Intestinal Microbiota Gross Anatomy Histology And Motility

An enlargement of the cecum in the Caesarean-derived guinea pigs was the first anatomical difference observed when the epoch of germ-free research started (1), and similar differences have been observed in all rodents so far investigated. This enlargement might partly be explained by an absence of mucin breakdown in the germ-free animals, partly by a reduced degradation of dietary compounds, such as fiber, and partly by a reduced sensitivity to biogenic amines in germ-free animals (11). Interestingly, it has been shown that a mono-association of germ-free animals with Clostridium difficile markedly reduced the cecum size (12).

For years, it was generally accepted that the villi were more slender and uniform in shape and that the crypts were shallower, containing less cells in germ-free animals as compared to their conventional counterparts. Moreover, the lamina propria was supposed to be thinner, and the turn-over rate of epithelial loss was slower. However, most recently it was shown—in germ-free, and conventional rats and mice—that age, gender, and the intestinal compartment actually under study have to be taken into proper consideration before stating significant differences (13-15).

Another striking difference than an enlarged cecum, is a reduction in spontaneous muscular activity in germ-free animals. This may in part be due to a reduced sensitivity to biogenic amines (11), partly also to a reduction in motor migrating complexes (16). Interestingly, it was found that mono-association of germ-free animals with some bacterial species, including a probiotic strain, switches the function from a GAC to an MAC pattern within a few days. Furthermore, the area of endocrine cells in the GI tract is enlarged in germ-free animals (17).

Most recently it has been found that experimental post-surgical intestinal adhesion formation is markedly reduced in germ-free rats (18). After mono-associated with lactobacilli, i.e., a probiotic strain, the animals reacted similar to the germ-free control, whereas they switched to a conventional pattern after being mono-associated with Escherichia coli. Obviously, germ-free animals should be used for solving this important question in surgery.

Additionally, germ-free animals may express a compartmentalized reduced osmolarlity in intestinal content, an increased colloid osmotic pressure, a higher oxygen tension, and a higher redox potential than their conventional counterparts. As a consequence of this, strictly anaerobes are often difficult to establish as a monoculture in germ-free animals (this is often a dose-dependency).

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