Immunological response of mice to members of the autochthonous intestinal microflora

Based on agglutination, complement fixation, bactericidal, and opsonization tests, it is evident that suspensions of a Bacteroides sp. isolated from the mouse intestine and given by parenteral routes do not initiate antibody formation in mice. Rats also respond poorly to the antigenic stimulus. In contrast, suspensions of other bacterial species isolated from the rodent intestine are highly immunogenic. Furthermore, high titers of antibody are produced in rabbits, guinea pigs, and sheep after parenteral injection of the Bacteroides suspensions. Marked differences in the absolute and relative numbers of Bacteroides in the intestinal contents of rodents and the latter three animal species have been demonstrated. It is proposed that the immunological unresponsiveness of rodents to this organism is related to its existence as a member of the autochthonous microflora in these animals.