Microbiological is the adjective for the science that studies very small living things such as bacteria — the science called microbiology. So a microbiological test is a test that looks for those tiny lives; microbiological safety is about keeping food, water, or a hospital free from the dangerous ones. The study itself is microbiology; this word, with -ical, describes things that belong to that study. You will see it in lab reports, food rules, and medical writing. It is a formal, scientific word, not a word for chat. Though actually, learners often grab the wrong member of the family: microbiology is the study, a microbiologist is the person, and microbiological is “about that study.”