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The most studied microbial gas product is hydrogen, or H2, gas. Many bacterial species, both helpful and harmful to humans, produce H2 gas as a byproduct of their metabolism, while others consume H2. While there is nothing intrinsically wrong or harmful about H2, gas, we can use the timing of H2, gas release in the breath to map out where in the GI tract H2,-producing organisms dwell. This is the principle used to chart the location of microbes in small intestinal bacterial overgrowth, SIBO, in which bacteria have ascended up into the upper GI tract. Methane gas, or CH4, is produced by a unique class of non-bacterial creatures called Archaea, species such as Methanobrevibacter smithii. Unlike H2, gas that has no known physiologic consequences for humans, methane slows the normal propulsive activity of peristalsis that propels digested food down the length of the GI tract. Because of this effect, overgrowth of methane-producing Archaea is associated with the signature issue of constipation. (It is not clear, however, whether upper GI colonization, not just colonic proliferation, is necessary for methanogenic consequences to be experienced.) There is a third gas produced by microbes: hydrogen sulfide, or H2S. Unlike H2, H2S can be toxic. “Sewer gas” is H2S with its characteristic unpleasant smell that emits from sewers and large collections of human waste and can cause respiratory illnesses in sewage workers. H2S is also produced by microbes in spoiled meat and rotten eggs in which they are agents of putrefaction—rotting. Other health conditions can also raise breath H2S levels. Chronic obstructive pulmonary disease (COPD), emphysema and bronchitis; emotional stress; laryngopharyngeal reflux; periodontal disease and a coated tongue, even bad breath (halitosis) are associated with increased breath H2S. Dentists use a device called a “Halimeter” to measure H2S in the breath produced by mouth microbes as a cause for bad breath, telling us that even the relatively low levels of H2S are detectable by human smell. It therefore came as a surprise when it was discovered that H2S is also a critical mediator of numerous physiological processes in the human body. H2S has been labeled “the third biological gas” that mediates bodily processes along with nitric oxide (NO) and carbon monoxide (CO)—also components of car exhaust—with effects that include vasodilation (relaxation of arteries), antioxidation, and anti-inflammation. H2S plays a role in blood pressure regulation and serves an important protective and healing role in the intestinal lining. H2S is being explored as a potential treatment for asthma, a condition in which acute exacerbations have been associated with reduced H2S levels. Administration of H2S can reduce intestinal inflammation, even protect from stomach ulcers caused by use of non-steroidal anti-inflammatory drugs. H2S is the signal that mediates intestinal nociception, signals from gut to brain that keep us aware of internal processes, including discomfort and pain. H2S plays important physiologic roles in many species of animals besides humans, including mediating the marked reduction in metabolic rate in hibernating bears, causing a form of suspended animation. There is an age-related drop in H2S levels as we age, and H2S has been administered in an experimental model that extended longevity. Enhanced levels of intestinal sulfide (via increased taurine intake) have also been shown to increase immunity against intestinal pathogens. There are two general sources of H2S: 1. Human intestinal conversion of the amino acids cysteine, methionine, and taurine from diet (fish, shellfish, poultry, beef, pork, other protein sources) 2. Bacterial conversion of sulfur-containing compounds from diet (onions, garlic, cruciferous vegetables), as well as bacterial metabolism of human intestinal mucus (sulfated mucin proteins) In other words, the human body produces H2S and bacteria produce H2S. As with NO and CO, H2S makes numerous physiologic processes possible. Without it, health and life would be seriously impaired, perhaps impossible. Despite its crucial role as a physiologic gas mediator, H2S has paradoxically been associated with unhealthy effects that include:
H2S gas is a critical signal transmitter for both the human body and microbes, mediating numerous processes such as immunity and inflammation. However, for unclear reasons, H2S-producing microbial species such as Desulfovibrio, Bilophila, and Fusobacteria can proliferate, then ascend into the upper GI tract, situations in which excessive quantities of H2S can be produced. This typically results in diarrhea or irritable bowel syndrome symptoms with diarrhea. Excessive H2S-producing species can also occur concurrently with methane- and H2-producing species. While lessons are still being learned on how to interpret H2S breath levels, there are several issues we should bear in mind when testing for H2S: