Gastrointestinal system

Dysbiosis
There is strong evidence that dysbiosis or an imbalance in the microbial ecology of the gut plays a role in the symptoms of ME/CFS. On average, ME/CFS patients have lower levels of Bifidobacteria, Escherichia coli and higher levels of aerobic bacteria. , in particular Enterococcus and Streptococcus species. The latter produce D-lactate, a form of lactic acid only produced by non-human cells that is poorly metabolized in humans. D lactate is associated with a wide variety of cognitive and neurological symptoms, such as in patients who suffer from D lactate acidosis. A study found that higher levels of enterococcus bacteria in CFS patients were associated with more severe neurological and cognitive dysfunction.

CFS patients may suffer from small intestinal bacterial overgrowth (SIBO) at high rates.

One hypothesized consequence of dysobiosis is an overproduction of hydrogen sulfide (H2S) by pathogenic bacteria. H2S can inhibit mitochondrial respiration by blocking cytochrome c oxidase.

A study of Norwegian and Belgian patients found significantly decreased proportions of Firmicutes genera Holdemania and increased proportions of Bacteroidetes genera Alistipes in the Norwegian but not the Belgian sample. Significantly increased proportions of Firmicutes genera Lactonifactor were found in both. .

Intestinal permeability
In a healthy digestive tract, the intestinal walls provide a tight, selective barrier to allow the absorption of nutrients prevent the entry of bacteria or pathogens. However, in CFS patients, there is evidence of increased intestinal permeability or "leaky gut." Intestinal permeability can allow for the translocation of bacteria across the mucosal lining and inside the blood stream.

One study found higher levels of serum IgA and IgM against lipopolysaccharides (LPS), a major component of the outer membrane of gram negative bacteria, indicating translocation. . Serum IgA was significantly correlated to the severity of illness.