In my initial installment on probiotics, I discussed how our intestine is home to trillions of microorganisms, primarily bacteria, referred to as the intestinal flora.  I noted that under normal conditions, most of the bacteria that comprise our flora are considered beneficial and help promote good health.  A normal intestinal anatomy, with normal physiologic function, acts to favor the growth and survival of a healthful flora.

Under normal conditions, the primary food source for the intestinal flora is fiber.  Fiber is an indigestible form of carbohydrate.  Due to our lack of the necessary enzymes to digest fiber, it travels unabsorbed, through the small bowel to the colon.  Once fiber reaches the colon, the normal home for the majority of our intestinal flora, it can be digested and metabolized by our flora, which does possess the necessary digestive enzymes.

Good bacteria appear to be able to inhibit pathogenic (disease causing) bacteria from inhabiting or increasing their numbers, within our intestine. The bacteria in a healthy flora produce substances that impair the ability of pathogenic bacteria to thrive.  Some of these substances change the pH within the intestine in a way that favors the growth of beneficial bacteria and discourages growth of unfriendly bacteria.  Resistance to pathogenic bacteria may further result from the ability of a healthy flora to successfully compete for and deny bad bacteria the nutrients needed for their survival. Once good bacteria have established themselves on the surface of the intestine, they defend their home turf against detrimental organisms by producing substances that impair the ability of pathogens to thrive.  

Additionally, some of the compounds produced by a healthy intestinal flora are known to enhance the health and function of the intestine.  Short-chain fatty acids are an example of a vital nutrient that isn’t found in the diet, but is produced by intestinal bacteria.  Bacterial fermentation of fiber in the colon produces short-chain fatty acids which are then absorbed and act to promote bowel health and function.  Finally, there is evidence that the intestinal flora interacts with and affects the immune system, potentially promoting our immune function.

However, when there are significant abnormalities in intestinal anatomy, motility, or absorptive capacity, this can result in an unhealthy intestinal flora.  Detrimental bacteria that would normally be unable to establish themselves in sufficient numbers to cause problems, can sometimes flourish in these situations.   Additionally, it is possible that a course of antibiotics can severely disrupt a healthy intestinal flora, allowing opportunistic, bad bacteria to gain a foothold in the intestine.  Once established, these bacteria can reduce the population of good bacteria and impair their normal beneficial affect upon bowel health and immune function.

Probiotics are often prescribed for individuals who have difficulty supporting a healthy intestinal flora, due to altered intestinal anatomy or function or for those who have had disruption of their flora as a result of antibiotic therapy.  These commercial bacteria and yeast preparations contain billions of microorganisms in each dose.  Probiotics are ingested in an effort to establish and maintain a beneficial intestinal flora, while inhibiting the survival of detrimental bacteria.  I will discuss the selection, purchase, and use of specific probiotic products, as we continue to explore this topic in upcoming postings.