Bacteria are predominantly single-celled microorganisms constituting the domain Bacteria. They are prokaryotes: their cells lack a membrane-bound nucleus and the typical membrane-bound organelles of eukaryotes. Although they share this cellular organization with archaea, bacteria form a distinct evolutionary lineage. They inhabit soil, freshwater, oceans, and other organisms, occupying environments with widely differing temperatures, nutrient supplies, and oxygen concentrations. Their activities include nutrient recycling, primary production, symbiosis, and, in some species, disease. (openstax.org)
Cellular structure
Many bacterial cells measure roughly one to several micrometres, although size varies considerably. Common shapes include spheres, called cocci; rods, called bacilli; and curved or spiral forms. Cells may occur individually, in pairs or chains, or in organized clusters. Shape alone rarely provides sufficient information to identify a bacterial species. (openstax.org)
A cell membrane encloses the cytoplasm and controls exchanges with the environment. DNA is concentrated in a region called the nucleoid rather than enclosed within a nucleus. Bacterial ribosomes, usually described as 70S ribosomes, synthesize proteins. Most bacteria possess a cell wall containing peptidoglycan, which provides mechanical support and helps prevent rupture caused by osmotic pressure. Wall-less bacteria, including Mycoplasma, are exceptions. (openstax.org)
The Gram stain distinguishes many bacteria according to their cell-envelope structure. Gram-positive bacteria generally have a thick peptidoglycan layer; Gram-negative bacteria have a thinner layer and an additional outer membrane. Some bacteria possess capsules, attachment structures called pili or fimbriae, or flagella used for movement. Certain groups form highly resistant endospores—dormant survival structures rather than reproductive spores. (openstax.org)
Reproduction and genetic exchange
Most bacteria reproduce through binary fission. DNA is replicated, the copies are separated, and the cell divides into two daughter cells. Other patterns include budding and fragmentation. Under suitable conditions, populations can increase exponentially, but growth rates depend on the organism, nutrients, temperature, and other environmental factors. In a closed culture, growth commonly passes through lag, exponential, stationary, and decline phases as resources and conditions change. (openstax.org)
Many bacteria have one circular chromosome, although linear chromosomes and multiple chromosomes also occur. Additional independently replicating DNA molecules, called plasmids, may carry genes affecting metabolism, virulence, or antibiotic susceptibility. (ncbi.nlm.nih.gov)
Bacteria also exchange genetic information through horizontal gene transfer. Transformation involves uptake of environmental DNA; conjugation transfers DNA through direct cell contact; and transduction transfers DNA through a bacteriophage, a virus that infects bacteria. These processes do not themselves increase cell numbers, but they can introduce inherited traits into a population. Together with mutation, they supply variation on which natural selection acts. (openstax.org)
Metabolic diversity
Bacterial metabolism encompasses numerous ways of obtaining carbon and energy. Heterotrophs acquire carbon from organic compounds, whereas autotrophs build organic material using inorganic carbon. Energy may come from light or chemical reactions involving organic or inorganic substances. These distinctions describe separate aspects of nutrition: an organism’s carbon source does not necessarily determine its energy source. (openstax.org)
Some bacteria use oxygen in respiration; others respire using alternative electron acceptors or obtain energy through fermentation. Obligate anaerobes cannot grow under oxygenated conditions, while facultative anaerobes can grow with or without oxygen. Cyanobacteria perform oxygen-producing photosynthesis. Other photosynthetic bacteria use different electron donors and do not release oxygen. This metabolic diversity allows bacteria to occupy many ecological niches. (openstax.org)
Ecological roles
Bacteria are major agents of decomposition, breaking down organic material and returning nutrients to circulation. Their transformations of carbon contribute to the carbon cycle. Through nitrogen fixation, certain bacteria convert atmospheric nitrogen into forms that can enter biological systems. Other bacteria participate in nitrification and denitrification, connecting different nitrogen compounds and reservoirs. Nitrogen-fixing bacteria may live freely or associate with plants, including species that inhabit legume root nodules. (openstax.org)
Many bacteria live in biofilms, surface-associated communities embedded in a matrix produced by their cells. Such communities occur in natural waters, soils, industrial systems, and living organisms. Within hosts, bacterial relationships range from mutually beneficial associations to parasitism. Human-associated bacteria can contribute to digestion and protection against invading organisms; their effects depend on the species, location, and host conditions. (openstax.org)
Disease and antimicrobial resistance
Some bacteria cause infections, although many are harmless or beneficial. Antibiotics act against susceptible bacteria by killing them or inhibiting their growth. Antibiotic resistance occurs when bacteria possess or acquire mechanisms that reduce a drug’s effectiveness. These mechanisms include drug destruction, alteration of the drug’s target, and removal of the drug from the cell. Resistance can spread through bacterial multiplication and genetic exchange; it is a property of bacteria, not of the infected person’s body. (cdc.gov)
Identification and practical uses
In microbiology, bacteria are characterized through microscopy, staining, cultivation, and biochemical tests. DNA sequencing, including analysis of ribosomal RNA genes, supports identification and classification when organisms have similar appearances. Sequence-based methods complement rather than simply replace observations of structure and physiology. (openstax.org)
Bacterial activities are used in food fermentation, wastewater treatment, and the breakdown of some pollutants. Engineered bacteria can also produce useful proteins: recombinant DNA methods enable bacterial production of human insulin. These applications exploit particular strains and metabolic capabilities rather than properties shared by every bacterium. (openstax.org)