Bringing water to boil kills all the bacteria, you often hear. Not quite. Some, including the rod-shaped Bacillus bacteria retrieved from many different environments, form small, robust survival structures when the going gets tough. These so-called endospores, needing neither water nor nutrition, can persist for decades or centuries, waiting for better times. When conditions improve, the endospores germinate to develop into proper bacterial cells. So they’re a bit like the seeds of plants, but even more resilient—and remarkably heat-resistant. This is why boiling won’t do for microbiologists. To break endospores down, they resort to a steam autoclave that heats their re-usable equipment up to 121 °C at high pressure. Only that will do.

Bacteria are sometimes named after the scientists who isolated or characterized them, but they usually have to wait for others to suggest the honor. The human pathogen Ralstonia pickettii actually has two “parents”, Ericka Ralston and M.J. Pickett. Ralston had received some Pseudomonas hospital strains from Pickett, and she discovered that these strains, although closely interrelated, differed markedly from most other species of that genus, so suggested a new species called Pseudomonas pickettii for them. These differences were later seen as so significant that the strains were placed into an entirely new genus, Ralstonia, along with the plant pathogen R. solanacearum and a few other species. Not that the dog would care, though.

Ordinary bacteria of the soil possess many capabilities that animals and plants don’t. Take Sphingomonas, for example, ovoid to cylindrical shaped microbes that produce pigmented colonies of yellow or off-white color. In 2008, a Canadian schoolboy won a prize for demonstrating that enriched soil bacteria can degrade 17 percent of carrier bag plastic within six weeks, with Sphingomonas having the biggest appetite for the polyethylene. Sphingomonas have been used in several biotechnological applications, including the degradation of soil contaminants and the production of extracellular polymers.

Pseudomonads are very widespread in natural environments and would almost certainly be found in the sort of puddle a dog would tread into. Many of the 200 or so species support plant growth or help degrade dead organic material in soil. But as so often, the few nasty ones get all the publicity. Pseudomonas aeruginosa is frequently found in hospitals and an “opportunistic” human pathogen, meaning it infects patients with severe wounds or a weakened immune system. Its cells often harbor plasmids, small circular DNA located outside the bacterial chromosome, that can carry genes giving the bacterium resistance against antibiotics.

Is there a microorganism that means to science what Muhammad Ali meant to sports? If so, the answer must be Escherichia coli. A true legend, E. coli (as it is famously abbreviated) is probably the best-known bacterium of all time. For millennia it was just one of many bacterial species that line the intestine and feature in the feces of human beings and numerous animals (including dogs) before it rose to fame in the 20th century to become a heavyweight champion of fundamental research, molecular techniques, and water quality monitoring. But beware: some E. coli can knock you out. The pathogenic strain O157:H7, for example, causes dangerous gastrointestinal disease.

The bacterial genus Proteus is named after Proteus, a mythological Greek god of the sea, characterized in Homer's Odyssey as highly changeable in appearance. Why? Proteus cells can vary in length and abilities. Under certain conditions, they alter their appearance and collectively swarm across an agar plate, creating visible patterns that change over time. In nature, Proteus bacteria tend to live in rather ungodly places, including decomposing animal matter, sewage, manure soil, as well as in the intestine and feces of human beings and animals. Some are opportunistic pathogens, causing urinary tract and other infections in hospitals and nursing homes.

Sound familiar? The family next door are alright, you get along with them most of the time. Well, except for ... that’s what you could say about Staphylococcus. These spherical bacteria, which like to form grape-like cell clusters, mostly live on the skin and mucous membranes of humans, dogs, and many other animals but usually don’t cause problems. Not so Staphylococcus aureus. Don’t be fooled by its good looks—golden yellow colonies, hence "aureus" ("gold" in Latin). This species is a major cause of food poisoning and dangerous infections worldwide, particularly among people with a weak immune system. Some strains, known as MRSA, are a huge health concern because they have become resistant to the antibiotics we use.

Corynebacterium are very irregularly shaped bacteria. Some look like the letter V under the microscope, others like a club (hence the name, coryneform means "club-shaped"). The species C. diphtheriae and C. ulcerans can cause diphtheria, a potentially deadly disease, if the bacterial cells are themselves infected by a certain virus that prompts them to produce a toxin. Pet dogs can carry corynebacteria into the house, so it’s a good idea to wash your hands after contact—as is, of course, to regularly get your booster injections that should keep you immune.

We give most of our crops a lot of fertilizer because plants cannot use atmospheric nitrogen (N₂) as their nitrogen source. What plants can’t do, the common soil-dwelling Bradyrhizobium bacteria can. Legumes such as lentil, pea and bean plants have found a clever way to make use of this ability. They maintain little “factories” on their roots in which bacteria such as Bradyrhizobium turn atmospheric nitrogen into ammonium that the plants take up. In return, the bacteria receive some organic substances that they need. This form of cooperation between species is called symbiosis.

If you think Aspergillus are a plain nuisance, we’ll excuse you. These molds infest bread, fruit, vegetables, nuts—basically any food you keep in the kitchen that contains a residual amount of water, making the food inedible for humans. Their filaments penetrate the infested item and spoil it before you can see or taste that this has happened. As you have probably experienced, they are almost everywhere, even if you don’t have a pet. Some may cause allergies or infections, and two species that can infest pistachio nuts produce carcinogenic aflatoxins. On the other hand, there are species that have practical uses in biotechnology, including the industrial production of citric acid.