Microorganisms found on the skin are usually regarded as innocuous symbiotic organisms (commensals), pathogens or potential pathogens. In recent years, we have radically revised our understanding of the host-microorganism interaction together with the mechanisms of bacterial virulence. Studies have shown that chronic wounds are colonised by multiple bacterial species, many of which persist in the wound. The presence of bacteria such as Pseudomonas aeruginosa can induce wound enlargement and/or delayed healing. It is this situation of delayed healing that we equate with critical colonisation.