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Man’s Microbes: the Pets You Never Knew You Had


Recently, I have posted a few times on recent scientific research involving the gut, or intestinal, microflora, that is microscopic living things in one’s intestines.  I neglected to mention a few elementary facts regarding human beings and their microbes, particularly those in our least favorite bodily excrescences, feces.

First, the human feces are by dry weight 60% bacteria, living and dead.   The stomach is nearly sterile; from there, bacterial populations gradually increase as one proceeds down the intestinal tract. There are, very roughly, 100 trillion (An old source from 1977 says, there are up to 10 to the 14 power (10^14) bacteria in the human intestinal tract, that is, ten times more than the 10^13 human cells in the body) microbes in your intestine.  What is more important, they represent about a hundred times the number of different genes as are in the human genome, and many of these genes have functions that impact the human body in dramatic fashions.

For example, the gut microbes absorb carbohydrates and peptidoglycans that the human intestinal endodermal cells cannot, and break them down to generate simple sugars that the intestine can absorb, and the body can use.  The presence of this type of bacteria accounts in large part for the presence of apparent errors in the estimation of calorie intake in obese people, especially those who claim they are eating little but continue to gain weight.

Another example is, broadly, the secretion of factors by gut microflora that influence the human immune system, intestinal growth, and most unusually, moods (by the secretion of serotonin, for example), as well as their possible effect on neoplasms in the rectum, colon, and rarely, small intestine.

First and foremost, however, the microbes of the intestine serve as a protective barrier to pathogenic bacteria when the pathogens gain access to the gastrointestinal tract.  Resident bacteria adhere to the cell walls of intestinal surface cells and prevent other bacteria from gaining access by “crowding them out” or competing for residence in the same ecological niche, reducing available energy sources, and even poisoning the invaders.

The residents of the intestinal tract form many large groups but Bacteroides and Firmicutes are two of the most populous.  The Firmicutes are usually gram-positive, include some pathogenic species, and they are known to be “promiscuous” sharers of genetic material by connecting to each other with thin hollow tubes that admit the nucleic acid chains and pass them from one to another.  Many are also capable of forming endospores, smaller cells with condensed genetic material that are essentially inactive and resistant to adverse environmental conditions like heat and cold.

Bacteroides, on the other hand, are gram-negative and do not form endospores.  They also protect the gut against invasion by other bacteria.   However, Bacteroides fragilis is responsible for opportunistic infections such as appendiceal abscesses.  Bacteroides are typically resistant to a wide variety of antibiotics and have recently shown resistance to more (erythromycin, clindamycin, and tetracycline.)  Bacteroides are typically found in people who eat plenty of meat and animal fats, and also in those who are lean.  The Firmicutes, conversely, especially those called Prevotella, are typically found in those who consume large quantities of carbohydrates, as well as vegetarians.

When people are fed a laboratory diet, their bacteria respond with changes in composition within 24 hours.  However, these changes are usually modest.  Normal weight people who eat a lot of fat and protein usually have Bacteroides in their intestines and do not change that composition when fed a laboratory diet over a period of ten days.

A study of European children versus children in Burkina Faso showed that the Europeans had mostly Bacteroides, while the Burkina Faso children had a predominance of Prevotella.  This was attributed to the diet in Burkina Faso, which is mostly carbohydrate, whereas the European diet has plenty of fat and protein.

It is entirely possible for animals to live without any bacteria at all in their intestines.  This type of animal can be grown and born without the presence of bacteria, and they can be raised in an essentially sterile environment.  Germfree mice, however, require supplements of Vitamin B12, Vitamin K, folate, and biotin.

The gut bacteria also digest polysaccharides and some oligosaccharides into short chain fatty acids(acetate, propionate, and butyrate), which stimulate the proliferation of the gut epithelium and cause it to differentiate.

The gut immune system is stimulated by enteric bacteria, and it forms into nodes along the intestinal tract; when fully developed, it is known as gut-associated lymphoid tissue or GALT.  When an antigen is consumed orally, the systemic response to it is prevented; this effect persists for more than a month in normal guts but only a few days in germ-free mice.

More fascinating facts later.

Sources: Science public access article 2011; Infection and Immunity 2008;

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