A reader sent this to me:
“This is another supplement [Active hexose correlated compound (AHCC) – a fermented mushroom extract] that helped me a lot and stopped working
Since I am working off a model, it gives a framework to evaluate such things and possibly identify the mechanisms involved. Fermentation often produce bacteria that produces antibiotics [In fact, I just heard a report that 25% of the bacteria in men’s beard produce antibiotics!!!].
Reading PubMed, the mechanism of AHCC effects is not clear, but what we do find in terms of results include:
- “the oral treatment with AHCC protected mice from lethal infection with Pseudomonas aeruginosa and intraperitoneal one also protected mice from infection with methicillin-resistant Staphylococcus aureus (MRSA).”
- “Supplementation with AHCC appears to modulate immunity and increase survival in response to acute infection and warrants further investigation.” 
- ” Our previous studies demonstrated that heat-shock protein 27 (HSP27) was involved in …. and it was down-regulated by AHCC-treatment.”
- “adenosine was identified as one of the NO suppressive components in AHCC” 
- “Herein we describe the synthesis and evaluation of a series of adenosine analogs for in vitro antibacterial activity against Staphylococcus aureus … compound c6 has much stronger antibacterial potency against Pseudomonas aeruginosa than ciprofloxacin” 
- “Our data provide a preclinical experimental basis for the synergistic effect of AHCC and B. longum BB536 on inflammatory bowel disease.” 
- “The study of colonic microflora indicated that rats treated with AHCC had higher aerobic and lactic acid bacteria counts as well as higher bifidobacteria counts,” 
The heat shock protein rang a bell, and checking Wikipedia we find “Production of high levels of heat shock proteins can also be triggered by exposure to different kinds of environmental stress conditions, such as infection, inflammation, exercise, exposure of the cell to toxins” Heat shock proteins are associated with CFS  ” Basal hsp27 was significantly higher among CFS patients compared to controls, and decreased immediately post-exercise, remaining below basal levels even at 7 days”  and even Cort Johnson has a post on them. The gotcha is that it appears to be seen in only 30-40% of CFS patients (which suggests that it maybe microbiome dysfunction centric).
It is a prebiotic that does influence bacteria growth. It appears to have antibiotic characteristics against a significant species for CFS. It also impacts heat shock proteins and appears to reduce chemicals produced by stress.
The reader’s report of it stop working is reasonable, because bacteria such as staphylococcus aureus would originally be reduced significantly and then the resistant strains would re-populate. A strong reason to do rotation with only 1-2 weeks on anything that had antibiotic characteristics.
If you take it, it should be taken with B.longum and other bifidobacteria probiotics.