Microflora and our health

Human flora

The human flora is the assemblage of microorganisms that reside on the surface and in deep layers of skin, in the saliva and oral mucosa, and in the gastrointestinal tracts. They include bacteria, fungi, and archaea.

Some of these organisms are known to perform tasks that are useful for the human host, however the majority have no known beneficial or harmful effect. Those that are expected to be present, and that under normal circumstances do not cause disease, but instead participate in maintaining health, are deemed members of the normal flora, or microbiota. 

It is estimated that 500 to 1000 species of bacteria live in the human gut   and a roughly similar number on the skin.  Bacterial cells are much smaller than human cells, and there are at least 10 times as many bacteria as human cells in the body (approximately 10¹⁴ versus 10¹³)

Intestinal flora

Gut flora consists of microorganisms that live in the digestive tracts of animals, and is the largest reservoir of human flora.

The metabolic activities performed by these bacteria resemble those of an organ, leading some to liken gut bacteria to a "forgotten" organ". It is estimated that these gut flora have around 100 times as many genes in aggregate as there are in the human genome.

Bacteria make up most of the flora in the colon and 60% of the dry mass of feces. Fungi and protozoa also make up a part of the gut flora, but little is known about their activities.

Research suggests that the relationship between gut flora and humans is not merely commensal (a non-harmful coexistence), but rather a symbiotic relationship.

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, and producing hormones to direct the host to store fats. However, in certain conditions, some species are thought to be capable of causing disease by producing infection or increasing cancer risk for the host.

Superorganism

Metagenomic Analysis of the Human Distal Gut Microbiome, Science 2 June 2006:

The human intestinal microbiota is composed of 10¹³ to 10^{14 *} microorganisms whose collective genome ("microbiome") contains at least 100 times as many genes as our own genome.

We analyzed 78 million base pairs of unique DNA sequence and 2062 polymerase chain reaction–amplified 16S ribosomal DNA sequences obtained from the fecal DNAs of two healthy adults. Using metabolic function analyses of identified genes, we compared our human genome with the average content of previously sequenced microbial genomes. Our microbiome has significantly enriched metabolism of glycans, amino acids, and xenobiotics; methanogenesis; and 2-methyl-D-erythritol 4-phosphate pathway–mediated biosynthesis of vitamins and isoprenoids.

Thus, humans are superorganisms whose metabolism represents an amalgamation of microbial and human attributes.

*100 000, 000 000 000 - hundred thousands billions.

Half cannot be cultured

Fergus Shanahan, MD, FRCP (Professor and Chair):

Colonization with bacteria is critical for the normal structural and functional development and optimal function of the mucosal immune system.

At least half of the resident flora cannot be cultured by conventional techniques but are identifiable by molecular methods. Collectively, the resident flora represent a virtual organ with a metabolic activity in excess of the liver and a microbiome in excess of the human genome. An improved understanding of this hidden organ holds secrets relevant to several infectious, inflammatory and neoplastic disease mechanisms.

Fascinating stuff !  I wonder if they like it when you swim in a lake or the ocean, or go out in the sun a while ? - Probably.  :-)  Certainly they would prefer you eat some foods over others...  So keep your microbes happy (or at least in sustainable balance).  :-)

I guess it is not easy to say what they like and whether they ever have any consensus on it :)

An interesting summary from a study on one type of the beneficial microorganisms:

"Humans and other mammals are colonized by a vast, complex, and dynamic consortium of microorganisms. One evolutionary driving force for maintaining this metabolically active microbial society is to salvage energy from nutrients, particularly carbohydrates, that are otherwise nondigestible by the host."

If it's true that carbohydrates "are otherwise nondigestible" except for the "vast, complex, and dynamic consortium of microorganisms" living within us, that would be huge.  True obligate symbiosis !  I had thought, and read, that it was "facultative", that "we could live without them", just not quite as... well.

Btw:  The appendix is now thought to be the "safe house" for our microflora in case of sickness or an attack of antibiotics...    

Some quick internet searching seems to confirm it !  We have obligate symbiosis with our anaerobic intestinal flora for breaking down carbohydrates.  Not just us, other animals, including some fish, have it too.  Life is far more interwoven, more interdependent, than we thought...

Exactly, and not only that! Regarding microflora, I read there are some animals they can keep sterile: keeping these mice in special conditions and providing them with specific diet, they learn over the decades to keep them alive longer.

Some quotations on appendix:

Appendix

William Parker, Ph.D., assistant professor of experimental surgery, who conducted a study on appendix in collaboration with R. Randal Bollinger, M.D., Ph.D., professor emeritus in general surgery, in Duke University Medical Center:

"Our studies have indicated that the immune system protects and nourishes the colonies of microbes living in the biofilm. By protecting these good microbes, the harmful microbes have no place to locate. We have also shown that biofilms are most pronounced in the appendix and their prevalence decreases moving away from it. ... Diseases causing severe diarrhea are endemic in countries without modern health and sanitation practices, which often results in the entire contents of the bowels, including the biofilms, being flushed from the body. ... Once the bowel contents have left the body, the good bacteria hidden away in the appendix can emerge and repopulate the lining of the intestine before more harmful bacteria can take up residence."

Biofilm is thin and delicate layer, an amalgamation of microbes, mucous and immune system molecules living together atop of the lining the intestines.

Loren G. Martin, professor of physiology at Oklahoma State University:

"Endocrine cells appear in the appendix of the human fetus at around the 11th week of development. These endocrine cells of the fetal appendix have been shown to produce various biogenic amines and peptide hormones, compounds that assist with various biological control (homeostatic) mechanisms. ... Among adult humans, the appendix is now thought to be involved primarily in immune functions. ... Researchers have also shown that the appendix is involved in the production of molecules that help to direct the movement of lymphocytes to various other locations in the body. ...This local immune system plays a vital role in the physiological immune response and in the control of food, drug, microbial or viral antigens. ... In the past, the appendix was often routinely removed and discarded during other abdominal surgeries to prevent any possibility of a later attack of appendicitis.