater. Gene transfer is optimal at high cell densities and under high
antibiotic concentrations. Under heterogeneous environmental conditions, this gene transfer can still occur at
signifiant levels and will contribute to the emergence and spread of resistant pathogens, such as methicillinresistant Staphylococcus aureus if the wastewater is not properly treated. Also, in some places, isolates of
vancomycin-resistant enterococci have been detected in samples of sewage obtained downstream from hospitals,
such as in the Porto area in Portugal