Spoilage bacteria / Spoilage microorganisms
Spoilage bacteria
Japanese: 腐敗菌（腐敗微生物）
Microorganisms that decompose food, producing unpleasant odors and harmful substances. Like fermenting microbes, they break down organic matter, but cause changes undesirable to humans.
Spoilage bacteria decompose food, producing foul odors and harmful substances. They break down proteins to generate ammonia and hydrogen sulfide, rendering food 'rotten.'
What is fascinating is that the boundary between fermenting and spoilage microbes is blurry. Foods like kusaya and funazushi involve changes that seem close to spoilage, yet they are beloved as fermented delicacies in certain cultures.
To the microorganisms themselves, both processes are simply a meal. It is always humans who decide whether to call it fermentation or decay.