Bacteria are single-celled prokaryotic organisms that are found in nearly every habitat on Earth. Key characteristics of bacteria include their small size (usually 1 micrometer in length), lack of a nucleus, use of binary fission for reproduction, and ability to thrive in diverse environments. Bacteria play important roles in nutrient recycling and are both beneficial and pathogenic to other organisms. While antibiotics have been used to treat bacterial infections, overuse has led to increased antibiotic resistance in many bacteria.