Bulletproof Your Intestines

Or, the importance of good bacteria on health and digestion. I'm going to paraphrase the Wikipedia entry on Gut Flora in a moment.
In summary:

• There are more bacteria in the human body than there are total cells.
• The relationship is symbiotic.
• Bacteria help digest things your body otherwise cannot.
• Bacteria improves digestion by up to 30% in rats (improves the body's ability to extract nutrition from food)
• Bacteria contribute vitamins to their host as they help digest foods.
• Bacteria prevent growth of pathogenic bacteria (reduce likelihood of becoming ill).
• Bacteria train the immune system and may prevent allergies.
• Bacteria may help prevent cancer by lowering pH to expel toxins and prevent tumor growth.
• The bacteria composition of human hosts is directly linked to obesity/bodyfat composition.

This doesn't require much editorializing, since the facts speak so loudly. Bacteria is an essential part of the human condition, improving it almost wholesale. Humans evolved in a much dirtier (less sterile) world than we inhabit today. In some ways, our bodies thrive under these conditions since we evolved to live in symbiosis with these bacteria.
Do not neglect your intestinal bacterial health. It is an important "organ," and benefits from being nurtured with helpful cultures such as those pictured below. Remember: every cell in your body is made up from nutrients that pass through your mouth, and through your digestive tract. Make the most of it and reap the benefits.

My current ferments: kombucha, kefir and sauerkraut.
Here's the promised Wikipedia paraphrasing:

Gut flora consists of microorganisms that live in the digestive tracts of animals and is the largest reservoir of human flora. Gut (the adjective) is synonymous with intestinal, and flora with microbiota and microflora.
The human body, consisting of about 100 trillion cells, carries about ten times as many microorganisms in the intestines. The metabolic activities performed by these bacteria resemble those of an organ, leading some to liken gut bacteria to a "forgotten" organ.
Research suggests that the relationship between gut flora and humans is not merely commensal (a non-harmful coexistence), but rather a symbiotic relationship. Though people can survive without gut flora, the microorganisms perform a host of useful functions, such as fermenting unused energy substrates, training the immune system, preventing growth of harmful, pathogenic bacteria, regulating the development of the gut, producing vitamins for the host (such as biotin and vitamin K), and producing hormones to direct the host to store fats.
Bacteria in the gut fulfill a host of useful functions for humans, including digestion of unutilized energy substrates, stimulating cell growth, repressing the growth of harmful microorganisms, training the immune system to respond only to pathogens, and defending against some diseases.
Without gut flora, the human body would be unable to utilize some of the undigested carbohydrates it consumes, because some types of gut flora have enzymes that human cells lack for breaking down certain polysaccharides. Rodents raised in a sterile environment and lacking in gut flora need to eat 30% more calories just to remain the same weight as their normal counterparts.
Another important role of helpful gut flora is that they prevent species that would harm the host from colonizing the gut, an activity termed the "barrier effect". Harmful yeasts and bacterial species such as Clostridium difficile (the overgrowth of which can cause pseudomembranous colitis) are unable to grow excessively due to competition from helpful gut flora species adhering to the mucosal lining of the intestine, thus animals without gut flora are infected very easily. The barrier effect protects humans from both invading species and species normally present in the gut at low numbers, whose growth is usually inhibited by the gut flora.
The process of fermentation, since it produces lactic acid and different fatty acids, also serves to lower the pH in the colon, preventing the proliferation of harmful species of bacteria and facilitating that of helpful species. The pH may also enhance the excretion of carcinogens.
Gut flora have a continuous and dynamic effect on the host's gut and systemic immune systems. The bacteria are key in promoting the early development of the gut's mucosal immune system both in terms of its physical components and function and continue to play a role later in life in its operation. The bacteria stimulate the lymphoid tissue associated with the gut mucosa to produce antibodies to pathogens. The immune system recognizes and fights harmful bacteria, but leaves the helpful species alone, a tolerance developed in infancy.
The resident gut microflora positively control the intestinal epithelial cell differentiation and proliferation through the production of short-chain fatty acids. They also mediate other metabolic effects such as the syntheses of vitamins like biotin and folate, as well as absorption of ions including magnesium, calcium and iron. The gut flora plays a major role in metabolizing dietary carcinogens, the microcomponents and the macrocomponents.
Bacteria are also implicated in preventing allergies, an overreaction of the immune system to non-harmful antigens. Studies on the gut flora of infants and young children have shown that those who have or later develop allergies have different compositions of gut flora from those without allergies, with higher chances of having the harmful species C. difficile and S. aureus and lower prevalence of Bacteroides and Bifidobacteria. One explanation is that since helpful gut flora stimulate the immune system and "train" it to respond properly to antigens, a lack of these bacteria in early life leads to an inadequately trained immune system that overreacts to antigens.
It is known from experiments on mice that obese mice lacking leptin, a lipid metabolism regulator (ob/ob mice), have a distinct gut flora compared to (normal) lean mice, reflected in a change in the ratio between bacteria from the divisions Bacteroidetes and Firmicutes, which is shifted towards fewer Bacteroidetes and more Firmicutes in obese mice.
The microbes occupying the human gut are also in direct relation to obesity. A shift in the ratio between bacterial divisions Firmicutes and Bacteroidetes can be observed in lean and obese individuals—in the latter, a shift towards Firmicutes can be observed. The ratio between Firmicutes and Bacteroidetes dynamically reflects the overall weight condition of an individual, shifting towards Bacteroidetes if an obese individual loses weight.