Histamine-Producing Bacteria
Certain gut bacteria express the enzyme histidine decarboxylase, which converts the amino acid L-histidine into Histamine. This microbially-produced histamine adds to Total Mediator Load independently of diet or mast cell activity.
Known Producers
Species and strains with documented histamine production:
- Certain Lactobacillus strains — This is the critical nuance. Lactobacillus is widely considered “probiotic,” but histamine production varies at the strain level. L. reuteri, L. hilgardii, and L. buchneri are known histamine producers. L. rhamnosus GG and L. plantarum are generally considered histamine-neutral or histamine-degrading. The species name alone is not sufficient — strain-level data matters.
- Enterobacteriaceae — including E. coli and Klebsiella
- Morganella morganii — a potent histamine producer, often involved in scombroid fish poisoning
- Enterococcus faecalis — certain strains
- Staphylococcus species — certain strains
Why This Matters
A person taking a probiotic containing histamine-producing strains may inadvertently increase their gut histamine load — worsening symptoms despite the probiotic being “good for gut health” in general terms.
This is why “take a probiotic” is not simple advice for someone with MCAS or Histamine Intolerance. The specific strains in the product need to be cross-referenced against histamine production data. See also Histamine-Degrading Bacteria.
The SIBO Connection
Small intestinal bacterial overgrowth (SIBO) places histamine-producing bacteria in the small intestine, where they encounter dietary histidine before it’s absorbed. This can dramatically increase histamine production in the exact location where DAO is supposed to be clearing it — overwhelming the clearance system at the point of first contact.