When does bacterial colonisation become infection?

Most lay people would be horrified if they knew that every square centimetre of their skin provided a home for millions of bacteria! The vast majority of these bacteria are completely harmless and never cause any problems. In certain circumstances, however, some can become pathogenic. The distinction between living happily and harmlessly on the surface of the skin and causing morbidity is the difference between colonisation and infection. Usually it is easy to differentiate between colonisation (bacteria present but doing no harm) and infection (bacteria present and causing harm), but there are many occasions when this division is not clear-cut and decisions concerning antibacterial therapy have to be made on both clinical and bacteriological grounds.