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 (Lucas et al., Appl Environ Microbiol 2005, PMID 15746344), and L. buchneri are known histamine producers.

The L. reuteri paradox

L. reuteri produces histamine but uses it as an anti-inflammatory signal — the bacterially-produced histamine activates H2 receptors to suppress TNF-α production (Thomas et al., PLoS One 2012, PMID 22384111). This means a “histamine-producing” bacterium can simultaneously be anti-inflammatory through a different pathway. The simplistic “histamine-producing = bad” framing doesn’t hold for every species.

L. rhamnosus GG and L. plantarum are generally considered histamine-neutral or histamine-modulating (see Histamine-Modulating Bacteria). 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-Modulating 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.