The rootlets of plants are surrounded by bacteria that perform a useful office for the plant by fixing the nitrogen of the air and converting it into compounds that the plant can use in the manufacture of protein and plant protoplasms.
Bacteria are evidently necessary for the promotion of plant life through the making of soil and the fixation of nitrogen, although it was long ago shown by the Pasteur Institute that some plants may grow in a perfectly sterile medium. Do bacteria perform an equally useful service for animals, or is their presence in the alimentary canal to be looked upon as an unfortunate accidenta condition to which the body has been obliged to accommodate itself, although not without suffering serious disadvantages? The studies of Tissier and Metchnikoff and numerous others have demonstrated that bacteria are not essential to animal life, notwithstanding the fact that they are commonly found present in the intestines of animals. The presence of bacteria, in other words, is in a sense accidental and in no way essential to life.
It is true, as already pointed out, that the intestine is invaded by acid-forming bacteria within a few hours after birth. But this seems to be a defensive arrangement that has been developed for the purpose of protecting the young organism against the destructive effects of putrefactive organisms that otherwise would quickly take possession of the whole digestive tract.
Pasteur held that animal life without bacteria would be found impossible but he did not under-take to test his theory by actual experiment. Experiments made by various investigators have not confirmed the views of Pasteur but have shown the opposite to be true.
Metchnikoff has long maintained that the colon bacilli are not helpful, but in the highest degree harmful, producing, through their pernicious influence, hardening of the arteries, pre-mature old age, and numerous degenerative disorders of the heart, liver, kidneys, and other vital organs.
Fortunately the animal world furnishes a sufficient number of examples of animal organisms which exist, either the whole or a part of their life-cycle, without the presence of bacterial life in their interiors.
Certain animals, as has been pointed out by Metchnikoff, are naturally free from bacteria. Among these are the large fruit eating bats of the tropics. These animals have very short colons and begin to evacuate the unusable remnants of food an hour after it has been eaten. The material does not remain long enough to permit of any fermentative changes. The digestive juices of these animals are found to be incapable of destroying bacteria. Their intestines are free from bacteria simply because they live upon food that contains no bacteria and discharge the unusable remnants of their food be-fore there has’ been time for bacteria to develop. No putrefactive products are found in the feces of the fruit-eating bat; that is, they contain no phenol, skatol or indol.
The scorpion affords another example of an animal whose intestine contains no microbes. The same is also true of certain maggots which are provided with digestive juices which are able to digest wool, seeds, and the most resistant microbes (Burnet).
A study of Arctic animals at Spitzenberg by Levin showed that the intestines of more than half the animals examined were free from bacteria.
The larve of various insects that burrow in thick leaves live without contamination by bacteria.
Many caterpillars are perfectly aseptic. A caterpillar that frequents rose treesNepticular -is always free from bacteria.
Wollmann succeeded in breeding sterile flies from aseptic larva.
Nuttall and Thierfelder reared guinea pigs without bacteria.
Says Burnet, of the Pasteur Institute, a pupil of Metchnikoff, “From the point of view of nature, it is quite normal for the albuminous excreta of our food to putrefy and thus return into the general circulation of matter ; but it is regrettable when this takes place in our bodies, for phenols, skatol, and indol, among other products, penetrate into our circulation and affect the cells of our arteries and brain. It would be to our advantage if the food-stuffs were expelled immediately after useful digestion, and before the terminal phase, the putrefaction, begins in the waste products. And since in the part of the intestine, which properly speaking is the digesting part, the small intestine, there are practically no bacteria, and since, on the other hand, they swarm in the large intestine where there are scarcely any digestive ferments, it is evident that, on the whole, the intestinal flora is injurious. The ideal condition would be to live free from bacteria while the world remained populated by them. Is a life of such purity possible?”