Chemical products . Basic research . Easy to find events that occur only 1 in a billion times if have 100 billion bacteria in test tube. Crucial to modern biology. E.g. genetic engineering, ability to move genes freely from one organism to another, select genes of interest and amplify their expression.
Bacteria are natural hosts for such activities.
Made simple microscope. Drew bacteria, yeasts, etc. View micrograph taken through Van Leeuwenhoek's microscope . Spontaneous Generation controversy. 1748, Needham boiled mutton broth, sealed container.