Protective effect of rabbit immune serum administered orally to rats infected by a human pathogenic strain of E. coli

1-week-old rats were inoculated orally with a strain of E. coli (serotype 078) isolated from the blood of a newborn baby who had died of septicemia. During the 3 weeks following inoculation, approximately 50% of the animals died of septicemia and 60% of the surviving rats had pathogenic bacteria in their rectum. Some of the surviving rats were severely impaired in their development. Autopsy showed evidence of active intestinal infection localized mainly in the ileum and cecum. A rabbit anti-E. coli (strain 23) serum (agglutinating titer: 1/2,500) afforded 100% protection when as little as 0.03 mg of serum protein per gram of rat body weight was orally administered in a single dose. The immune serum had an effect both on the mortality rate and on the growth of the rats. However, it never affected the survival of pathogenic bacteria in the rectum, even when administered at a daily dose of 1.5 mg of serum protein per gram of rat body weight on 4 consecutive days. The immune rabbit serum had only a weak bactericidal effect in vitro. The hemagglutination test showed the presence in the immune serum of antibodies against the fimbriae of the pathogenic E. coli strain (titer: 1/1,000). The role of antibody in inhibiting the adherence of bacteria to epithelial cells and/or their progression across the mucous layer are discussed as possible immune mechanisms in the intestinal lumen.