<human>:  I have a question about the book The Rise of Microorganisms. What is the definition of a bacterium?
<bot>: A bacterium is a single-celled microorganism that is able to reproduce asexually. Bacteria are found in almost every environment on Earth, and they are the most abundant form of life on Earth. Bacteria are also the most diverse form of life on Earth, with more than 3 million species.