Probiotics and Microflora

Max Sherman, RPh

Disclosures

US Pharmacist. 2009;34(12):42-44. 

In This Article

Genesis and Evolution

Most of us are aware that bacteria are a part of a healthy human ecosystem (i.e., an assembly of species and the organic and inorganic constituents characterizing a particular site). According to one author, the armies of bacteria that sneak into our bodies the moment we are born are the "primal illegal immigrants."[5] Most are industrious and friendly, minding their own business in tight-knit, long-lived communities, doing the grunt biochemical work we all rely on to stay alive.[5] The ecosystem forms at birth, but the human-microbe alliance begins months before. Midway through pregnancy, a hormonal shift directs the cells lining the vagina to begin stockpiling sugary glycogen, the favorite food of sausage-shaped bacteria called lactobacilli. By fermenting the sugar into lactic acid, these bacteria lower the pH of the vagina to levels that discourage the growth of potentially dangerous invaders.[6]

The infant mouth's first inoculation of bacteria includes a generous sampling of the lactobacilli present in the mother's birth canal. With the first gulp of breast milk, these lactobacilli are joined by millions of bifidobacteria, a related group of acid-producing microbes.[6] The source of these bacteria is the mother's nipples, where the bacteria appear during the eighth month of pregnancy. Bifidobacteria secrete acids and antibiotic chemicals that repel potentially dangerous organisms, including Staphylococcus aureus. Bifidobacteria and lactobacilli are soon joined by acid-tolerant Streptococcus salivarius bacteria, which appear on a baby's tongue during the first day of life. Bifidobacteria are anaerobic, pleomorphic rods that break down dietary carbohydrate and synthesize and excrete water-soluble vitamins.[7] Their name is derived from the observation that they often exist in a Y-shaped, or bifid, form.[8] These organisms predominate in the colons of breastfed babies, account for up to 95% of all culturable bacteria, and protect against infection.[9] Strangely, they do not occur in such high numbers in adults.[8] Several other streptococci, along with one or more kinds of Neisseria bacteria, settle in during the first week. The vast majority emanate from the mother's mouth, which is always within reach of a nursing baby's fingers.[10]

As the baby begins nursing or drinking formula, the bacterial population inside the mouth increases. These bacteria consume enough oxygen to create a zone where anaerobic bacteria can thrive. By the time the baby is 2 months old, a microscopic close-up of the gums will reveal clusters and chains of bacteria and fungi. Another wave of bacteria arrive when the first teeth appear. The first is Streptococcus sanguis, followed by Streptococcus mutans. By middle childhood, the diversity inside the mouth surpasses a hundred species, and their total number is greater than 10 billion.[6] Bacteria also settle in the nasal cavities, which are connected to the mouth via the upper respiratory tract. The bacteria eventually lodge in the intestinal tract. In the small intestine, incoming microbes engage the infant's dormant immune system. Pits on the surface of the Peyer's patches (aggregated lymphoid tissue in the ileum) capture passing bacteria, where they are ushered into the underlying lymph tissue. Interaction on the Peyer's patches triggers the production of an abundance of immunoglobulin A (IgA) antibodies. Instead of marking the bacteria for destruction, IgA clusters across the bacterial surface, preventing the bacteria from attaching to the intestinal wall. This action also leads to the proliferation of T and B cells that will marshal an attack against these same bacteria should they turn up in the blood or other forbidden areas.[6] The small intestine must provide a platform for nutrient absorption, but at the same time the epithelium and its associated immune cells must keep out pathogens that escape the inhospitable environment of the stomach. To satisfy these responsibilities, small intestinal epithelial cells divide at a rate of 13 to 16 cells every hour.[6] When the child reaches adulthood, his or her intestine becomes home to an almost inconceivable number of microorganisms. The size of the population—up to 100 trillion—far exceeds all other microbial communities associated with the body's surfaces and is more than 10 times greater than the total number of our somatic and germ cells combined.[11] (There is a significant variation in both the total number of bacteria and the composition of the bacterial flora in different body regions.[12]) Since humans depend on their microbial inhabitants (microbiome) for various essential services, a person should really be considered a superorganism, consisting of his or her own cells and those of all the commensal bacteria. Humans are not inherently endowed with a healthy immune or digestive system. Fortunately, the microbiome in our intestinal tract provides us with genetic and metabolic attributes we have not been required to evolve on our own, including the ability to harvest otherwise inaccessible nutrients and to modify host immune reactivity.[11]

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