How Not to Grow Bacteria

The hot and the cold of bacteria

The hottest place on earth?


Death Valley, establishing the world record when its air temperature reached a roasting 134°F in July 1913.

Of course if your body temperature reached that level, you’d be a goner.

We humans are incredibly good at self-regulating our body temperature to one degree above or below 98.6°F (37°C).

Anything above 100.4°F (38°C) is regarded as a fever, so the remarkable 115°F recorded when 52-year-old Willie Jones of Atlanta was admitted to hospital with heatstroke, was just about as high as you can go and still survive.

Mr. Jones did — making it into the Guinness Book of World Records for the highest recorded body temperature.

At the opposite end of the scale, a body temperature below 95°F (35°C) doesn’t look too promising for someone.

It’s where hypothermia kicks in.

So we’ve now established that humans have three key temperatures — a maximum, a minimum, and an optimum.

And in fact this is a model that applies to most organisms, including those tiny ones we specialise in at uBiome. Bacteria.

Bacteria and temperature

It’s what led the U.S. Department of Agriculture (USDA), among others, to establish what’s popularly known as the “Danger Zone” when it comes to food safety.

The zone runs from 40°F to 140°F (4°C to 60°C), as it’s the range that matches the conditions in which most of the common pathogenic (disease-causing) bacteria can flourish.

Leaving food out too long at room temperature can cause Staphylococcus aureus, Salmonella enteritidis, E. coli, and Campylobacter to grow dangerously.

Bacteria can reproduce like rabbits (well, like rabbits on steroids to be honest) doubling in number in as little as 20 minutes, given the right conditions.

And here’s the thing. As these nasty bacteria grow, it doesn’t generally affect the taste, smell, or appearance of food, making it hard for humans to know whether pathogens are present or not.

So in general, it makes a good deal of sense to keep hot food hot (over 140°F, or 60°C) and to keep cold food cold (below 40°F, or 4°C).

Bacteria which thrive at temperatures of around 98.6°F (37°C), the body temperature of humans and most other warm-blooded animals, are known as mesophiles.

But some species of bacteria thrive at higher, or lower temperatures, and they are collectively labelled extremophiles.

Those that enjoy high temperatures are known as thermophiles, while those which prefer chillier conditions are classified as psychrophiles.

Note the “r”—it’s “psychro” rather than “psycho“, the psychro coming from the Greek word “psukhros,” meaning cold.

Cold-loving, rather than Alfred-Hitchcock-infatuated, then.

Among the bacteria that enjoy icy conditions is a species called Moritella profunda, found in deep oceans, and having a maximal growth temperature of 36°F (2°C).

The species name “profunda” means “from the deep.”

Interestingly, the genus name “Moritella” is in honor of the marine biologist Richard Morita, a Japanese-American born in Pasadena, California in 1923.

Although this has nothing to do with cold-loving bacteria, I can’t mention “Dick” Morita without acknowledging the shocking fact that he (an American citizen) and his family were incarcerated in what amounted to a prison camp, midway between San Francisco and Los Angeles during the Second World War, for the simple “crime” of being Japanese.

While Prof. Morita gave his name to a genus of psychrophiles, his oceans are also home to thermophiles—those heat-seeking microbes we were just talking about.

In fact, prokaryotes have been found frolicking happily around hydrothermal vents in the deep sea at temperatures of at least 248°F (120°C).

And while we’re on the subject of bacteria and high temperatures, we should briefly touch on something else.

The spontaneous combustion of haystacks.

When hay is gathered too wet, the growth of aerobic bacteria can contribute to the stack becoming super-hot, sometimes to the extent that it bursts into flames of its own account.

Apparently farmers know to look out for signs such as steam rising from a stack, or unusual odors such as pipe tobacco, or caramel.

Let’s end by going from hot back to cold again.


Remarkably there’s a species of bacteria known as Pseudomonas syringae that’s used in the production of artificial snow at ski resorts.

It uses special proteins to alternately repel and attract water molecules, resulting in the water freezing at higher temperatures and under conditions not normally conducive to freezing.

This process, known as “biogenic ice nucleation,” resulted in a commercial product named Snomax, which was launched in 1984 by a company based in Berkeley, California where — it must be said — you are likely to wait a long time if you hope to build a snowman in your backyard.

Further reading

A close up look at how bacteria make ice

Bizarre Ice-Forming Bacteria’s Secrets Revealed

Body Temperature

Can bacteria grow in the refrigerator?

Danger Zone

Haystack fires (spontaneous combustion)

Hypothermia—Symptoms, Causes and Risk Factors

Life in Extreme Heat

Nutrition and Growth of Bacteria

Preliminary Experimental Evaluation of Snomax (TM) Snow Inducer, Nucleus Pseudomonas syringae, as an Artificial Ice for Weather Modification


Richard Yukio Morita Oral History


The World’s Most Extreme Places

You Might Be Surprised When You Take Your Temperature