Mechanisms of host defense and quantitative comparisons of bacterial populations in experimental peritonitis

A model for the quantitative study of bacterial levels in blood, ascitic fluid, and liver, induced by Escherichia coli in the rat, has been devised. Three experimental situations were then studied: non-fatal peritonitis, fatal peritonitis induced by bacteria rendered more virulent by serial passage through test animals, and fatal peritonitis using haemoglobin adjuvant with the more virulent strain. Results indicate that a variety of defense mechanisms are operant in the host animal. In the non-fatal peritonitis, clearance of free bacteria from the peritoneum is observed with a late rebound in local and systemic populations. These phenomena correlate well with in vitro studies of bacterial uptake by peritoneal macrophages. In fatal peritonitis without adjuvant, much larger numbers of bacteria seem to escape initial clearance in the peritoneum and proximal reticuloendothelial system with resultant overwhelming septicaemia. In fatal peritonitis with adjuvant, much less clearance of organisms from the peritoneum is observed, with resultant overgrowth of bacteria and host death. It thus seems that the initial host defenses center around peritoneal clearance of introduced organisms, and that processes which interfere with this clearance prove fatal.