Around 500 species of bacteria, as well as many species of yeast and other organisms, inhabit the human gastrointestinal tract and make up the "gut flora". In an average adult the bacteria weigh about 1kg and the number of individual organisms easily outnumbers the total number of cells in the human body.
Luckily for us a large number of these organisms don't cause disease – instead, they help protect us from disease. The gut flora perform many functions that contribute to the health of the human host; "friendly bacteria" mostly have names beginning with 'Lactobacillus' or 'Bifidobacteria'. You may have noticed food products like yogurt that contain these bacteria. It is becoming more common for them to be added, just as vitamins and minerals are added to some food products.
The friendly bacteria aren't the only type of microorganism present in the human gastrointestinal tract however. There are also a number of organisms that don't provide the human host with any benefits; these include coliform bacteria, yeasts and bacteroides. Usually these other organisms are kept in balance with the friendly bacteria.
Lactobacillus type strains are capable of fermenting food stuffs to lactic acid. L. acidophilus and similar strains are used in cultured dairy products such as yogurt. Other major useful functions of friendly bacteria include:
For many years, the addition of these "good" bacteria to our intestinal tract has been used to promote health. As a class, they have been referred to as probiotics, since they stimulate organismal growth is the intestines. Uses of probiotics have been especially helpful when antibiotic use kills many of the good bacteria, allowing other less beneficial bacteria to take up residence in the gut.
Lactobacilli have active anti-microbial characteristics and are able to kill various pathogens such as Streptococcus, Staphylococcus, E. coli and the yeast Candida. The benefits of lactobacillus and other probiotics are only now coming to full light, so expect much more research to be forthcoming in the next few years.
Illness can occur when the amount of friendly bacteria is reduced and the other organisms are able to increase their numbers and become the majority. There are a number of factors that can disrupt the balance of organisms in the intestines and lead to overgrowth of the less desirable species.
The most important factors are:
Probably the most important factor is the use of broad spectrum antibiotics. These medications don't discriminate between friendly and unfriendly bacteria. This means that every time you take one of these drugs for an infection your are wiping out large numbers of the friendly bacteria in your intestines that give protection from other, potentially harmful, intestinal residents. This fact has long been known by researchers and is taught to microbiology students.
Adding sufficient acidophilus bacteria or commercial preparations of the enzyme lactose to milk-containing meals will prevent symptoms and improve nutrient absorption for individuals with lactose intolerance, but not for those allergic to milk.
Acidophilus is essential in the absorption of nutrients from our food. It colonizes the walls of the small intestine, secretes substances that help maintain a balance of intestinal microflora (by creating a favorable environment for the growth of beneficial flora), and helps keep the intestinal wall functioning properly by keeping clear the spaces between the microvilli so that they can function efficiently. Acidophilus also promotes proper peristalsis – the normal movement of food through the small intestine. The beneficial flora dramatically influence metabolism and overall health.