Colonization Versus Infection

Most organisms which colonize are harmless commensals and are best left alone. They may, in fact, prevent invasion by pathogens. This happens by competition for nutrition and by producing inhibitory metabolic products. Some commensals can nevertheless invade if they have an opportunity. It is important to remember that most pathogenic organisms set up colonization first and switch to an invasive mode after sensing a quorum and suitable environmental conditions. These help them to fine tune expression of metabolic and resistance determinants. It is difficult but is critically important to pinpoint exactly when colonization turns to invasion.