Bacteria
About four billion years ago, the first life forms appeared on Earth as unicellular microorganisms. These early organisms were bacteria and archaea, dominating biological existence for three billion years. Stromatolites serve as bacterial fossils from this ancient era, though their lack of distinctive morphology makes dating specific species difficult. Gene sequences allow scientists to reconstruct the bacterial phylogeny, showing that bacteria diverged first from the archaeal-eukaryotic lineage. The most recent common ancestor of bacteria and archaea likely lived between 2.5 billion and 3.2 billion years ago. This ancestor was probably a hyperthermophile capable of surviving extreme heat. Life on land may have begun with bacteria approximately 3.22 billion years ago. Bacteria also played a central role in the second great evolutionary divergence involving archaea and eukaryotes. Ancient bacteria entered endosymbiotic associations with proto-eukaryotic cells to form mitochondria or hydrogenosomes. Later, some eukaryotes engulfed cyanobacteria-like organisms to create chloroplasts in algae and plants. This process is known as primary endosymbiosis.
Bacterial cells typically measure between 0.5 micrometres and 5.0 micrometres in length. A few exceptions exist, such as Thiomargarita namibiensis which reaches half a millimetre long. Epulopiscium fishelsoni extends even further to 0.7 mm, while Thiomargarita magnifica can grow up to 2 cm. Mycoplasma species represent the smallest bacteria at just 0.3 micrometres. Most bacterial shapes fall into categories like spherical cocci or rod-shaped bacilli. Some species appear as curved vibrios or tightly coiled spirilla. Star-shaped bacteria also exist within this diverse group. The cell wall determines shape and influences nutrient acquisition and predator evasion. Peptidoglycan forms the structural basis of bacterial cell walls. Gram-positive bacteria possess thick layers of peptidoglycan and teichoic acids. Gram-negative bacteria have thinner peptidoglycan layers surrounded by a second lipid membrane containing lipopolysaccharides. Penicillin kills bacteria by inhibiting peptidoglycan synthesis. Flagella serve as rigid protein structures about 20 nanometres in diameter for motility. Fimbriae are fine filaments that help bacteria attach to surfaces. Pili transfer genetic material between cells during conjugation.
Bacteria inhabit acidic hot springs, radioactive waste, and deep Earth crust environments. Approximately 2 times 10 to the power of 30 bacteria exist on Earth today. They form a biomass exceeded only by plants. Oceans harbour around 3 times 10 to the power of 26 bacteria providing up to 50% of human oxygen. Soil contains roughly one thousand million bacteria per few grams. One cubic metre of air holds approximately one hundred million bacterial cells. Cyanobacteria fix carbon dioxide while methanotrophic bacteria use methane as an electron source. Diazotrophs fix nitrogen gas using the enzyme nitrogenase. Sulfate-reducing bacteria produce toxic forms of mercury like methylmercury. Nonrespiratory anaerobes generate energy through fermentation producing ethanol as waste. Facultative anaerobes switch between fermentation and different terminal electron acceptors based on environmental conditions. Bacterial metabolic processes drive responses to pollution and sustain biological stability. Extremophile bacteria convert dissolved compounds such as hydrogen sulphide into energy near hydrothermal vents. These organisms provide nutrients needed to sustain life in extreme environments.
Most bacteria possess a single circular chromosome ranging from 160,000 base pairs to 12.2 megabase pairs. Some Streptomyces species contain linear chromosomes instead. Plasmids are small extra-chromosomal DNA molecules carrying genes for antibiotic resistance or virulence. Bacterial genomes encode hundreds to thousands of genes usually arranged in continuous stretches. Mutations arise from errors during DNA replication or exposure to mutagens. Mutation rates vary widely among different bacterial clones. Three main methods transfer genetic material between cells: transformation, transduction, and conjugation. Transformation involves taking up exogenous DNA from the environment. Transduction occurs when bacteriophages integrate foreign DNA into the bacterial chromosome. Conjugation transfers genetic material through direct cell contact via pili. Horizontal gene transfer allows acquisition of traits like antibiotic resistance across species boundaries. Stress-directed mutation increases error rates in genes involved in growth-limiting processes. Binary fission produces two identical clone daughter cells under optimal conditions. Some populations double every 17 minutes. Myxobacteria form fruiting bodies containing approximately 100,000 cells when starved of amino acids.
Several bacterial species cause infectious diseases including cholera, syphilis, anthrax, leprosy, tuberculosis, tetanus, and bubonic plague. Respiratory infections represent the most common fatal bacterial diseases globally. Clostridium botulinum produces a powerful exotoxin causing respiratory paralysis. Salmonellae generate endotoxins leading to gastroenteritis. Helicobacter pylori was discovered as a pathogen for peptic ulcer disease many years after its initial observation. Staphylococcus and Streptococcus can cause skin infections, pneumonia, meningitis, and sepsis. Rickettsia species are obligate intracellular parasites causing typhus or Rocky Mountain spotted fever. Chlamydia causes pneumonia or urinary tract infection and may contribute to coronary heart disease. Opportunistic pathogens like Pseudomonas aeruginosa affect immunosuppressed individuals or those with cystic fibrosis. Antibiotics treat bacterial infections by inhibiting specific processes different from host mechanisms. Chloramphenicol and puromycin inhibit bacterial ribosomes without affecting eukaryotic ones. Antibiotic use in intensive farming contributes to rapid resistance development. Disinfectants like bleach kill bacteria on surfaces to prevent contamination. Antiseptic measures sterilize skin prior to needle insertion.
Antonie van Leeuwenhoek first observed bacteria in 1676 using a single-lens microscope of his own design. He referred to all microorganisms including bacteria as animalcules. No one else saw them again for over a century following his observations. Christian Gottfried Ehrenberg introduced the word bacterium in 1828. Louis Pasteur demonstrated in 1859 that microorganism growth causes fermentation rather than spontaneous generation. Robert Koch proved the germ theory through research into tuberculosis, anthrax, and cholera. Koch received a Nobel Prize in 1905 for this work. His postulates remain criteria today for testing if an organism causes disease. Ferdinand Cohn began studying bacteria in 1870 and classified them based on morphology. Paul Ehrlich developed the first antibiotic in 1910 targeting Treponema pallidum. Carl Woese recognized archaea as a separate evolutionary domain from bacteria in 1977. This discovery depended on sequencing 16S ribosomal RNA. The three-domain system now divides prokaryotes into Bacteria and Archaea. Modern classification emphasizes molecular systematics using genetic techniques like genome-genome hybridization.
Lactic acid bacteria such as Lactobacillus species have fermented foods like cheese and yogurt for thousands of years. Bacteria capable of digesting petroleum hydrocarbons clean up oil spills. Fertilizer was added to beaches after the 1989 Exxon Valdez spill to promote bacterial growth. Bacillus thuringiensis serves as an environmentally friendly insecticide under trade names like Dipel. Escherichia coli metabolism models help scientists understand gene function and enzyme kinetics. Biotechnology uses engineered bacteria to produce therapeutic proteins including insulin and antibodies. Biological Resource Centres preserve bacterial strains for global scientific availability. Bacteria degrade organic compounds used in waste processing and bioremediation. They synthesize enantiomerically pure chemicals for pharmaceuticals or agrichemicals. Nitrogen-fixing bacteria provide absorbable nitrogen forms for plants in soil rhizospheres. Over 1,000 bacterial species exist in normal human gut flora contributing to immunity. These beneficial bacteria synthesize vitamins like folic acid, vitamin K, and biotin. Probiotic dietary supplements sell these gut flora benefits to consumers.
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Common questions
When did the first bacteria appear on Earth?
The first life forms as unicellular microorganisms appeared about four billion years ago. These early organisms were bacteria and archaea, dominating biological existence for three billion years.
How large are bacterial cells compared to other organisms?
Bacterial cells typically measure between 0.5 micrometres and 5.0 micrometres in length. Exceptions include Thiomargarita magnifica which can grow up to 2 cm while Mycoplasma species represent the smallest at just 0.3 micrometres.
What is the most recent common ancestor of bacteria and archaea?
The most recent common ancestor of bacteria and archaea likely lived between 2.5 billion and 3.2 billion years ago. This ancestor was probably a hyperthermophile capable of surviving extreme heat.
Who discovered bacteria and when was this discovery made?
Antonie van Leeuwenhoek first observed bacteria in 1676 using a single-lens microscope of his own design. He referred to all microorganisms including bacteria as animalcules.
Which diseases do specific bacterial species cause?
Several bacterial species cause infectious diseases including cholera syphilis anthrax leprosy tuberculosis tetanus and bubonic plague. Respiratory infections represent the most common fatal bacterial diseases globally.