pathogenic bacteria; bad bacteria; harmful bacteria
Often contrasted with 善玉菌 (beneficial bacteria) in discussions of gut flora and health. Used in both scientific and everyday contexts.
See also: 善玉菌
When bad bacteria in the gut increase, you become more prone to getting sick.
ヨーグルトは悪玉菌を減らすのに役立つと言われている。
Yogurt is said to help reduce harmful bacteria.