The ecological balance between the human host and associated micro-organisms described above can be disturbed by several factors, most dramatically by administration of antimicrobial agents. This perturbation mainly manifests as decreased colonization resistance of members of the commensal microbiota, which leads to varying states of disease as well as emergence of antibiotic-resistant strains. Shortterm changes in the quantity and composition of the bacteria comprising the normal human flora as a response to antibiotic exposure have been extensively recorded. However, only a few recent studies have investigated the long-term impacts of antibiotic administration, including development of resistance.