ABSTRACT
The human host and the microorganisms colonizing skin and mucous surfaces constitute
dynamic biological communities or ecosystems. The composition of the microbiota is
relatively stable but fluctuations occur intra-individually over time, and there are also large
inter-individual differences. The specific microbiota at each ecological habitat is referred to
as the normal microbiota. The numerically and the most diverse human normal microbiota
is found in the gastrointestinal tract, and some of the species are potential pathogens that
may cause disease under certain circumstances (1). One of the functions of the
gastrointestinal microbiota is to act as a barrier against overgrowth of such organisms
and also to prevent colonization of pathogenic bacteria from the environment. This
phenomenon is termed, “colonization resistance” (2). Treatment with antimicrobial agents
disturbs the ecological balance between the host and the normal microbiota and overgrowth
of yeasts and Clostridium difficile, or of intrinsically or acquired resistant microorganisms,
may occur. Horizontal spread of resistance genes by conjugation or transformation to other
microbial species can take place. The gastrointestinal normal microbiota plays an important
role in this development (3).
