The phrase "certain microorganisms" refers to specific types of tiny living things that are being discussed or referred to in a particular context. Full definition
Certain microorganisms produced them naturally when fed starch, whose molecules are made up of long strands of sugar units. (newscientist.com)
This report will tell you whether you have certain microorganisms associated with certain healthy (or unhealthy) lifestyle habits. (mindbodygreen.com)
It's the same basic principle that explains why oxygen is toxic to certain microorganisms because they evolved in places where there's little or no O2. (treehugger.com)