Because antibiotics used as human and veterinary medicines specifically target undesired microorganisms, the potential impacts of these biologically active compounds are of particular concern for beneficial microbial communities and other organisms in aquatic and terrestrial ecosystems. For example, fluoroquinolones exert their toxicity to bacteria through inhibition of DNA gyrase, while ^-lactams target transpeptidase and transglycosylase, ultimately affecting cell walls. Microorganisms are critically important to the functioning of aquatic and terrestrial systems. Because the most probable route of antibiotics into the aquatic environment is via wastewater effluents, it is reasonable that effects may be greatest in aquatic systems receiving these inputs (e.g., streams, lakes). Within a stream ecosystem, it is possible that antibiotic impacts could affect processes such as micro-bial decomposition of organic matter, macroinvertebrate nutrition including either bacteria directly ingested or those acting as symbionts within the invertebrate gut, and/or nitrification and denitrification processes.
The goal of an ecological risk assessment is protection of populations, communities, and ecosystems and related functions. Measures of effect are selected to support an assessment endpoint, which is a clearly defined ecological value that is to be protected such as ecosystem structure (e.g., organisms, populations, communities) or function (e.g., leaf decomposition, nutrient cycling). However, functional responses to stressors are routinely not observed before structural changes occur in an ecosystem. Previous authors have noted that the critical functions of ecosystems regulated by microorganisms may be more subtly affected over longer time periods by antibiotics released to the environment.
Thus, it is critical that measures of effect be selected carefully during risk assessment if the potential hazards of antibiotics in the environment are to be appropriately characterized. For example, a major concern is that antibiotics found in the environment may cause increased resistance in natural bacterial populations.
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