Bacteria are single celled organisms belonging to the domain Bacteria and characterised by a prokaryotic cellular organisation. Unlike eukaryotic cells, bacterial cells generally lack a membrane bound nucleus and most membrane bound organelles. Their genetic material is located in a nucleoid region and usually consists of a circular chromosome, although many bacteria also contain plasmids, which are smaller DNA molecules capable of carrying additional genes.
Bacteria have diverse shapes, including spherical cocci, rod shaped bacilli and curved or spiral forms, although bacterial morphology is much more varied than these basic categories suggest. Bacterial cells have evolved enormous metabolic diversity and inhabit virtually every environment in which life is possible, including soil, water, sediments, extreme environments, plants, animals and the human body.
Bacterial cells contain a plasma membrane surrounded in most species by a cell wall, with peptidoglycan being a defining component of bacterial cell walls. Gram positive bacteria generally possess a relatively thick peptidoglycan layer, while Gram negative bacteria have a thinner peptidoglycan layer located between the inner membrane and an outer membrane containing lipopolysaccharide. Gram staining exploits differences in cell envelope structure and remains an important laboratory classification method.
Some bacteria possess capsules or other surface layers that can help resist environmental stresses or immune attack. Flagella can provide motility, while pili and fimbriae can participate in attachment, movement or DNA transfer. These structures contribute to bacterial survival and interactions with hosts and environments.
Bacteria reproduce primarily through binary fission, in which one cell replicates its DNA and divides to produce two daughter cells. Under favourable conditions some bacterial populations can increase extremely rapidly, although actual growth rates depend on nutrients, temperature, pH, oxygen availability, waste accumulation and competition. Bacteria can exchange genetic information through mechanisms such as transformation, transduction and conjugation.
Transformation involves uptake of environmental DNA, transduction involves DNA transfer mediated by bacteriophages, and conjugation commonly involves direct cell to cell transfer through specialised molecular systems. Horizontal gene transfer can spread traits such as antibiotic resistance among bacterial populations and contributes substantially to bacterial evolution. Mutation and natural selection further modify bacterial populations over time.
Bacteria display remarkable metabolic diversity. Some are obligate aerobes requiring oxygen, some are anaerobic, and others can switch between metabolic strategies depending on environmental conditions. Bacteria can obtain energy from organic molecules or inorganic substances and can use a wide range of carbon and electron sources. Photosynthetic bacteria contribute to primary production in some environments, while chemolithotrophic bacteria can obtain energy by oxidising inorganic compounds. Bacterial metabolism drives major biogeochemical cycles involving carbon, nitrogen, sulfur and other elements. Nitrogen fixing bacteria can convert atmospheric nitrogen into biologically usable forms, while other bacteria participate in nitrification and denitrification. This metabolic versatility allows bacteria to occupy ecological roles ranging from decomposers and symbionts to pathogens and primary producers.
Many bacteria are beneficial or essential to ecosystems and hosts. Environmental bacteria decompose organic material and recycle nutrients, making elements available for continued biological use. Bacteria associated with humans and other animals form microbiomes that can influence digestion, metabolism, barrier function and immune development. Some bacteria produce vitamins or metabolites useful to their hosts, while others compete with potentially harmful organisms.
In agriculture, bacteria contribute to nutrient cycling and plant associations, while industrial biotechnology uses bacterial enzymes and metabolic pathways to manufacture chemicals, foods and pharmaceuticals. Therefore, bacteria should not be equated with disease. Pathogenic bacteria represent only a subset of bacterial diversity, and many bacterial species have neutral, beneficial or essential relationships with other organisms.
Pathogenic bacteria can cause disease through several mechanisms. Some invade tissues and damage cells directly, while others produce toxins or manipulate host signalling pathways. Bacterial infections can trigger innate and adaptive immune responses, including inflammation, antibody production and cellular immunity. Examples of bacterial diseases include tuberculosis, cholera, bacterial meningitis and some forms of pneumonia, although disease outcomes depend on species, strain, host condition and route of infection.
Antibiotics can treat susceptible bacterial infections by targeting processes such as cell wall synthesis, protein synthesis, nucleic acid replication or essential metabolic pathways. Antibiotics do not generally work against viruses because viruses lack many of the cellular structures and metabolic pathways targeted by antibacterial medicines.
Antibiotic resistance occurs when bacteria acquire or evolve traits that reduce their susceptibility to antimicrobial drugs. Resistance mechanisms include enzymes that chemically modify or destroy antibiotics, changes in drug targets, reduced permeability, active efflux and metabolic bypass pathways. Resistance genes can spread through horizontal gene transfer as well as inheritance during bacterial reproduction. Selection for resistance is increased when susceptible bacteria are killed while resistant variants survive and reproduce. Antimicrobial resistance is therefore an evolutionary and ecological phenomenon rather than simply a property created by an individual patient taking a particular drug. Appropriate antibiotic use, infection prevention, surveillance, laboratory testing and development of new treatments are important components of controlling resistant bacterial infections.
Bacteria are distinct from viruses because bacteria are cellular organisms with their own ribosomes, membranes, metabolism and mechanisms for producing energy and synthesising proteins, whereas viruses are acellular infectious entities that depend on host cells for replication. Bacteria are also distinct from archaea, another major domain of prokaryotic life, because their molecular and cellular characteristics differ in important ways. Bacterial diversity is immense, and many species remain poorly characterised.
Modern bacterial classification increasingly uses genomic information rather than relying solely on physical appearance or biochemical tests. Techniques such as DNA sequencing and metagenomics have revealed extensive bacterial diversity in environments that are difficult to culture in laboratories. Bacteria therefore represent one of the major branches of cellular life and are fundamental to ecology, evolution, medicine, agriculture, biotechnology and global biogeochemical cycles.
