Gut microbiome reports often describe some bacteria as beneficial and others as potentially harmful.
But bacteria do not always fit neatly into these two groups.
Some gut bacteria perform functions associated with health. They can help break down fibre, produce useful metabolites and support the intestinal barrier.
Other bacteria may be associated with inflammation or disease.
But the effect of a bacterium can depend on which species or strain is present, how much is present and the other microorganisms in the gut.
Your diet, medications, age and health can also affect its role.
Can a Common Gut Bacterium Be Both Helpful and Unhelpful?
Yes.
Bacteroides is a good example.
These bacteria are common in the human gut. Some Bacteroides species help break down complex carbohydrates and interact with other gut microorganisms.
But higher levels are not always better.
In research on healthy ageing, persistent Bacteroides dominance in older people was associated with a less favourable ageing pattern.
Among older men, higher levels of Bacteroides were also associated with higher mortality.
This does not mean that Bacteroides caused poorer health or shorter survival.
The relationship may also work in the opposite direction. Disease can change the microbiome. Medications, diet and reduced physical activity can change it too.
What Does This Mean for Healthy Ageing?
There is no simple list of bacteria that everyone should increase or reduce.
A low level of a bacterium considered beneficial does not automatically mean that you need to replace it. A higher level of another bacterium does not automatically mean that it should be reduced.
For healthy ageing, the overall microbiome and its function are more informative than one bacterial result.
Your microbiome should also be considered together with your diet, medications, lifestyle, age and health.
The aim is not to achieve a fixed list of “good” bacteria. It is to support a gut microbiome that functions well as part of your overall health.
