The normal intestinal microflora: ecology, variability and stability.

The composition and functions of the intestinal floras from humans and other animals is compared and contrasted. Intrinsic and extrinsic factors, such as anatomy, age, and diet, help define the nature of the flora typical to each species of animal. Stable differences, such as the ability to reduce cholesterol or not, between the flora of individuals of the same species are described. The difficulties of characterizing a flora by cultural taxonomy are discussed and alternatives are suggested. These alternatives, collectively known as microflora associated characteristics (MACs), are measures of the biochemical activity of the bacterial flora. How MACs can be used to measure the effects of subtherapeutic levels of antibiotics on a flora is described.