Microbes

This page explains what microbes are and how they are studied. It gives no medical advice.

What microbes are

Microbes, also called microorganisms, are living things mostly too small to see without a microscope, and microbiology is the science that studies them.^{[1, 2]} The US Department of Energy (DOE) counts viruses, bacteria, archaea, fungi, protists and some green algae as microbes, and notes that they can be single cells, many-celled or not cells at all.^[1]

• Bacteria are single cells with no nucleus.^{[1, 6]}
• Archaea are single cells too, but form one of the three domains of life, next to bacteria and the eukaryotes, whose cells have a true nucleus.^{[12, 6]}
• Fungi cover yeasts and molds as well as mushrooms.^{[1, 3]}
• Protists are single-celled organisms that do have a nucleus.^[1]
• Viruses are tiny parcels of DNA or RNA inside a protein coat that can multiply only after getting into a living cell.^{[4, 1]}

Whether viruses are alive is still argued over. The DOE notes that some biologists do not count them as living organisms.^[1] An article on virus classification published by the CDC, the US public-health agency, lists what viruses share with life (genes, copying themselves, evolving) but concludes they are not alive, since they cannot gather and store energy and stay inactive outside a host cell.^[23] Its authors hold that a cell is the simplest thing that can be called alive, so they call viruses nonliving entities that lead, at best, a kind of borrowed life.^[23]

How small they are

A nanometre is a billionth of a metre, about ten hydrogen atoms placed end to end.^[10] The smallest bacteria measure about 200 nanometres, and a panel of 18 experts judged that this is probably the floor for life as we know it.^[10] Even tinier “nanobes”, 20 to 150 nanometres long, were reported, but microbiologists were not convinced that they were living things able to reproduce.^[10]

NIOSH, the US National Institute for Occupational Safety and Health, explains that a light microscope cannot pick out anything below 0.2 micrometres, whereas an electron microscope can.^[11]

Bacteria compared with nucleated cells

A bacterium has none of the inner compartments, such as a nucleus or mitochondria, found in plant and animal cells.^[6] Its DNA, a circular double helix, sits in a zone called the nucleoid.^[6] Bacteria multiply by binary fission: a cell copies its chromosome and splits into two.^[6] An actively growing bacterium may finish its life cycle in roughly 30 to 60 minutes, against about 16 to 24 hours for a typical eukaryotic cell.^[7] Bacteria can also swap genes, for instance across a bridge formed between two cells during conjugation, which spreads variety through a population.^[8]

Archaea and life at the extremes

Archaea were first grouped with bacteria, but by 1990 they had been shown to be a third domain, sharing some genes and chemical pathways with eukaryotes that bacteria lack.^{[6, 13]} The scientist Carl Woese proposed the archaea branch in the 1970s.^[14]

Some archaea live in boiling, acidic Yellowstone springs and others in Antarctic cold, while plenty more live in soils, marshes, the oceans and even the human colon.^{[13, 6]} Thermophiles grow best at 40 °C or above, and hyperthermophiles at 80 °C or above.^[6]

Where microbes live

Microbes live in air, soil and water, and on and inside people and animals, often in communities known as microbiomes.^{[2, 3]} NOAA, the US ocean and weather agency, describes microbes as helping to steer the flow of energy and matter on Earth by recycling nutrients and shaping the air’s make-up.^[19] Long ago, cyanobacteria started building the oxygen-rich atmosphere that human life later came to depend on.^[14]

Microbes in and on the body

The CDC says the microbiomes on our skin and in our gut help keep us healthy and protect us from infection.^[2] Bergtrom’s textbook estimates that our gut bacteria may outnumber our own cells by up to about ten to one.^[6] A research article on the CDC site reports that archaea, once thought to live only in extreme places, are important members of the human microbiome, mostly seen as harmless and still poorly understood.^[12] In the guts of cows, humans and other mammals, methane-making archaea help with digestion.^[6]

The DOE describes complex life, people included, as barely able to survive without microbes, and notes that we use them to make medicines and biofuels, clean polluted places and raise crops.^[1]

When microbes cause disease

MedlinePlus explains that many microbes are harmless and some even help, and it uses the term infectious diseases for illnesses that germs cause.^[3] The CDC separates colonisation, when a microbe lives on or in the body without causing symptoms, from infection, when it causes disease.^[2] Germs can pass between people by direct contact, by touching things that carry them, through insect or animal bites, and in contaminated food, water or soil.^[3] Only a small fraction of viruses can infect humans, and only a few hundred of the millions of fungal species make people ill.^{[4, 5]}

Antibiotic resistance

Antimicrobial resistance means that microbes no longer respond to the medicines meant to kill them, so they keep growing.^{[20, 22]} A drug acts as a selection pressure: microbes that withstand it survive.^[2] Resistant germs can pass resistance genes to other germs.^[21] The WHO says misuse and overuse of these medicines drive the problem, which reaches every country.^[22] It estimates that bacterial resistance was associated with (linked to, not proven to cause) more than 4.7 million deaths worldwide in 2021, and that about 1 in 6 laboratory-confirmed bacterial infections resisted antibiotics in 2023.^[22]

How scientists study microbes

NIOSH describes phase-contrast microscopes, which reveal nearly see-through microbes such as E. coli without staining, and stains such as the Gram stain that sort microbes by how they react.^[11] Classic microbiology grows pure cultures on agar, yet some heat-loving bacteria can be difficult or even impossible to grow in the lab.^[18] Metagenomics gets round this by collecting DNA directly from an environmental sample.^[18] The DOE Joint Genome Institute reads the genomes of single microbes and of whole communities.^[1]

Microbes and life beyond Earth

Scientists searching for signs of life on other planets study Yellowstone’s hot springs.^[14] The National Park Service notes that the organisms nearest life’s common ancestor are hyperthermophiles, which it reads as a hint that life may have begun in hot places on the young Earth; this is a hypothesis, not a settled fact.^[14] Because Mars’s surface receives far more radiation than Earth’s, researchers test whether radiation-tough Earth microbes could survive there.^[16]

Common misconceptions

“Microbes are all germs that make us sick.” Many are harmless, and the CDC describes our own microbiomes as protective.^{[3, 2]}

“Antibiotics work on viruses.” MedlinePlus states that antibiotics do not work for viral infections.^[4]

What this page does not cover

This overview leaves out diagnosis, treatment, vaccines and any medical advice. It says little about protists, fermentation and food, the nitrogen cycle, ocean food webs, soil and deep-sea life, or exact cell counts, because its sources do not cover them in enough depth.

Going further

See Cells, Genetics, Immune System, Medicine, Ecology and Planets for the neighbouring parts of the map.