Mechanisms of bacterial superinfections in viral pneumonias

Although it was once thought that bacterial infection was merely a function of the virulence of the microbe it is now known that other pathogens can alter host resistance. With respect to bacterial superinfection during viral pneumonias, three important factors must be considered; the role of the virus, the role of the bacterium, and the immune status of the host. The fact that no one bacterial species is reponsible for all human cases of postinfluenzal bacterial pneumonia indicates that there is a general impairment of pulmonary antibacterial defenses brought about by the viral infection. The fact that the rate of intrapulmonary killing varies with different bacterial species indicates that the superinfecting organism can itself play a role in the dual disease process. Finally, it has been amply demonstrated that the resistance of the host is dependent on a variety of factors which include innate variables such as genetic endowment and a multitude of imponderable variables acquired through life experiences which can be considered under the general category of 'host factors'. All three factors interact and collectively impinge upon the resistance of the host. Lastly, as influenza virus infections occur most frequently in epidemic outbreaks, the relationship between influenza virus and secondary bacterial infections is the classic example. However, there is growing evidence that an association exists between other virus groups and bacterial pathogens in respiratory tract infections. Adenovirus, parainfluenza virus, and rhinovirus are among the agents that appear to pave the way for bacterial pneumonias. Mycoplasma pneumoniae, once considered to be a virus and the cause of primary atypical pneumonia, may also render the respiratory tract susceptible to bacterial invasion.