“Eating a bowl of porridge oats a day could alter gut microbial functions, according to Scandinavian researchers who have tipped the ingredient as a possible prebiotic.”
“People with better control of blood sugar after eating barley kernel bread also have a different balance of microbes in the gut, a new study in Cell Metabolism reveals..Metagenomic analysis showed that the gut microbiota of responders was enriched in Prevotella copri and had increased potential to ferment complex polysaccharides after BKB” 
“Low counts of a bacterial species generally have been discounted in importance, but slight shifts in the ratios of abundant microbe populations have been thought to have roles in obesity, diabetes and inflammatory bowel diseases such as Crohn’s disease.”
“Supplements containing Lactobacillus gasseri KS-13, Bifidobacterium bifidum G9-1, andBifidobacterium longum MM2 may shift the microbiota of elderly adults to those of healthier and younger people, says a new study”
“Diagram showing how various species gut bacteria matches other species:” [Source]
“Intestinal worms manipulate their host’s immune system to ensure their survival, in part by changing the metabolism of the host’s gut microbiome. The worms, called helminths, infect around 2 billion people around the world, and are able to block harmful inflammatory responses in humans and mice.” 
“Bifidobacteria are common and frequently dominant members of the gut microbiota of many animals, including mammals and insects. Carbohydrates are considered key carbon sources for the gut microbiota, imposing strong selective pressure on the complex microbial consortium of the gut. Despite its importance, the genetic traits that facilitate carbohydrate utilization by gut microbiota members are still poorly characterized. Here, genome analyses of 47 representative Bifidobacterium (sub)species revealed the genes predicted to be required for the degradation and internalization of a wide range of carbohydrates, outnumbering those found in many other gut microbiota members.”  NOTE: Bifidobacteria are often very low with CFS — which implies carbothydrates are not handle, possibly leading to gluten and other issues with starch.
“Saccharomyces cerevisiae CNCM I-3856 in irritable bowel syndrome: improvement in abdominal pain and bloating in those with predominant constipation”