Bacteria exist in our guts in a symbiotic relationship with us; they utilize the food we eat, and in return, help digest our food and produce certain compounds, like vitamin K, that we need. As soon as the baby begins swallowing breast milk or formula, any bacteria in the mouth are washed into the stomach. Much of the time, the acid of the stomach kills bacteria, but a few slip through. Plus, if the flushing rate of the stomach is rapid, as it often is in infants, not all the mouth bacteria are killed in the stomach, and many can go on to flourish in the small and large intestine. There they help us as much as we help them. In fact, sometimes newborns develop vitamin K deficiencies and hemorrhage because their intestines do not yet have the proper bacteria to produce this vitamin. Older people receiving antibiotic therapy sometimes have a similar problem when their intestinal bacteria are killed off.
As many as 300 different species of bacteria have been identified from human feces. An adult can defecate 3 x 1013 bacterial cells per day. The types of bacteria present in the gut are highly influenced by the types of foods one eats. If cramping and bloating are frequent problems for you or someone you know, try eating yogurt or drinking acidophilis milk. The species present in these products often help with those types of problems. Also, when taking antibiotics, diarrhea often results. Rather than taking anti-diarrheal medications, some physicians are now recommending yogurt to help replace microorganisms lost through antibiotic therapy.
Gnotobiotic animals are those that are raised without any symbiotic organisms, like the bacteria that would normally inhabit their guts, or raised with only certain specific microorganisms. The purpose of raising animals in the laboratory this way is to determine the nature of the interactions occurring between the animal and its microflora. Raising gnotobiotic animals is not an easy task, since every aspect of a normal environment contains microorganisms. Using very aseptic techniques, technicians begin with cesarean sections of pregnant animals within a special isolation chamber. The baby animals are then transferred to other isolation chambers where they are fed food and water that has been sterilized. Researchers can then start a microorganism-free colony of animals that can mate and give birth normally.
The interesting thing that scientists have found in gnotobiotic animals is that they do not develop normally. As adults, they do not have the same anatomy and physiology as their counterparts with symbiotic microbes. Their basic anatomy is the same of course--same numbers of legs, a tail, and so on. But their lymphatic systems do not develop as they should, and they have poorly developed intestinal walls and enlarged areas of their large intestines. Vitamin K and the B complex vitamins must be given to these animals to ensure their survival. They are more susceptible to disease when they finally do encounter it. On a brighter note, microbe-free animals do not have cavities in their teeth.
After sharing this information with your students, discuss with them the necessity of having symbiotic microorganisms inhabiting our bodies and the consequences of an interruption in our normal microflora.