From the microbiologist's point of view, the area of the monogastric gut most amenable to manipulation is the colon, since this is the main site of microbial fermentation, although the ileum also harbours a significant microbial population. The colon, however, by virtue of the enormous numbers of bacteria it contains, offers the greatest potential for the application of microbial changes which may have a significant effect on the overall health and productivity of the animal. However, the application of these isolates to live animal studies generally produces highly variable results - a preparation which is effective in one herd may be ineffective in another. As an example, isolates of the bacterial species Lactobacillus acidophilus can be obtained from many sources, including silage, milk, the rumen of cattle and sheep, and the lower intestine of most animals, including pigs. The age of the bacterial cultures, or more accurately the number of bacterial generations between isolation and application.
Bacteria reproduce quickly, and adapt rapidly to the environment in which they find themselves.
An isolate of Lactobacillus spp. In the complex microflora of the intestine, these bacteria are in constant competition with other microbial species for nutrients, so that the production of compounds inhibitory to other species confers a survival advantage. However, in isolation, the bacteria are grown in pure culture in a nutrient-rich medium. In this case, the production of an inhibitor active against an absent competitor represents a waste of resources for the bacterial cell. The stability of the intestinal fermentation, whereby the addition of large numbers of a single species may result in a population which cannot be maintained on the nutrients available.