With the antibiotics persisting in the environment for long periods, there is ample opportunity for microbial resistance to develop, which is the major concern for public and animal health. The prevalence of resistant bacteria in the environment has increased dramatically as a consequence of the widespread use of antibiotics (Kruse & Sørum 1994). Resistance can theoretically develop in two ways. small proportion of microbes is naturally resistant to antibiotics, particularly at low concentrations. If antibiotic residues are introduced into the environment, they provide a selective pressure such that the resistant strains survive and successfully compete against non-resistant strains or types, and eventually replace those bacteria which are susceptible to control by antibiotics. A second mechanism whereby resistance can develop is through horizontal gene transfer. Conjugation and transfer of R plasmids (containing genes for antimicrobial resistance) can occur between bacteria living in the environment and those associated with humans and animals, regardless of the evolution or ecology of the species (Kruse and Sørum 1994).
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