Pulmonary bacterial susceptibility in the burned rat.

An in vivo model system, using the burned rat as a model of altered host resistance, was developed and studied to investigate the lung's ability to handle an aerosolized bacterial insult of either P. aeruginosa or S. aureus. Rates of lung bacterial clearance were correlated with mortality, bacteriologic and histologic results. Susceptibility of the lung to sepsis was shown to be directly related to host resistance and was significantly increased following cutaneous thermal injury. Exposure of burned, non-seeded rats to a bacterial aerosol on day one post burn resulted in a mortality of 50% (P. aeruginosa) and 11.1% (S. aureus). This mortality clearly demonstrates an increased susceptibility at day one post burn. In contrast, this susceptibility gradually decreased with time; no mortality in the burned rat when exposed on day six post burn or in the normal, unburned rat. Histologic findings correlate well with the mortality results showing a gradually decreasing severity of pneumonitis if aerosol exposure was further delayed. Lung bacterial clearance studies revealed that the initial good response of the pulmonary defense mechanisms immediately following aerosol challenge are short-lived and that a marked increase in pulmonary bacterial susceptibility occurs as early as 24 hours following thermal injury.