A bacterium is a tiny, single-celled organism that can be found almost everywhere on Earth. These microorganisms are so small that they can only be seen under a microscope. Bacteria come in various shapes, such as rods, spheres, and spirals, and they play essential roles in many processes, including breaking down organic matter and helping with digestion in humans and animals. Some bacteria are beneficial, while others can cause diseases. For example, E. coli is a type of bacterium that can be found in the intestines of humans and animals, where it helps with digestion. However, certain strains of E. coli can lead to food poisoning. Understanding bacteria is crucial for health, agriculture, and environmental science.