Bacteria relevant to humans

A gut resident with roles in breaking down carbohydrates.

Many strains are harmless residents. Some cause diarrheal illness.

Often carried without harm; infections can become serious.

The bacterium behind TB. Active disease differs from latent infection.

Pneumococci can live in the nose and throat without causing illness.

Meningococci can be carried in the back of the nose and throat without illness.

A stomach colonizer associated with inflammation and ulcer disease.

Strain-dependent intestinal illness, sometimes causing rapid dehydration.

A spore-forming bacterium with distinct colonization and infection states.

A common nose and throat resident that can also cause ear, lung, bloodstream and meningeal infection.

Group A Streptococcus: a throat and skin organism behind strep throat, scarlet fever, impetigo and rare severe infections.

The diphtheria bacterium: illness comes from a toxin that only some strains produce.

The whooping cough bacterium: a cold-like start that can turn into weeks of coughing fits.

A water-system bacterium that causes Legionnaires’ disease when contaminated mist is breathed in.

A species of thousands of serotypes; the non-typhoidal ones are a leading cause of food-related diarrheal illness.

A spiral gut pathogen and one of the most common causes of diarrheal illness, often traced to poultry.

A dysentery bacterium that needs only a tiny dose; the leading Shigella species in lower-income settings.

A cold-tolerant gut pathogen, often traced to pork, whose pain can be mistaken for appendicitis.

A gut resident that turns opportunist in healthcare settings, and a focus of carbapenem-resistance concern.

The most common bacterium on human skin, and the most common source of medical-device infections.

A normal gut resident found in most people, and an opportunist in urine, wounds and the bloodstream.

A small share of the gut community that helps ferment carbohydrates, and the anaerobe most often isolated from infections.

One of the most abundant gut bacteria in healthy adults, and a major supplier of butyrate to the colon.

A mucus-layer specialist that lives on mucin and is present in most healthy adults.

An environmental water-and-soil bacterium that mainly causes infections in hospital patients and damaged lungs.

A hardy hospital bacterium that often just sits on skin or equipment but can cause hard-to-treat infections in intensive care.

A foodborne bacterium that most people shrug off but that can cause severe illness in pregnancy, newborns and older adults.

A human-only bacterium that causes gonorrhoea and very often produces no symptoms at all.

The spiral bacterium behind syphilis, an infection that moves through stages and can go quiet for years.

An obligate intracellular bacterium behind both the world’s leading infectious cause of blindness and its most reported bacterial STI.

A wall-less bacterium that causes chest colds and the mild pneumonia people often keep walking around with.

A spore-forming soil bacterium of grazing animals that causes anthrax in people who handle infected animals or their products.

A soil bacterium whose toxin causes tetanus — an illness you cannot catch from another person and that vaccination prevents.

A spore-forming bacterium whose toxin causes botulism — rare, preventable and a medical emergency when it happens.

A fast-growing anaerobe behind one of the commonest causes of food poisoning, and behind gas gangrene in damaged tissue.

The plague bacterium: a rodent-and-flea organism that still causes a few thousand human cases a decade, mostly in three countries.

The corkscrew-shaped bacterium behind Lyme disease, carried by ticks rather than passed between people.

A tick-borne bacterium that lives inside cells and causes Rocky Mountain spotted fever, where treatment delay is the main danger.

A tough, inhaled bacterium of farm animals that causes Q fever — usually a flu-like illness, occasionally a long one.

A goat-and-sheep bacterium that reaches people through unpasteurised dairy and causes a long, relapsing fever.

A rabbit-and-rodent bacterium so infectious that a handful of cells can cause illness, yet it is not passed between people.

A hook-ended spirochete spread through animal urine in water and soil, causing everything from a flu-like week to organ failure.

The bacterium of leprosy: slow, curable, and far less contagious than its history suggests.

A common gut and genital-tract resident in adults that occasionally causes serious infection in newborns.

A normal gut resident that has become one of the harder hospital organisms to treat.

A mouth resident strongly associated with tooth decay — though decay has more causes than one bacterium.

The most abundant bacterium on healthy oily skin, and a contributor to skin balance rather than simply a cause of acne.

A lactic-acid-producing resident that dominates one of the most common and most acidic vaginal community types.