With a slight travesty of the well-known terminology introduced by Claude Bernhard, the mammalian organism itself—a mouse, a rat or a human—can be characterized as a Milieu interieur (MI), a normal microbiota as a Milieu exterieur (ME) and the macroorganism and its microbiota as a Milieu total (MT) (3). In studies on the interplay between MI and ME, two terms, i.e., Microflora Associated Characteristic (MAC) and Germ-free Animal Characteristic (GAC)—have been found to be of considerable value (4). A MAC is defined as the recording of any anatomical structure, physiological, biochemical or immunological function in a macroorganism, which has been influenced by the microbiota. When microorganism(s) influencing the parameter(s) under study are absent—as in germ-free animals, newborns, or in relation to ingestion of antibiotics—the recording of a MAC can be defined as a GAC. Consequently, a germ-free organism is a sum of all GACs, and a normal macroorganism is a sum of MACs. Studies in germ-free animals and healthy newborns have given us the values of GACs, i.e., the MI. When we are investigating conventional organisms—MT—the question "what have the microbes done," can be answered by the equation MT minus MI = ME. A gnotobiotic animal harboring a known microbiota, may present a set-up of some MACs and some GACs, depending on the specific activity of its microbiota.

Over the years, the MAC/GAC concept has been applied in several studies (5). So far, most studies have been related to a phenotypic expression of what the microbes have done. However, the concept is applicable also when studying host-microbe cross-talk on a molecular, genotypic level (6-8). In the following, some major discrepancies between germ-free and conventional animals will be highlighted (Table 1).

Pregnancy And Childbirth

Pregnancy And Childbirth

If Pregnancy Is Something That Frightens You, It's Time To Convert Your Fear Into Joy. Ready To Give Birth To A Child? Is The New Status Hitting Your State Of Mind? Are You Still Scared To Undergo All The Pain That Your Best Friend Underwent Just A Few Days Back? Not Convinced With The Answers Given By The Experts?

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