[Sources of antibiotics in natural environments and their biological role].

Nowadays antibiotics are broadly used not only for treatment of bacterial infections but also in nonmedical applications. For many years they have been added as livestock and poultry growth supplements, and they are applied similarly in fish farming. In basically unchanged form they may get into the natural environment and remain there for a long time. Excessive use of antibiotics leads to widespread of antibiotic resistance among clinical and environmental bacterial strains. Subinhibitory concentrations of antibiotics, which do not inhibit growth of bacteria, are often found in soil, water or even in the tissue of different organisms. Such low concentrations affect many bacterial genes through changes in their transcription level and increase of the mutation rate, and as a consequence lead to many bacterial adaptations to environmental stresses. There is also evidence that subinhibitory concentrations of antibiotics induce transfer of mobile genetic elements through horizontal gene transfer pathways, and therefore enhance antibiotic resistance, also among environmental strains. The analyzed data suggest the necessity of restriction and regular monitoring of antibiotics, which may be considered as environmental pollutants.