Beneficial bacteria / Harmful bacteria
Beneficial / Harmful bacteria
Japanese: 善玉菌・悪玉菌（有用菌・有害菌）
Colloquial classification of gut bacteria based on their effects on human health. Beneficial bacteria contribute to health while harmful bacteria have adverse effects, though these are not strict scientific terms.
Good bacteria and bad bacteria—easy-to-understand labels, but more a human-centered convenience than a scientific classification.
The gut is said to harbor roughly 1,000 species and 40 trillion individual bacteria. Among them, those with beneficial health effects like Bifidobacterium and lactic acid bacteria are called good, while those producing harmful substances like Clostridium perfringens are called bad.
However, the same bacterium can behave differently depending on circumstances. The line between good and evil is not as simple as it may seem.