Bacteriological means “about the scientific study of bacteria,” or “done with the methods of that study.” Bacteriology is the science that looks closely at those tiny living things: how they grow, how they spread, how they make people sick, and how we can stop them. So a bacteriological test is a lab check for bacteria in water, food, or a sample from a patient. A bacteriological laboratory is the room where that work happens. The word is formal and scientific; you will meet it in reports, hospitals, and school biology, not in chat. Though actually, in everyday talk people often just say “bacteria test” and skip this long adjective.