Bacteria

Bacteria are a large domain of unicellular prokaryotic microorganisms. They are among the oldest and most abundant life forms on Earth and inhabit nearly every environment, from soil and water to extreme habitats and the bodies of plants and animals. Typical bacterial cells are small, usually 0.2 to 5 micrometers in length, and lack a membrane-bound nucleus or other organelles, with a cytoplasmic region bounded by a cytoplasmic membrane and surrounded by a cell wall.

Many bacteria have cell walls containing peptidoglycan, which helps distinguish them from archaea. Based on the Gram stain, they are commonly categorized as Gram-positive or Gram-negative, reflecting differences in cell envelope structure. Some bacteria lack a cell wall altogether, such as Mycoplasma.

Bacteria reproduce mostly by binary fission, enabling rapid population growth. They also exchange genetic material through transformation, transduction, and conjugation, contributing to genetic diversity. Most bacterial genomes are circular DNA molecules located in the nucleoid; many also carry plasmids.

Metabolic diversity is broad; bacteria can be photo-, chemo-, or heterotrophs, and may be aerobic or anaerobic. They contribute to nutrient cycling, decomposition, and symbiotic relationships. Cyanobacteria perform photosynthesis; nitrogen-fixing bacteria convert atmospheric nitrogen to forms usable by living organisms. In biofilms, bacteria form structured communities on surfaces.

Humans interact with bacteria in many ways. Some species are pathogens that cause diseases, while others are beneficial members of the gut microbiota, participate in fermentation, or produce enzymes, vitamins, and antibiotics. Antibiotic resistance poses a major public health challenge. Advances in culture-independent methods have expanded knowledge of bacterial diversity and ecosystems.