Article by Amber Dance
They’re too small to see with the naked eye, but these microbes have abilities that put superheroes to shame.
Chestburster: Photorhabdus luminescens
Remember that scene in Alien where the baby alien erupts from John Hurt’s chest? Photorhabdus luminescens does something similar to its victims, and it glows an unearthly green while doing it. Fortunately for us, its victims are insects.
Photorhabdus acts as accomplice to a tiny worm called a nematode. The nematode totes the bacteria around in its gut until it finds a suitable victim, such as a fat caterpillar.
It worms its way into the insect’s bloodstream, then spits up the bacteria. P. luminescens spews toxins that kill the insect, enzymes that liquefy it, and antibiotics that prevent any other microbes from getting in on the feast. The worm and bacteria reproduce inside the carcass until finally new nematodes burst forth.
Scientists aren’t sure why P. luminescens glows. There’s a suitably grim possibility: it may be ordering its next meal. Living insects might be drawn in by the light, only to meet a horrific fate.
P. luminescens can get into human wounds, but it usually doesn’t cause disease. In fact, having a wound that glows ghastly green is a good sign for recovery.
During the American Civil War, soldiers called it the “Angel’s Glow”. Presumably, the antibiotics P. luminescens makes to ward off freeloader microbes also keep the wound clean.
Rocket Man: Listeria monocytogenes
Listeria infections are not so desirable. The bacteria can be found in soil and water or on plants, but it really shows its stuff when it lands in an animal or person.
Then it commandeers components of the animal’s cells. It uses the “bones” of a cell to build itself a comet tail and rocket around inside.
Unlike many bacteria living in our food, Listeria grows pretty well in the refrigerator. Its presence in foods like raw milk or soft cheeses can cause food poisoning, with flu-like symptoms or diarrhoea. Usually it isn’t serious, but listeriosis can cause pregnant women to miscarry, and it can be dangerous for people with weakened immune systems.
When it lands in a cell, Listeria immediately starts collecting bits of the cellular skeleton, called actin, around itself. Eventually it will build a ladder out of those actin bits, and ride the top of the ladder to move around the cell. Under a microscope, the bacterium looks like the head of a comet with an actin tail trailing behind it.
When Listeria reaches the cell’s edge, it pushes and pushes, making a long skinny finger sticking out of the cell. Another cell usually eats that finger: a grave mistake, as it gives the bacterium a new place to play.
The Blob: Myxococcus xanthus
Bacteria are single-celled organisms, but that doesn’t mean they have to live and work alone. Myxococcus xanthus bacteria travel in a wandering, rippling swarm that some scientists refer to as a “wolf pack”.
Each individual moves by sticking out a long, skinny “pilus”, which looks like a hair. It attaches the pilus tip to another bacterium, or to the surface it’s crawling on. Then it sucks back the pilus, dragging itself forward.
They don’t do it for exercise: this pack hunts other microbes. Like the creature from The Blob, the swarm preys on everything in its path.
The Myxococcus release antibiotics and digestive enzymes to paralyse the unlucky victim and break it down. All the nearby Myxococcus slurp up the resulting slurry.
This strategy works well when the bacteria are patrolling well-stocked hunting grounds. But if they run out of prey, they tighten their belts.
The bacteria slow down and start to build a tower. Stacking on top of another, layer by layer, about 100,000 bacteria assemble themselves into a yellowish, globular fruiting body.
Some of the bacteria in the fruiting body are special: they form round spores with a tough outer coat. These spores don’t need food or water, and can wait around until food is plentiful again. At that point, they germinate and start the swarm anew.
Radioactive Man: Deinococcus radiodurans
Myxococcus spores can resist heat and dryness, but they look pretty wimpy compared to Deinococcus radiodurans. This is one tough microbe, and it has an entry in Guinness World Records to prove it.
D. radiodurans holds the title of “most radiation-resistant lifeform”. Not only can it can withstand an amount of gamma radiation that would kill a human, it can handle 3,000 times that amount.
The bacterium was discovered in 1956, by scientists trying to sterilise canned food with radiation. To their surprise, one can of meat still spoiled. It was inhabited by a reddish organism.
How does D. radiodurans do it? For one, it’s chock-full of antioxidants, such as the carotenoids that give it its scarlet hue. These counter dangerous chemicals called free radicals, which are made by radiation.
But the real problem with radiation is that it rips apart DNA. D. radiodurans solves this challenge by keeping four or more backup copies of its genome, separated in four compartments. Even if radiation shatters its DNA, the bacterium can glue the bits back together, or copy sequences from the backups.
D. radiodurans’ amazing resilience has led some scientists to suggest that it came from outer space, where cosmic radiation is high. However, it probably evolved its defenses for a more terrestrial threat: dehydration, which slices up DNA just like radiation.
D. radiodurans is found all over the world in many kinds of environments, probably because dried-up bacteria ride the wind.
For these gooey bacteria, home sweet home is a deep, dark cave dripping with acid. An old mine is ideal. The bacteria live together in slimy “snottites” — yes, that’s what scientists call them — dangling from the cave walls.
Acidithiobacillus ferrooxidans and Acidothiobacillus thiooxidansdine on minerals containing sulphur or iron, such as pyrite (“fool’s gold”). They can survive surrounded by toxins like arsenic and mercury.
Acidithiobacillus also makes caves sparkle. As they digest those yummy metals, the bacteria excrete sulphuric acid. The acid dissolves limestone, which creates tiny gypsum crystals.
Green Lantern: Aliivibrio
If a slimy cave doesn’t sound like home to you, how about the insides of a bobtail squid?
Aliivibrio likes that environment just fine. The squid likes Aliivibrio too, because it provides camouflage so the squid can cruise the night-time ocean undetected by predators.
A newly-hatched squid will collect bacteria from surrounding seawater into special cavities, called the light organ, on its underbelly.
Not just any bacteria, though, Aliivibrio (formerly called Vibrio) are especially adept at swimming through the squid’s mucus. They can also withstand the antimicrobial chemicals, such as hydrogen peroxide, that it makes to kill off undesirables.
In return for a cozy place to live and all the sugar and protein they can eat, the bacteria protect the squid.
Normally, a little bobtail squid would appear as an obvious shadow to anything lurking below. But thanks to Aliivibrio, the squids glow blue-green, so they blend right in with the moonlight. The squid can safely putter about looking for shrimp to eat.
Different kinds of Aliivibrio inhabit different squids in different oceans. A. fischeri likes warm water, and tends towards the Pacific, while cold-loving A. logei prefers the Atlantic.
The Kragle: Caulobacter crescentus
In The Lego Movie, the good guys fear a superweapon called the “Kragle” – which turns out to be a tube of glue. Those blocky characters would hate Caulobacter: this aquatic bacterium makes a sugary goop that is phenomenally sticky.
The C-shaped Caulobacter crescentus uses its glue to attach to surfaces in the watery environments where it lives. When a team of biologists and physicists measured how much force it took to yank C. crescentus off a surface, it was about 5 tonnes per square inch.
C. crescentus doesn’t spend all its life stuck down. A newborn C. crescentus, called a swarmer, swims around powered by a rotating tail called a flagellum.
When it finds a nice place to settle down, such as a pebble or a plant, it drops the flagellum and grows a stalk. At the tip of that stalk there is a bit called the holdfast, which lives up to its name with the help of those sticky sugars.
Then C. crescentus gets down to the business of making baby bacteria. It splits in half, and the half that isn’t stalked heads off as a new swarmer.
C. crescentus can live in fresh water, salt water and even tap water (don’t fret, it doesn’t typically cause disease). It thrives in places where there’s almost nothing for it to eat. Scientists suspect that the stalk, in addition to being sticky, is also good at picking up nutrients.
Electro: Geobacter metallireducens
Underneath that Caulobacter-infested water, the mud buzzes with electricity. In the sediments, where oxygen is scarce, Geobacter is using electrical energy to breathe.
When living cells make energy, they make electrons, and these must be dumped somewhere. Us air-breathers use oxygen as to mop them up, but Geobacter and some other underground bacteria dump their electrons on metals instead.
Their idea of “fresh air”, then, is a place with lots of dissolved metals like iron or manganese, or even uranium or plutonium.
If they run out of dissolved metals, they have a Plan B. They grow a flagellum and swim around seeking any metal, dissolved or solid.
To transfer their electrons to solid metals, they grow nanowires called pili. In other words, they’re electric, rather like Spider-Man’s nemesis Electro – albeit with a more convincing motivation.
Metal-breathing microbes are also good sharers. Different Geobacter species can build a network, allowing them to trade resources.
For instance, one might be able to eat the surrounding nutrients, but have nowhere to dump its electrons. The other might have nothing to eat, but the right wires to dispose of electrons. Together, in a pili network pulsing with electricity, they work together to stay alive.
Magneto: Magnetospirillum magneticum
How is a swimming bacterium to navigate through the water? Magnetospirillum carries a compass.
This corkscrew-shaped bacterium, and others such “magnetotactic” species, align themselves to Earth’s magnetic field. They tend to live in brackish water, like bogs, or the sediments at the bottom. They prefer very specific oxygen levels: some, but not too much.
Magnetospirillum could find the optimal oxygen environment by wandering randomly through the water, but it’s taken the approach ofX-Men villain Magneto and found a more efficient way.
Magnetotactic bacteria collect iron from their environment, and make magnetite nanocrystals in little sacs. They use their cellular skeleton to arrange the crystals in a neat line. The Earth’s magnetic field pulls the magnetic crystals so the bacteria line up pointing north and south.
By waggling its flagellum, Magnetospirillum travels back and forth along the magnetic line, looking for the oxygen to rise or fall until it finds a happy medium. Instead of wandering in three dimensions, it just has to go back and forth in one.
Mud Monster: Streptomyces
Step outside on a rainy morning, and you’ll likely notice the earthy smell of wet soil. But it’s not really the soil you’re smelling: it’s the Streptomyces living in it.
These bacteria, which grow in long filaments in the earth worldwide, exude a chemical called geosmin. It is this we recognise in a freshly-turned field. Geosmin is also responsible for the earthy flavor of beets – and, unfortunately, the muddy taste of some fish, wine and water.
People and other animals are exquisitely sensitive to the smell of geosmin. Humans can pick it up at concentrations as low as five parts per trillion.