Bacteria

• 1. Introduction

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  Bacteria were the first life forms to appear on Earth, arriving about 4 billion years ago. For roughly 3 billion years afterward, most living things were microscopic, and bacteria and archaea ruled the planet. A few micrometres in length and usually built from a single cell, they spread into nearly every habitat that exists. They float in the air, saturate the soil, and persist in acidic hot springs, radioactive waste, and the deep rock of Earth's crust. There are thought to be roughly 2 followed by 30 zeros of them, a biomass that only plants exceed. Yet for all that abundance, most bacteria remain strangers to science. An estimated 43,000 species have been named, but only around 2% have been fully studied, and many cannot be grown in a laboratory at all. How does a cell with no nucleus and a single circular chromosome run so much of the living world? Why does the body carry trillions of them without harm? And how did one of the smallest things alive help build the cells of plants and animals?

• 2. Strangers In The Census

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  Just 10 bacterial species account for half of all scientific publications, while nearly 75% of all named bacteria have no academic research devoted to them at all. The imbalance runs deeper still. About 90% of scientific studies on bacteria focus on less than 1% of species, mostly pathogens relevant to human health. The best-studied organism, Escherichia coli, has more than 300,000 published studies. Many of those papers use it only as a cloning vehicle to investigate other species, revealing nothing about its own biology. Even E. coli keeps secrets. A quarter of its roughly 4,000 genes are poorly studied or remain uncharacterised. Bacteria with minimal genomes tell the opposite story. Organisms like Mycoplasma, carrying fewer than 600 genes, usually have most of their genes functionally understood, because nearly all of them are essential and conserved across many species. When researchers isolate DNA from the environment and mass-sequence it, the count of candidate species runs into the thousands, perhaps millions. Estimates of true bacterial diversity range from 10 million to a billion total species, and even those figures may be off by many orders of magnitude.

• 3. Architects Of The Cell

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  Eukaryotes, the cells of animals, plants and other complex organisms, owe their existence to ancient bacteria. Proto-eukaryotic cells engulfed alphaproteobacterial symbionts, and those captives became mitochondria or hydrogenosomes. Every known member of Eukarya still carries them, sometimes in highly reduced form, as in certain amitochondrial protozoa. The arrangement was not a single event. Some eukaryotes that already held mitochondria later engulfed cyanobacteria-like organisms, which became the chloroplasts of algae and plants. Scientists call this primary endosymbiosis. The favour bacteria did did not stop at building cells. Nearly all animal life depends on them for vitamin B12, also known as cobalamin, because only bacteria and some archaea carry the genes and enzymes to make it. That vitamin acts as a cofactor in DNA synthesis and in fatty acid and amino acid metabolism. It matters especially for the nervous system, where it helps synthesise myelin. Bacteria diverged first from the archaeal and eukaryotic lineage, and their most recent common ancestor with archaea was probably a hyperthermophile living about 2.5 billion to 3.2 billion years ago.

• 4. Shapes And Walls

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  Thiomargarita magnifica can reach 2 centimetres in length, roughly 50 times larger than other known bacteria and easily visible to the naked eye. Thiomargarita namibiensis grows up to half a millimetre, and Epulopiscium fishelsoni reaches 0.7 millimetres. At the other extreme sit members of the genus Mycoplasma, just 0.3 micrometres across, as small as the largest viruses. Most bacterial cells fall between 0.5 and 5.0 micrometres, about one-tenth the size of a eukaryotic cell. The shapes carry names drawn from old languages. Spherical bacteria are cocci, from the Greek for grain or seed, while rod-shaped ones are bacilli, from the Latin for stick. Curved rods are vibrio, spiral forms are spirilla, and tightly coiled ones are spirochaetes. The wall that fixes these shapes also splits bacteria into two great camps. Most carry a thin layer of peptidoglycan wrapped in a second lipid membrane, the Gram-negative pattern. Only the Bacillota and actinomycetota groups use the thick-walled Gram-positive arrangement. The difference has clinical teeth. Vancomycin kills only Gram-positive bacteria and does nothing against Gram-negative pathogens such as Haemophilus influenzae or Pseudomonas aeruginosa.

• 5. The Crowd Behaves

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  When myxobacteria are starved of amino acids, individual cells detect their neighbours through quorum sensing, crawl toward one another, and assemble into fruiting bodies up to 500 micrometres long that hold about 100,000 cells. Inside, the bacteria divide the labour. Roughly one in ten cells climbs to the top and hardens into a dormant myxospore, resistant to drying and other harsh conditions. This is bacteria acting as a society rather than a smear of identical clones. Quorum sensing lets a population measure whether it is dense enough to bother with shared work, like excreting digestive enzymes or producing light. Bacteria often live bound to surfaces in dense aggregations called biofilms, which can range from a few micrometres thick to half a metre deep and may mix bacteria, protists and archaea. Within a biofilm, cells can carry more than five hundred times the resistance to antibacterial agents that lone planktonic bacteria of the same species show. That toughness explains why biofilms haunt medicine, clinging to chronic infections and the surfaces of implanted devices. A few bacteria add another social trick: bioluminescence, often produced by species living with fish, where the glow likely lures larger animals.

• 6. Survivors And Toolmakers

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  Endospores can survive for millions of years showing no detectable metabolism, shrugging off UV light, gamma radiation, detergents, heat, freezing, pressure and desiccation. Genera such as Bacillus, Clostridium and Heliobacterium form these dormant cores of DNA and ribosomes, wrapped in a cortex and a rigid multilayer coat. The toughness is so extreme that spores have endured the vacuum and radiation of space, raising the idea that bacteria might ride dust, meteoroids or comets between worlds. The same endurance turns deadly when spores germinate. Inhaled Bacillus anthracis spores cause anthrax, and Clostridium tetani spores in a deep wound bring on tetanus through a toxin released by the growing bacteria. Human industry has learned to put bacterial talents to work. After the 1989 Exxon Valdez oil spill, fertiliser was spread on beaches in Prince William Sound to feed naturally occurring bacteria that digest petroleum hydrocarbons, and the effort worked where the oil was not too thick. Bacillus thuringiensis, a soil-dwelling Gram-positive bacterium sold under names like Dipel and Thuricide, serves as an insecticide so specific it barely touches humans, wildlife or pollinators. Bioengineered bacteria now manufacture therapeutic proteins such as insulin, growth factors and antibodies.

• 7. The First Glimpse

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  In 1676 the Dutch microscopist Antonie van Leeuwenhoek peered through a single-lens microscope of his own design and saw bacteria for the first time. He did not class them apart, calling every microorganism he found, bacteria, protists and tiny animals alike, animalcules. He sent his observations to the Royal Society of London in a series of letters. Then came one of the strangest gaps in the history of science. The cells sat right at the limit his simple lenses could resolve, and for over a century no one else saw them again. The word itself arrived later. Christian Gottfried Ehrenberg introduced bacterium in 1828 as a genus of non-spore-forming rods. Louis Pasteur showed in 1859 that microbial growth, not spontaneous generation, drives fermentation, and with Robert Koch he championed germ theory. Koch proved that theory through his work on tuberculosis and won a Nobel Prize in 1905, leaving behind postulates still used to test whether an organism causes a disease. In 1910 Paul Ehrlich built the first antibiotic by reworking dyes that stained Treponema pallidum, the syphilis spirochaete, into compounds that killed it. The deepest shift came in 1977, when Carl Woese, reading 16S ribosomal RNA, recognised that archaea descend from a line separate from bacteria, splitting the prokaryotes into two domains.