Synergistic protective effect of antibodies against Escherichia coli enterotoxin and colonization factor antigens.

We studied the ability of antisera against different Escherichia coli surface antigens, both alone and in combination with anti-enterotoxin, to decrease fluid secretion induced by intestinal challenge with enterotoxigenic E. coli in rabbits. Antiserum against lipopolysaccharide protected significantly against O group homologous bacteria. Monospecific antisera against pilus-associated colonization factor antigens CFA/I and CFA/II were also effective, giving highly significant protection against enterotoxigenic E. coli strains bearing the corresponding colonization factor antigens. Protection was also observed with Fab fragments of the CFA/I antibodies. Addition of the anti-lipopolysaccharide serum to a protective antiserum against purified heat-labile enterotoxin resulted in an antisecretory effect which slightly exceeded the sum of the effects obtained with each preparation alone. The combination of antiserum against CFA/I or CFA/II with anti-enterotoxin gave protection that equaled the product of the effects obtained with each antiserum alone; i.e. the antisera cooperated synergistically.