Antibiotics and Antibiotic Resistance Genes in Water

Water ecosystems comprise various habitats, trophic sources, and provide drinking water for all terrestrial organisms. Owing to the extensive use of antibiotics (ABs) among humans (particularly in hospitals), farm animals and aquaculture, these substances and their related antibiotic resistance genes (ARGs) can enter natural ecosystems and different compartments (e.g., water and sediment) in different ways. In fact, antibiotics are only partially metabolized by treated organisms, and residual concentrations are excreted, either unchanged or as active metabolites, via urine and faeces. These residual antibiotics reach various ecosystems through discharge from hospitals and domestic sewage wastewater treatment plants (WWTPs), as well as from livestock waste and effluents. AB residues can affect negatively natural microbial communities, which play a key role in fundamental ecological processes, such as the maintenance of water and soil quality. At the same time, AB can stimulate development and spread of antibiotic resistance. Indeed, ABs and other resistance cofactors can promote the development of ARGs in chromosomes or their transfer through mobile genetic elements between pathogens and natural bacteria and vice versa. It has been generally recognized that, e.g. through drinking water consumption, ARGs can be transferred from environmental bacteria to pathogens in animal and human microbiomes. However, antibiotics and their ARGs are not yet included in priority lists with specific legislative threshold limits. Although antibiotic resistance is a natural phenomenon very few studies have focused on pristine ecosystems to assess background ARG levels in water. This information could offer valuable insights for future environmental regulations and legislation. In this chapter, the most commonly measured and detected antibiotics and associated ARGs in rivers are discussed. Additionally, other factors that could contribute to antibiotic resistance are reported, including the impact of chemical mixtures, the possible effects of climate changes and the presence of micro- and nanoplastics.