Colonization and Infection

Physiologically, internal organs such as lower airways and bladder are sterile. However, colonization of lower airways and bladder by potentially pathogenic micro-organisms (PPMs) is common in critically ill patients on the intensive care unit (ICU) [1]. Colonization of the internal organs generally follows the impaired carriage defense of the digestive tract, which promotes carriage and overgrowth of PPMs (see Chapter 2), and the impaired defenses of the host against colonization, due to illness severity. Failure to clear colonizing microorganisms from the internal organs invariably leads to high concentrations of PPMs, predisposing to invasion. The host mobilizes both humoral and cellular defense systems to hinder the invading micro-organisms. However, infection will require not only invasion, but also severity of the underlying disease, which jeopardizes immunocompetence. The aims of this chapter are to define the concepts of colonization/infection and to describe the defense mechanisms of the host and the interventions for control of colonization/infection.