The publication of evidence that gum disease bacteria may cause Alzheimer’s has prompted questions on social media about how seriously we should take the results. Here’s what you need to know about the landmark study.

Does the study show that the bacteria cause Alzheimer’s disease, or just that the two are linked in some way?

Any well-informed reader will know that correlation doesn’t mean causation, and not every link between two factors implies that one causes the other. But some do. Taken together, the new mouse experiments do suggest a causal link between the gum disease bacterium Porphyromonas gingivalis and Alzheimer’s.

When the team gave these bacteria to mice, the animals developed dementia that looked like human Alzheimer’s. The gingipain toxins produced by these bacteria activate certain parts of the immune system, and were found in this study to degrade tau protein. Both effects are known to produce Alzheimer’s symptoms.

And when the team gave these mice a drug that kills the bacteria, either directly or by blocking the gingipains they use to feed, there was a reduction both in bacteria and in symptoms. Together, these satisfy most of the conditions required to attribute a disease to a microorganism.

But that’s in mice – what about humans?

There are hints that the same may be true in humans. Nearly all of the 54 Alzheimer’s brain samples studied by the team contained gingipains, and the more gingipains a person had, the more degraded tau protein they had – a sign of worse Alzheimer’s symptoms while alive.

The team also found low levels of gingipains and damage in the brains of some people who didn’t have the condition, consistent with the fact that Alzheimer’s is the result of years of accumulating brain damage.

Isn’t this study too small to say anything?

While 54 people isn’t a huge sample, it is large by the standards of much brain research, where tissue can be hard to get. Larger study sizes tend to come into play in later-stage clinical studies.

Can we trust a study from a pharmaceutical firm?

The study involved researchers from Cortexyme, a pharmaceutical firm in San Francisco that is developing treatments that interfere with P. gingivalis and its toxic gingipains. This led some online commenters to express doubts over whether the study can be “trusted”.

But the study was accepted by the peer-reviewed journal Science Advances, and 14 of the 26 authors of the paper aren’t directly employed by Cortexyme.

Besides, many big pharma companies have pulled out of less profitable drug-development areas, including antibiotics and Alzheimer’s, and Cortexyme is typical of the smaller firms that now do most early drug development. It is healthy to be sceptical, and much research is needed to confirm the study, but if we don’t trust commercial firms on principle, we will struggle to find ways to develop badly needed drugs.