The role of the PCF8775 antigen and its antigenic subcomponents, in particular, the coli surface (CS) antigen CS6, as colonization factors and protective antigens was studied in the reversible intestinal tie adult rabbit diarrhea model. This was done by testing the abilities of different mutants which carried one or two of the CS components to colonize the intestine and to induce protective immunity against reinfection with PCF8775-positive enterotoxin-producing Escherichia coli. Infection with enterotoxigenic E. coli carrying CS4-CS6, CS5-CS6, or CS6 alone induced diarrhea in 75% or more of the rabbits, whereas the corresponding nonenterotoxigenic mutants, as well as enterotoxigenic but CS-negative strains, induced diarrhea in only a few cases. Mutants carrying CS6 alone colonized the intestine equally as well as strains carrying CS4-CS6 or CS5-CS6 did, whereas CS-negative mutants were excreted in the stool for a significantly shorter period. Rabbits previously infected with mutants carrying CS6 alone or CS6 in combination with CS4 or CS5 developed diarrhea with a significantly lower frequency after reinfection with a normally highly diarrheagenic dose of enterotoxigenic CS4-CS6-positive E. coli bacteria than did animals immunized with corresponding CS-negative mutants. Fecal excretion of the rechallenge strain was also of considerably shorter duration than that observed after initial infection with corresponding strains in 27 of the 30 animals (90%) immunized with strains carrying CS6 alone or in combination with CS4 or CS5. Such reduced shedding of the challenge strain was only seen in a few rabbits (3 of 12) initially infected with CS-negative bacteria. These results suggest that the CS6 component of PCF8775 is a colonization factor in rabbits and that it is also capable of inducing protective immunity
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