Bacterial adherence to respiratory tract mucosa-a dynamic interaction leading to colonization

Normally, the mucosa of the nasooropharynx, trachea, and, perhaps, the major bronchi is colonized with aerobic and anaerobic microbes. This epithelial surface coexists with the microbial flora and is not overgrown with it. Moreover, the physiologic functions of the mucosa-including a protective barrier, mucociliary clearance and humidification, and warming of respired air-are not impeded. How this flora is controlled and what is amiss when virulent or pathogenic bacteria can cause infection are fascinating questions. A balance is maintained during health in which epithelial cell integrity-a function of proper nutrition, available secretory immunoglobulins and glycoproteins, and ciliary motion-resists the microbe's attempt to attach via specialized receptors (pili) or by proteolytic destruction of local proteins. These interactions are reviewed in detail. When colonization is excessive and aspiration of more microbes into the lower airway occurs, infection is more probable. Certain bacteria such as Streptococcus pneumoniae and Hemophilus influenzae, which are associated with chronic bronchitis, illustrate a mechanism in which the host-microbial balance may be upset by selective impairment of a host protein, secretory IgA₁. Alternatively, viral infection or cilotoxic microbes (mycoplasma) can favor colonization of bacteria when mucosal clearance mechanisms are impaired. Last, mucosal integrity can be breached by noxious gases or inflammation that may allow bacteria entry into the submucosal that provides a nidus for infection.