Hospital-acquired gram-negative rod pneumonias: an overview.

Because of a high incidence and case fatality rate, nosocomial infections of the lower respiratory tract due to aerobic gram-negative rods are important, particularly in patients bedded in intensive care units. Risk factors include severity of illness, antimicrobial therapy and respiratory tract instrumentation. Respiratory tract colonization, which invariably precedes infection, seems to be a general characteristic of patients with severe illness, whereas normal oral defenses clear gram-negative bacteria very efficiently. Pneumonia follows after failure of pulmonary antibacterial defenses to cope with aspirated inoculums. Attempts to block colonization with local antimicrobials result not only in a transient decrease in colonization and pneumonia but also in the appearance of resistant organisms. Attempts to enhance human lung antibacterial defenses by bacterial interference or immunization have not been well studied. Despite some progress, the general outlook for immediate control of these infections is not encouraging.