On the Nature of Bacteremia in Experimental Pneumococcal Pneumonia in the Dog

Abstract

In the preceding study of the relationship of the natural pneumococcidalpromoting activity of the serum to bacteremia in dogs with experimental pneumococcal pneumonia (1), it was found that the blood of non-bacteremic dogs retains its natural humoral immunity with a remarkable degree of constancy. The occurrence of bacteremia was followed by wide individual variations in the concentration of these antipneumococcal properties, ranging from complete disappearance in the presence of relatively few circulating pneumococci to their persistence in blood containing great numbers of microorganisms. These fluctuations were interpreted to signify differences in the body's capacity to elaborate new antibodies in response to the continued escape of pneumococci from the pulmonary lesion. That the observed variation in pneumococcal immune substances represented corresponding degrees of pneumococcidal power of the blood seemed evident from our tests performed with the shed blood; but to what degrees this action of the blood in vitro reflects the ability of the body to deal with pneumococci in the circulation, our observations failed to elucidate, nor did they provide any data as to whether the pneumococcusclearing action of the blood is more or less effective in the diseased animal than in the normal one.It was felt that a profitable approach to these questions could be made by observing the effects of artificially induced pneumococcemia in normal and diseased (pneumonic) animals and that such information would contribute to the solution of certain other fundamental problems presented by the bacteremic state, such as the rate of escape of pneumococci from pulmonary lesions, the capacity of the blood stream to dispose of them, and the mechanism by which the body controls the degree of blood invasion in the absence of demonstrable humoral immunity.

Host Mechanisms Which Act to Remove Bacteria From the Blood Stream

“…Many years ago Wright showed that the blood stream clearance of a particular strain of pneumococcus could be directly related to the bactericidal power of the peripheral blood in vitro (14). This finding has been confirmed by others (8).…”

“…It is generally agreed that the histiocytic cells of the vascular and lymphatic endothelium, characterized by Aschoff (4) by their ability to sequester vital dyes and particulate substances, are also responsible for the trapping of circulating microorganisms (5). The cells of the reticulo-endothelial system which appear most active in dye studies, the Kupffer cells lining liver capillaries, and the phagocytic cells in splenic sinuses appear to be the most important trapping sites in the early clearance of bacteria, although it is probable that the bone marrow, lymph node sinusoids, and other fixed phagocytic cell depots also participate (6)(7)(8)(9)(10)(11)(12)(13)(14). Quantitative studies on the organ distribution of intravenously administered microorganisms indicate that the liver collects the majority of bacteria when clearance is rapid.…”

Studies on Bacteriemia

“…The rate of clearance might abruptly decline or cease if raiculoendothelial removal mechanisms were fully saturated.--There is abundant evidence to suggest that this is not the case. Clearing mechanisms have been shown to operate effectively after secondary injections of bacteria, or in the presence of overwhelming bacteriemia (6,11,13,14).…”

Studies on Bacteriemia

“…The hepatic trapping of E. coli is less effective than that observed with staphylococci (6,10). Studies on pneumococcal bacteriemia in rabbits and dogs have suggested that clearance of this microorganism depends upon circulating leukocytes rather than the splanchnic tissues (14,15). The studies of Wood and his associates have dearly documented the intravascular phagocytosis of this microorganism during pneumococcal bacteriemia (16).…”