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Bacteria are single-celled microorganisms that belong to the domain Bacteria. They are among the most diverse and abundant life forms on Earth and are found in a wide range of environments, including soil, water, air, and living organisms. Bacteria have a simple cellular structure characterized by lacking a membrane-bound nucleus and membrane-bound organelles, making them prokaryotic cells.

Bacteria play essential roles in various ecological processes, such as nutrient cycling, decomposition, and symbiotic relationships with other organisms. While some bacteria are harmful and can cause diseases in humans, animals, and plants, many bacteria are beneficial and contribute to human health, agriculture, and biotechnological applications. Bacteria can have various shapes, including spherical (cocci), rod-like (bacilli), and spiral (spirilla or spirochetes). They reproduce through binary fission, a process where a single cell divides into two identical daughter cells. Bacteria have a wide range of metabolic capabilities, allowing them to obtain energy from various sources, such as sunlight (photosynthesis), organic matter (heterotrophy), or inorganic compounds (chemosynthesis).