No abstract

Escherichia coli are relatively avirulent organisms, but evidence derived from clinical observations and epidemiological studies has shown that some "O" serotypes (1) more frequently cause parenteral disease (2-9), and others more frequently cause enteric disease (10) in man than do the remaining O serotypes of E. coli. In addition, certain E. coli strains are more lethal for mice than others, and there is evidence that strains virulent for man are virulent for mice (11,12).Investigations in this laboratory have revealed that susceptibility to phagocytosis is one important difference between a mouse-virulent strain of E. coli 07 recovered from an infant with pyelonephritis and sepsis, and a mouseavirulent strain of enteropathogenic E. coli O l l l recovered from a diarrheal stool (13,14). The 07 strain resisted phagocytosis both by macrophages in vivo and polymorphonuclear leukocytes in vitro, persisted in the peritoneal cavity, and multiplied there until sufficient endotoxin was present to kill the animal.The polysaccharide side chains of the cell wall lipopolysaccharide (LPS, endotoxin) of E. coli determine its specitic phage susceptibility (15), its O antigen specificity (16); anti-O antibodies are opsonins (16). It seemed reasonable, therefore, to assume *