Roles of spleen and liver in the clearance of Escherichia coli K1 bacteraemia in infant rats.

The age-dependent increase in resistance of infant rats to K1 E. coli infection was studied by analysing the clearance of intravenously injected radiolabelled O18:K1 E. coli bacteria. In susceptible 7-day-old rats, the rate of reticuloendothelial clearance could not compete with bacterial multiplication, while the resistance of 18-day-old rats was attributed to the increased sequestration of bacteria in the spleen. After passive immunization with rat monoclonal anti-O18 IgG2b, the O18:K1 E. coli were rapidly cleared by the liver in both age groups. O1:K1, O18:K5 and O18:K- E. coli, which activate complement in an antibody-independent manner and are apathogenic for 7-day-old rats, were also rapidly sequestered in the liver. 7-day-old rats developed a fulminant bacteraemia after receiving 100 O18:K1 E. coli intravenously. After several hours a transient decrease in the level of bacteraemia was observed. However, the clearance remained incomplete, resulting in persistent bacteraemia and death. Endotoxin-responsive infant mice are able to clear the bacteraemia completely after a similar initial phase of bacterial multiplication while no clearance is observed in endotoxin-hypo-responsive mice. The marked host specificity of K1 E. coli infection thus appears to be related to differences in a clearance mechanism which may be sustained by an endotoxin-induced inflammatory response.