American Journal of Clinical Nutrition, Vol 25, 1364-1371, Copyright © 1972 by The American Society for Clinical Nutrition, Inc.

Immunological responses and microorganisms indigenous to the gastrointestinal tract

¹ From the Department of Microbiology, University of Texas, Austin, Texas 78712

Specific pathogen-free (SPF) mice parenterally injected with particular indigenous microbes isolated from the GI tract produce immunoglobulins reacting with antigens of the homologous bacteria. Nonvaccinated, SPF mice also contain natural antibody-forming cells (AFC) in their spleens reacting with these indigenous bacterial antigens. These natural AFC are not detected in newborn mice until 7 days after birth. SPF mice, therefore, exhibit both immune and natural antibodies reacting with antigens of certain members of the GI microflora.

Natural AFC are present in these mice, however, in only low numbers. More natural AFC are present that react with certain nonindigenous coliform strains than with the indigenous microbes tested. In addition, a much greater immunological response is produced after vaccination with nonindigenous coliforms than with the indigenous microbes. Germfree mice monoassociated with an indigenous Bacteroides strain exhibit only a slight increase in PFC although 10⁸ Bacteroides are cultured per gram of cecal tissue. Consequently, the mouse may be less responsive immunologically to antigens of certain indigenous GI bacteria than to those of nonindigenous microbes.

The number of spleen PFC reacting with indigenous Bacteroides antigens in monoassociated germfree mice does increase, however, to levels found in conventional animals. Thus, at least some natural antibodies reacting with certain indigenous bacteria may be induced by those bacteria in their natural habitat in the alimentary tract.

Many other unresolved questions remain concerning the immune response of an animal to its indigenous GI microflora. Whether or not the immune response of an animal has an influence on GI microbes is unresolved. Little is known as well about secretory immunoglobulins and indigenous microorganisms. Animals in which the indigenous microorganisms have been extensively studied are attractive experimental animals for examining these and related question.

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