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The Role of the Microbiome in Gut Immune System Development in Newborn and Mature Cattle
There is increasing evidence in a variety of mammalian species that the commensal microbiome has diverse effects on mucosal immune system development and function. It is difficult, however, to clearly delineate the effects of the microbiome from other contributing factors, such as diet, environment, and host genetics. The bovine gastrointestinal tract (GIT) is rapidly colonized during birth and these pioneer species are then replaced by a succession of changes, involving both increased microbioal density and diversity. This succession occurs rapidly during the first week of life and then progresses much more slowly. Characterization of the microbiome in the neonatal bovine GIT at both a family and species level revealed marked bacterial variation among individual animals. Furthermore, the composition of the microbiome varied significantly when comparing ingesta- and mucosa-associated communities within individual GIT regions. The first week postpartum is also a very dynamic developmental period in the bovine GIT with significant changes in both mucosal barrier and immune function. These developmental changes were analyzed by profiling miRNAs expressed throughout the small intestine. This analysis confirmed the greatest changes in GIT development occurred during the first week of life with differential expression of miRNAs involved in regulating a broad range of GIT developmental and immunological processes. Relatively few miRNAs were differentially expressed when comparing tissues collected from 6 week old calves and 3 week old calves. Correlation analyses between total bacterial numbers and specific families revealed significant associations between the commensal microbiome and the expression of genes involved in regulating both mucosal barrier and innate immune function. It appears the microbiome is an important factor influencing age-dependent changes in the expression of immune function genes. These correlation analyses also suggest that regional differences in the microbiome may be associated with significant regional differences in the expression of innate immune genes. This information provides the baseline to begin analyzing the role of individual bacterial species and interactions among bacterial species in regulating mucosal immune system function in healthy animals and during enteric infections. New experimental models will be required, however, to clearly delineate the role of specific bacterial species in the complex interaction between microbiome and host.
Keywords: microbiome, innate immunity, miRNA