Microbial ecosystems as targets of antibiotic actions

The old representation of the microbe-host interactions as a fight between two contenders has to be considered obsolete. Ecological studies have opened new routes to interpret symbiosis phenomena and to understand in more detail the significance of the microbial communities which are present in various niches of the skin and mucous membranes. In these ecosystems, the resident microorganisms play an important physiological role, frequently with mutualistic relationships. The great stability of these ecosystems is due to the constant composition of microbial communities. One of the main benefits that humans derive from their microbiota is protection from infections. Every change in this biological equilibrium, i.e. modifications of microbial communities (caused by exogenous or endogenous factors) can carry several negative consequences. In this context, exposure to antibiotics (either in the course of therapy or not) can produce direct changes in the target bacterial cells (adhesion, etc.) or modify the whole life of ecosystems by altering the quantitative or qualitative balance within different microbial populations. A selection of antibiotic-resistant organisms can propitiate the genetic transfer of resistance to other autochthonous or allochthonous microorganisms. Multiple-resistant organisms such as those involved in nosocomial infections have amplified the magnitude of this problem. In addition an impairment of host defences can favor infections caused by microorganisms originating from resident communities whose equilibrium has been altered by the antibiotic.