Viral-Bacterial Synergistic Interaction in Respiratory Disease

Humans and animals are constantly inoculated with various microorganisms resident in the upper respiratory tract and by inhaled aerosols, yet pneumonia is a relatively rare event. This implies the existence of very efficient defense mechanisms that are capable of eliminating the vast majority of microorganisms before they colonize and multiply to sufficient levels, resulting in clinical diseases. In order to overcome this continuous barrage of microorganisms, there is a complex array of defense mechanisms present in the upper and lower respiratory tract capable of eradicating these organisms. However, in individuals suffering from a variety of diseases, including virus infections, colonization occurs rapidly with subsequent development of pneumonia. Thus, it is estimated that 90% of bacterial pneumonias develop after a viral infection. Furthermore, individuals suffering from a viral pneumonia have a 40% chance of developing bacterial pneumonia. The reasons for the increased colonization of the lung by bacteria following virus infections are related to the surface properties of epithelial cells lining the respiratory tract, the physiological environment of the respiratory tract, and the alteration of the specific and nonspecific defense mechanisms of the lung that occurs as a result of virus infection.