"More good bacteria is better" — is it?
The entire probiotic category rests on one assumption: that adding more "good" bacteria improves gut health. That can be true in specific, studied situations. But it quietly assumes the problem is a shortage of bacteria. When the actual problem is overgrowth in the wrong place — the small intestine, where bacterial density should be low — adding more live bacteria is adding fuel, not fixing a deficiency.
SIBO is not "too few bacteria." It is too many, in the wrong place. → What SIBO is · the diet that starves it
Live bacteria are not automatically safe.
These are not hypotheticals — each is described in the medical literature:
Probiotic-associated sepsis
Bloodstream infection caused by the supplement organism itself (Lactobacillus, or the yeast Saccharomyces boulardii). Whole-genome sequencing has matched the infecting strain to the exact probiotic a patient was taking — most dangerous in the critically ill, immunocompromised, or those with a leaky gut barrier.
A trial that reported higher mortality
The PROPATRIA randomized trial (2008) gave a multi-strain probiotic to patients with predicted severe acute pancreatitis. Mortality was higher in the probiotic group than in placebo, with bowel ischemia among the deaths. A specific critically-ill population — but a documented signal that "harmless" is not a given.
Antibiotic-resistance gene transfer
Lactobacillus and other probiotic strains can carry antibiotic-resistance genes and, in the gut, potentially hand them to other bacteria — the opposite of what you want in an era of resistant infections.
D-lactic acidosis & brain fog
Certain probiotic bacteria produce D-lactate. In people with overgrowth, this has been associated with brain fogginess, bloating, and gas that improved when the probiotics were stopped and the overgrowth was treated.
Kombucha and kimchi run on the same logic.
The concern is not the probiotic pill specifically — it is introducing external live microbes into a gut that is already overgrown. Yogurt, kombucha, kimchi, and other fermented foods do the same thing. For someone whose symptoms point to overgrowth, "eat more fermented foods for gut health" can make things worse for the same reason routine probiotics can.
Not fear — just the missing half.
This is not a claim that probiotics are dangerous for everyone, or that they never help — in some studied situations they do. It is a correction to the default assumption that they are always harmless and always beneficial. If your symptoms point to overgrowth — bloating, foul gas, bad breath, brain fog — "add more bacteria" deserves skepticism, and the decision belongs with a clinician, not with the marketing.
Read the two-part Lactobacillus deep-dive.
The full evidence — from probiotic sepsis to what these bacteria do that you don't see — is laid out in the essay series on Substack.
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