To grow and produce crops under almost any condition, plants need to make good use of all nutrients available to them. While they are masters at absorbing some nutrients from the air and soil, they are struggling with others.

One such problematic element is nitrogen. Even though nitrogen makes up about 80% of the atmosphere, it is mainly present as dinitrogen gas N₂.

This means two nitrogen atoms are tightly bound to one another via three strong and energy-rich bonds. In this form, plants can neither take up the nitrogen nor use any of the nitrogen atoms to make other molecules from them.

Yet, they need nitrogen since it is part of every DNA molecule, protein, the energy provider ATP and many vitamins. Hence, plants need a way to acquire that element in a simple way that does not cost them too much energy.

Enter bacteria.

Diazotrophic bacteria fix nitrogen

The so-called diazotrophs have developed a highly efficient enzyme complex to capture, or fix, dinitrogen from the atmosphere and break up its energy-rich bonds. This complex is the nitrogenase, and all diazotrophs use one of three types of nitrogenase.

The most efficient nitrogenase contains a molybdenum ion at its core, while other nitrogenases use vanadium or iron. These metals are extremely rare in the environment. Hence, depending on which one is available, bacteria regulate which of the three nitrogenases to produce.

After capturing a dinitrogen molecule, the nitrogenase enzyme transfers energy in the form of protons and electrons to it. This eventually breaks up the bond between the two nitrogen atoms and produces two ammonium ions NH₃⁺.

Bacteria then use the ammonium ions for their own growth and share the surplus with their friends and partners. In Nature, several symbiotic relationships exist between bacteria and other organisms which are based around the nitrogen-fixating superpower of bacteria.

Soil bacteria share fixed nitrogen with plants

The best known nitrogen-fixing organisms are soil bacteria from the families Bradyrhizobium, Frankia, Bacillus, Clostridium, Burkholderia and Pseudomonas. These either live freely in the soil or form symbiotic relationships with plants.

Especially important are symbiotic rhizobia like Bradyrhizobium and Frankia. Plants attract these soil bacteria to their roots by sending out special molecules, which the bacteria respond to via their quorum sensing receptors. Within the root network of legume plants, the bacteria then build little nodules in which they live protected from the surrounding.

Within the nodules, bacteria fix and convert nitrogen with their enzyme complexes, which requires a lot of energy. Gladly, the host plant provides this energy in the form of sugars and organic acids that it produces with photosynthesis. The plant then transports these molecules into the root nodules, where the bacteria break them up, extract their electrons and thus gain the necessary energy.

After breaking up the nitrogen using these very electrons, the bacteria transport the produced ammonium from the nodules into the plant. With the ammonium, the plant makes DNA, proteins and vitamins; everything that it needs to grow and produce crops and fruiting bodies. Hence, rhizobia bacteria are highly important for the health of plants as well as crop production and yield.

Marine bacteria can fix nitrogen

Soil bacteria are not the only nitrogen-fixing organisms; marine bacteria are also important for global nutrient cycles. For example, cyanobacteria form long filamentous multicellular organisms, with some cells specialised in nitrogen fixation.

Often, cyanobacteria are closely associated with other marine bacteria with which they share nitrogen. So far, scientists do not fully understand these types of interactions but are sure that nitrogen-fixing organisms are crucial for the marine food web and the survival of many species under water.

When temperatures are high enough and nitrogen concentrations are optimal, you can pretty much see the nitrogen-fixation process. A green blanket on the water surface is a sign for cyanobacteria that power both photosynthesis and nitrogen fixation with the carbon dioxide and nitrogen from the air. This so-called algae bloom is mainly due to cyanobacteria like Aphanizomenon, Dolichospermum, Anabaena and Synechococcus bacteria.

Soil bacteria as biofertilisers for sustainable food production

Since some soil bacteria are so efficient in fixing nitrogen and providing it to the plant, they have also become valuable in agriculture. Some so-called biofertilisers consist of bacteria that are added to soil or plants to build symbiotic relationships with them, helping them grow better and produce bigger crops.

Hence, biofertilisers containing bacteria are an efficient and sustainable way to produce more food and in higher quality. With this, farmers will rely less on synthetic fertilisers while maintaining high crop yields. Additionally, using nitrogen-fixing bacteria as biofertilisers helps protect the health of the soil and the environment.

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Bacterialworld - A blog about bacteria: from scientific studies to vivid stories about the fascinating bacterial world

But what do bacteria and fungi use antibiotics for? Why do they produce them? And what are the advantages of microbes having antibiotics as molecular weapons?

What are antibiotics?

The father of antibiotics, Selman Waksman, first used the word antibiotics for any small molecule made by a microbe that can inhibit the growth of other microbes.

So, microbes – especially bacteria and fungi – use antibiotics to kill other microbes. These other microbes can be bacteria, fungi or bigger organisms. Not viruses though!!!

Why not viruses?

Because antibiotics bind and inhibit cellular machines in living organisms. These molecules often bind to so-called targets. Antibiotic targets can be proteins or enzymes that make for example the cell wall, other proteins or components of the respiration complex.

These proteins are generally essential. So, when antibiotics inhibit the proteins, the cells are missing these essential functions. And without them, they cannot survive and die.

Hence, like other bacterial toxins, antibiotics are lethal.

Interestingly though, bacteria and fungi make antibiotics from simple building blocks. These are present in every cell and can be amino acids, lipids or even sugars.

But instead of using these building blocks for their normal functions, microbes link them together in different ways. With this, they create new – and fancier – molecules that barely resemble the original blocks.

Then, they transport these antibiotics to the outside or send them off in outer membrane vesicles. When the antibiotic hits another microbe, there are two possibilities: either the microbe is resistant to the activity of the antibiotic or it dies from it.

But what about the microbe that produces the antibiotic? Is it resistant to the antibiotic itself?

Why are microbes that produce antibiotics not get killed?

Since antibiotics are meant to KILL other microbes, then why do producing microbes not get killed by their own antibiotics? The answer is self-protection!

Whenever bacteria or fungi produce antibiotics, they always also produce some sort of self-protective means. Just as when bacteria produce other toxins, they always need to make sure they are not killed by their own weapons.

These self-protectors usually keep the antibiotic in an inactive state. For example, they completely surround the antibiotic molecule so that it cannot bind to its usual target within the cell. Another strategy is to add a small molecule to the antibiotic – again to keep it from binding to its target.

Then, when the microbe is ready to transport the antibiotic outside of the cell, it takes the self-protection off the antibiotic. This releases only the toxic part – the antibiotic itself – into the surrounding.

Note, however, that these self-protection mechanisms are not antibiotic resistance mechanisms. Self-protection mechanisms are meant to inactive antibiotics only temporarily. Hence, these mechanisms are reversible. The antibiotic can still become active and thus toxic.

Resistance mechanisms, on the other hand, are meant to inactive antibiotics permanently. Hence, these mechanisms are irreversible. Since this usually completely destroys the antibiotic, it cannot become active anymore.

But what triggers microbes and especially bacteria to produce antibiotics? How do antibiotics help the producing cell in their daily circumstances?

Why do bacteria produce antibiotics?

To answer this question, we need to look at where the bacteria live that make antibiotics. And two-thirds of the known antibiotics are made by bacteria from the Actinobacteria family. Within this family, Streptomyces is the best-known member that produces half of all known antibiotics.

Another example is bacteria from the Myxococcus family. So, where do Streptomyces and Myxococcus bacteria live? Interestingly, these bacteria call the soil their home.

And in the soil, they often confront lots of friends and foes. And they need to constantly fight for their own survival.

Myxococcus is known as a wolf-pack predator because it kills its prey in massive attacks. Colonies of Myxococcous roll over their prey, secrete antibiotics and thus kill them and feed on them.

Streptomyces, on the other hand, uses its antibiotics a bit more civil.

To move in the environment, Streptomyces bacteria grow as long filaments throughout the soil. They build long chains and branch out into the soil as multicellular organisms. These branches are filled with Streptomyces cells but also spores so that the bacteria can extend to new places.

When the bacteria hit a period of bad weather or don’t find much food, they release their spores as a survival strategy. Plus, they start releasing nutrients for the spores. But these nutrients also attract other organisms like bacteria.

Hence, at the same time, Streptomyces produces a huge amount of antibiotics to fend off these putative food-stealers. Like this, Streptomyces makes sure their spores are safe and can survive in their new homes for a while.

How do antibiotics produced by bacteria help others?

Like Streptomyces, lots of bacteria use antibiotics to fight off predators. This assures their own survival and that of their species.

Yet, more and more research finds that bacteria not only kill other species with antibiotics so they can survive. The killing also benefits their hosts.

For example, the bacterium Janthinobacterium lividum lives on frogs where it produces the antibiotic violacein. This antibiotic kills fungi so that the bacterium protects the frog from deadly fungal infections.

Also, a bacterium that lives in our noses is the harmless Staphylococcus lugdunensis. This bacterium produces the antibiotic lugdunin. That inhibits the harmful Staphylococcus aureus from settling down in our noses. Now, scientists look into how we could use the harmless Staphylococcus lugdunensis to protect us from infections.

Another example of microbes that produce antibiotics to help others is the three-member association of ants, Streptomyces and a fungus. Several species of ants grow fungi for food. They feed their fungi with fresh plants and let them grow in special underground gardens.

To not contaminate these fungal gardens, ants carry symbiotic Streptomyces that produce antibiotics. Like this, the antibiotics kill other microbes and keep the fungal gardens free of harmful intruders. As a thank you, the ants feed the Streptomyces and give them a place to live.

About antibiotic-producing microbes

So, just as we use antibiotics to kill harmful bacteria and fungi, antibiotic-producing microbes do the same. They want to fight off predators and assure their own survival.

When you think about it: we use their own killer weapons against them. Poor microbes!

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Bacterialworld - A blog about bacteria: from scientific studies to vivid stories about the fascinating bacterial world

So, they talked of cocci and bacilli based on the spheres and rods that they saw under the microscope.

And they classified microbes and bacteria based on these shapes.

It came only with later, modern technologies that scientists learned that there was more to bacteria than their shapes. Even though bacteria looked similar, they had different superpowers.

Yet, some of these bacterial superpowers are indeed influenced by their cell shapes.

So, what is it about bacterial shapes? Why do bacteria look differently? And how do the different shapes of bacteria help them survive and thrive?

What gives bacteria their shapes?

To protect themselves from the environment, bacteria as well as all other organisms have cell envelopes. These keep the cellular machines and internal parts together so that a bacterium can function within this envelope.

And this envelope also gives bacteria their shape.

Both Gram-positive and Gram-negative bacteria have a layer of so-called peptidoglycan within their envelope. This peptidoglycan layer is made of sugars that are linked together by very strong bonds. This is why the peptidoglycan layer is pretty rigid and stiff and has a specific shape in each bacterium.

Either on the inside or on the outside, the peptidoglycan layer is linked to the cellular membranes. Together, these make up the bacterial envelope with a specific cell shape.

What different shapes do bacteria have?

Microbiologists have different ways to classify known bacterial shapes. Here, I will introduce you to the bacterial shapes according to what makes the most sense to me.

Rod-shaped bacteria

As the name suggests, these bacteria have a rod or cylindrical shape. Examples of rod-shaped bacteria are Escherichia coli and Bacillus subtilis.

Scientists are also convinced that rod-shaped bacteria are the evolutionary ancestors of all other bacterial shapes.

The shape comes from proteins that form long cables within the bacterial cell. These span out the whole bacterium from one end to the other.

Rod-shaped bacteria grow by two modes that we talk about in Why bacteria divide into two and grow with the help of a strong ring: First, they extend their cell size by growing the peptidoglycan, the cable proteins and the membrane.

Second, the cable proteins determine the middle of the cell, where the bacterium produces a special ring. Eventually, this ring narrows so that the bacterium divides and two bacterial cells form.

Spherical bacteria

The spherical bacteria – or so-called cocci – include many pathogenic bacteria like Staphylococcus aureus, Streptococcus pneumoniae and Neisseria gonorrhoeae.

Microbiologists think that spherical bacteria were once rod-shaped as well. However, spherical bacteria do not have these long cable proteins that extend their cell bodies. Thus, they stay spherical and grow by dividing their spherical cells right in the middle.

However, sometimes the two daughter cells do not completely divide and they stay attached to each other. This is why some spherical bacteria live as so-called diplococci.

Curved bacteria

Curved bacteria have the shape of a comma or banana and are sometimes also slightly twisted. Examples of curved or banana-shaped bacteria are Caulobacter crescentus and Vibrio cholerae.

These curved bacteria usually live in watery environments where there are flows. Here, the curved shape helps the bacteria to align with the flow while staying attached to a surface.

In the case of Caulobacter crescentus, one end of the bacterium is glued to a surface with a strong super glue. When this bacterium divides in the middle, one daughter cell remains attached to the surface, while the other one can swim away and find a new location to settle down.

Spiral bacteria

Spiral bacteria are a mix of rods and curves which give them a helical twist. Hence, these bacteria have a corkscrew shape.

Many pathogenic bacteria use their corkscrew shape to swim through gel-like solutions. This includes Helicobacter pylori and Campylobacter jejuni.

Since spiral – or helical – bacteria are also thinner, they can reach locations that are too narrow for other bacteria to reach. They also use their flagella to push themselves forward and “wriggle” through narrow pores.

Star-shaped bacteria

Some bacteria look even fancier than others: They are real stars – yes, bacteria with a star shape.

While we don’t know much yet about star-shaped bacteria, they belong to the so-called Stella species or are Methylomirabilis oxyfera. These usually grow in freshwater, soil and sewage.

The star shape comes from six little arms that extend out of the bacterial cell. These push and grow to the outside giving these bacteria a shiny star shape.

Why do bacteria have different shapes?

Now that we have seen the different shapes of bacteria, you might ask yourself, why do bacteria have these different shapes? How do they help them?

As always in biology, it comes down to how a property helps a bacterium survive in a certain location. Often, the cell shape gives a bacterium advantages over other bacteria and it is easier for them to settle down and face harsh environments.

For example, spherical cells have the lowest surface-to-volume ratio. This means they have a large envelope surface through which they can take up a lot of nutrients. All this while their cell volume is relatively small. So they don’t actually need that many nutrients. This helps cocci to grow in locations where there are little amounts of nutrients.

On the other hand, rod-shaped bacteria often have flagella. And thanks to their shapes, they are efficient swimmers. This allows them to swim to new places in cases of danger or the lack of nutrients.

Bacterial cell shapes help face harsh environments

Also, straight rod cells can pack into biofilms more efficiently and build organised structures. This helps them colonise different locations and resist dangerous environments.

Many rod-shaped bacteria also form longer filamentous organisms. These stronger and larger structures protect bacteria from being eaten by other organisms. Another advantage of these multicellular organisms is that they allow more cells to attach to surfaces and colonise hosts.

Lastly, both curved and helical bacteria use their shapes to get better around their environments. Curved bacteria grow in watery environments but also in our guts. Here, their shapes help them align with the flow of water or our gut content while they stay attached to a surface or the gut wall. This keeps them at their preferred location and protects them from being flushed away.

Spiral bacteria use a fascinating helical movement to screw through gel-like or viscous fluids. This for example helps pathogens swim through the mucus of our stomach and guts and colonise us and make us sick.

Bacteria and their shapes

Here, we looked at the different shapes that bacteria have and how these help them survive. Bacteria always face harsh and new environments and conditions and only survive if they have the right tools or means.

So, by adapting their shapes, bacteria often have advantages over other bacteria. Plus, they look cool and fabulous!

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Bacterialworld - A blog about bacteria: from scientific studies to vivid stories about the fascinating bacterial world

So, yes, also bacteria are always trying to find their siblings and communicate with them. And once they know they are not alone, they start reacting as a group.

Some bacteria start building biofilms – houses to keep the bacteria inside safe. Others like to talk to each other and produce goodies that everyone can enjoy. And other bacteria even form multicellular organisms with new superpowers.

Yet, some bacterial species like to move only in groups. Researchers call this bacterial movement twitching.

Bacteria can only twitch and move in groups when they have so-called twitching pili. Not all bacteria have these types of pili and – unfortunately for us – many bacterial pathogens produce them. And these bacteria use their pili to infect us and make us sick.

So, let’s have a look at what these bacterial pili are.

What are bacterial pili?

Bacterial pili look like little hair that grow out of bacterial cells.

This hair is anchored to the bacterial cell envelope and can be attached to any site of the bacterial surface. Some bacteria only have on pilus, others have two pili at opposite ends and some bacteria even produce bundles of pili that work together.

The pilus hair is a helix of an endless number of the same protein: the so-called pilin protein. This pilin works like a perfect puzzle piece: Each end of the pilin fits the next pilin piece. Like this, endless pilin puzzle pieces attach to each other in a circular manner and form a stable hair-like helix structure.

But not to lose their precious hair, bacteria need to attach the pilus to their cell envelope. For this, bacteria have a huge anchoring complex on the inside of their cell envelope. And this anchor holds the pilus at the correct location.

To make this pilus dynamic, bacteria link the anchor to a tiny motor. This motor has a ring shape that surrounds the anchor and thus the hair. And bacteria need this motor for the actual moving process.

How do bacteria move with pili?

This circular motor on the inside of the cell envelope has two main functions: to extend and retract the pilus. Endless circles of extending the pilus, attaching to a surface and retracting the pilus allow bacteria to move.

To extend or lengthen the pilus hair, the motor (orange) binds the pilin proteins inside the bacterium (grey circles) and transports them outside of the cell. This costs energy, which is why bacteria need this little motor. Hence, by adding more pilin protein to the pilus from the inside, the pilus hair (grey) extends towards the outside.

On the outside at the end of the pilus hair sits a protein (green) that can stick to surfaces. When this protein attaches to a surface, the motor on the inside of the bacterium changes its direction. Instead of adding pilins to the pilus and lengthening the hair, the motor takes pilins off the pilus and thus shortens the hair.

Now, the bacterium is attached to a surface while the pilus shortens. Like this, the bacterium pulls itself towards that surface.

This means that the attachment to the surface has to be so strong, that it can pull the bacterial cell towards this new location. This works like the bacterial superglue that some bacteria use to grow and survive.

What is the function of bacterial pili?

Bacterial pili can attach to all sorts of surfaces. Mainly, bacteria use this movement in environments of low water or on wet surfaces like human tissue.

For example, a bacterium can connect with its pilus to another bacterial cell. Now, when the bacterium retracts the pilus, it pulls the other bacterium closer. Like this, bacteria can form aggregates which helps them in the first steps of settling down and building biofilm houses.

Also, when several bacteria stick together and form bigger groups, they can move along a surface in a coordinated manner. This helps bacteria conquer new environments quicker and find new resources. For example, the bacterium Pseudomonas aeruginosa can reach out in swarms trying to find more space and new places to live in.

Interestingly, multicellular Myxobacteria move as huge cell aggregates to attack their prey. These bacteria use their twitching pili to glide along a surface, attach to a prey and pull the whole aggregate towards the prey. Like this, the Myxobacteria quickly run over their prey so it does not stand a chance.

However, bacterial pathogens also use pili to infect us. The bacterium Neisseria gonorrhoeae can attach its pilus to human epithelial and endothelial cells. When the bacterium then retracts the pilus, it pulls itself closer to the cell and attaches to it more tightly. Now, it can infect the cell and eventually the host.

But not all is lost with bacteria and their pili. Currently, researchers are trying to better understand how bacteria use their pili and how this machine works mechanistically. They will then try to find drugs that inhibit the pili. This could be an alternative way to inhibit bacterial pathogens and maybe even drug-resistant bacteria.

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Bacterialworld - A blog about bacteria: from scientific studies to vivid stories about the fascinating bacterial world

Do bacteria always only live in one form; either they are single cells or multicellular?

How do we distinguish between unicellular and multicellular bacteria?

Which advantage do bacteria gain from sticking together and forming multicellular organisms?

What are some examples of multicellular bacteria?

In this article, we will answer exactly these questions!

Let’s dig in!

What makes multicellular bacteria?

Scientists define multicellularity as a form of “biological organisation in which a permanent cell aggregate exhibits an activity more complex than that of the individual cells“.

This means that multicellular bacteria are only present in their multicellular forms. True multicellular organisms cannot go back being single-celled.

Hence, bacterial biofilms are no true multicellular organisms. Bacteria can decide between these two lifestyles; they actively produce the biofilm when needed and they break it off and become single cells again.

Also, a bacterial colony in a petri dish is not a multicellular organism. In a colony, a bunch of bacterial cells grow on top of each other. But the cells in the colony are not organised and they do not function in an organised manner.

Multicellular bacteria are organised

The difference here lies in the term biological organisation. Multicellular bacteria are organised due to two different concepts:

They work in an organised manner; bacteria within the multicellular organism need to communicate with each other. Thanks to communication, every bacterium within the organism knows what is going on, so it can react in an organised manner.

Just as when your stomach is empty, it tells your brain that you’re hungry and you react accordingly by eating. Your stomach and your brain are communicating with each other.

The second way to organise multicellular bacteria is by using different functions to advance the whole organism. Within a multicellular organism, some bacteria undergo a process called cell differentiation. Cell differentiation is what makes a human stem cell develop into a skin cell or a blood cell. And this skin or blood cell has more specialised functions than the stem cell that it was before.

The same can happen in multicellular bacteria. Some bacteria develop into specialised cells. These specialised bacterial cells have functions that other cells (or the single version of the bacteria) do not have.

Now, some of the bacteria have additional functions or abilities. And thus, the whole multicellular organism gains new bacterial superpowers that can advance the organism.

Why do bacteria form multicellular organisms?

Here, evolution plays a massive role since multicellularity has so many advantages.

In multicellular organisms, the labour is divided. Just as it is easier for you and your co-workers to work in a team with everyone doing what they are best at. With bacterial cells taking on new functions through cell differentiation, the whole organism profits.

Another advantage is that when bacteria cluster together, they can protect their core. And some multicellular bacteria keep their spores within the core for protection. Like this, their most vulnerable members are protected.

Also, multicellular bacteria are generally bigger than single bacterial cells. This makes it more difficult for attackers to prey on this organism. And we know how much bacterial warfare is going on in the microbial world.

What are some cool examples of multicellular bacteria?

Researchers have not found that many yet. But those multicellular bacteria, that they started to investigate, are pretty cool.

Well, that’s what I think, but see for yourselves.

Multicellularity in chains: filamentous cyanobacteria

Filamentous cyanobacteria are Earth’s oldest multicellular organisms. And thanks to them, we have all this precious oxygen on our planet.

Some cyanobacteria form long chains, so-called filaments. In such an organisation, the whole chain of cyanobacteria is surrounded by one common outer membrane. This means, that all cyanobacteria cells within the filament share one periplasm. And they use this periplasm to communicate with each other and exchange nutrients.

Also, filamentous cyanobacteria like the Anabaena species can undergo cell differentiation. In the picture above, you can see a chain of Anabaena cells. Some cells are smaller, which are the undifferentiated cells, and some are bigger blobs.

The normal-sized cells have photosystems and they perform photosynthesis to produce oxygen.

But when cyanobacteria do not have enough nitrogen, they start to differentiate into those bigger cells, so-called heterocysts. And these heterocysts are now able to fix nitrogen. This helps the organism with its nitrogen limitation.

The reason why Anabaena needs these two cell types is because the chemical processes of oxygen production and nitrogen fixation interfere with each other. They can not happen within one cell, which is why cyanobacteria need to have a different cell type for each process.

In the end, the cells share the produced oxygen and the fixed nitrogen with the whole filament. So everyone is happy with this arrangement.

Multicellular bacteria as electricity producers: cable bacteria

Cable bacteria form – similarly to cyanobacteria – long filaments that are surrounded by one common outer membrane. And they use this arrangement to transport electrons and conduct electricity.

We talked about multicellular cable bacteria in detail in the article Cable bacteria – unusual microbes conducting electricity. Head there to read about this special kind of multicellular bacteria.

Multicellular organisms in cell aggregates: Myxobacteria

Some bacteria, like the well-characterised Myxobacteria, can form huge cell aggregates of up to 100’000 cells. These cell aggregates are called fruiting bodies and their main function is to feed and transport their spores.

The spores have a special place within the Myxococcus fruiting body: They are kept at the core of the fruiting body. Here, they are safe and protected from the surrounding.

Interestingly, myxobacteria are also known as wolf-pack predators, because of the way they attack their preys. They kill their preys by launching a massive attack and secreting lethal bacterial toxins. This kills the prey instantly and the whole fruiting body can feed on the prey.

Multicellular organisms forming hyphae networks: Streptomyces bacteria

Streptomyces bacteria develop a complex network of hyphae within the soil. With this network, Streptomyces bacteria can branch into different directions and elongate the branch tips.

Within the branches, some hyphae within the soil have secluded compartments with walls to separate them from the rest of the network. Yet, Streptomyces uses the hyphae to transport nutrients and chemicals and to communicate.

But when nutrients are missing, the branches grow out of the soil and into the air. Here, they form spores and produce geosmin and antibiotics. This geosmin attracts insects that distribute the spores in the environment.

Plus, by producing antibiotics, Streptomyces tries to kill those microbes that want to eat the spores.

The superhero of multicellularity: Magnetotactic multicellular prokaryotes

Ever since I heard about these bacteria, they became my favourites. And not only because these multicellular bacteria cannot even survive as single cells.

All cells within the magnetic berry are connected to a common core. On the outside of the berry, bacteria have flagella.

And because many of these bacteria assemble together and each one has several flagella, the whole berry is basically covered in bacterial flagella. When all of these flagella start rotating together, the whole berry becomes incredibly fast.

The second feature is, that these magnetotactic bacteria sense the Earth’s magnetic field lines thanks to their magnetosomes. Hence, this magnetotactic superorganism is even more sensitive to the Earth’s magnetic field, which gives it probably even more superpowers.

Lastly, the multicellular magnetotactic bacteria respond to blue light and swim away from it. This is a completely new bacterial ability and researchers are still not sure why these bacteria do that.

Unfortunately, we do not know much about these fascinating organisms, because they are incredibly difficult to grow in the lab. Until now, researchers could only image these bacteria from environmental samples as they still do not know what these bacteria need to survive in the lab.

Multicellular bacteria – an advanced lifestyle

As we have seen in this article, bacteria can grow either as single cells or as multicellular organisms.

By teaming up with their sibling cells, multicellular bacteria gain new superpowers, they can spread out and protect their weakest team members.

From an evolutionary point of view, forming multicellular organisms was a super important step. Only thanks to this, highly-developed animals with all their different cells and organs could develop.

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Bacterialworld - A blog about bacteria: from scientific studies to vivid stories about the fascinating bacterial world

Geosmin is one of these bacterial products that easily vaporise into the air and often have distinct smells. They are called volatile compounds.

Some other volatile compounds make the taste of chocolate or the smell of Cheddar cheese. And others, like geosmin, the smell of rain.

Bacteria produce geosmin – a volatile organic compound

Geosmin has this specific earthy or musty odour. It is the natural smell after a refreshing summer rain; the earthy smell of beets or carrots but also the off-tastes in water or wine.

You probably know what I am talking about.

Geosmin itself is not toxic, however, toxic fungi and bacteria produce geosmin. To our brain, the smell of geosmin signals that toxic fungi or bacteria are growing. So, our brain protects us from those by telling us not to eat the food. Thanks, brain!

Animals can also “smell” geosmin. For example, when the vinegar fly senses geosmin, it understands that toxic fungi are growing. In this situation, geosmin also acts as a repellent and the fly chooses not to eat or live around that place.

On the contrary, some mosquito species are attracted to the smell of geosmin. For them, the geosmin smell means that they are close to a lake, so the mosquito chooses to lay its eggs in the vicinity. Later, the mosquito larvae grow in that lake where they already have a food source. They will eat the bacteria. So, to mosquitoes, the geosmin smell is a sign of food.

Streptomyces – a geosmin producer

There are a few families of bacteria that produce geosmin. One of them is bacteria from the Streptomyces family. And Streptomyces already has a pretty interesting lifestyle.

Streptomyces not only grows as a bacterial cell, but it also produces very long and thin arms that grow out of the bacterial cell; so-called mycelia. The mycelia from one bacterium form a connected network with mycelia from other bacteria. And this complex mycelia network can extend into the soil, spread around soil particles and even wrap around tiny organisms.

After forming an interconnected mycelial network, the Streptomyces bacteria produce spores. And interestingly, Streptomyces produces these spores at the end of the mycelia arms.

Bacterial spores are non-viable versions of bacterial cells, just like a plant seed is a non-viable version of a plant. Spores generally only contain the genomic DNA of the bacterium, proteins to stabilise the DNA and proteins to react to the environment.

Similar to plant seeds, spores are wrapped in a thick envelope to protect the spore from the surrounding. Then, when the environmental conditions are better, the spore – just like the plant seed – germinates and forms a viable bacterial (plant) cell. This cell can then grow and metabolise and form new mycelia.

Why do Streptomyces bacteria produce geosmin?

Interestingly, when Streptomyces starts forming spores, the bacteria also produce geosmin.

Research found that a little insect-like animal, the springtail, is actually attracted to geosmin-producing bacteria. These tiny invertebrates – also known as snow flies or Collembola – live in the soil. And here, they are especially attracted to Streptomyces spores.

Researchers saw that the springtail uses its tiny antennae to smell the geosmin. The insect then follows the geosmin smell and once it found the Streptomyces spores, it starts eating them.

What is the advantage of being eaten by springtails?

Yes, it seems that bacteria produce geosmin to attract animals and insects to be eaten by them. To understand the reason for that we have to know that the springtail is covered with a waxy outer layer. Spores, on the other hand, have a hydrophobic envelope that easily sticks to the waxy springtail. This makes the spores stick to the springtail.

And when the animal moves around in the environment, it carries the spores around. So, it seems that bacteria use the animal as transport vehicle.

Also, the researchers saw that the springtails absolutely love eating those Streptomyces spores. They even found that the spores are not digested by the springtails. Instead, the springtails had viable spores in their faeces from which the bacteria could grow again. This finding gave the researchers another clue that Streptomyces spores might use the animals for transport.

So, it seems that Streptomyces bacteria produce geosmin to attract insect-like animals to attach to them and use them to hitchhike to different places. In a new place, there might be more nutrients for the spores to germinate, form viable bacteria, grow and reproduce.

From this, the cycle starts over again, as the bacteria form mycelial networks again to produce spores. Then, the bacteria produce geosmin to attract insects to get carried somewhere else again.

Bacteria live with many more players in the environment

Yet, one thing to take into account at this point:

Both Streptomyces and springtails live in the soil and in this study, the researchers only looked at how these two species interact with each other.

In the soil, there are several other bacteria, fungi, viruses, insects or tiny animals. So far, we have no idea about how any of these other organisms could impact the interaction between Streptomyces and springtails. The researchers did this study in the controlled environment of the lab. But the whole game might completely change in the wild environment of the soil.

However, I still think this is a cool example of how bacteria trick animals for their own good.

Take away from this article:

• Bacteria produce volatile compounds like geosmin that animals can taste or smell and be attracted to or repelled from
• Bacteria specifically produce geosmin to attract springtails to eat the bacterial spores
• Springtails transport the bacterial spores to new places

The post Bacteria produce geosmin to trick bugs into hitchhiking appeared first on Bacterialworld.
Bacterialworld - A blog about bacteria: from scientific studies to vivid stories about the fascinating bacterial world

Yes, you read correctly.

So-called cable bacteria transport electrons from cell to cell over centimetre-long distances and thus conduct electricity.

Mind-blowing.

It all started when in 2001, researchers measured electric currents in aquatic sediments. Soon afterwards, they found that this current is due to specific bacteria that exchange electrons with the surrounding.

And they called these bacteria cable bacteria.

So, let’s look at these fascinating species in more detail.

What are cable bacteria?

Cable bacteria are multicellular bacterial organisms meaning many bacteria from the same family are living very closely together. They usually share the same food and nutrients and help each other out. This tightness makes the whole bacterial community stronger and often gives them new superpowers – like in this case, the multicellular cable bacteria can conduct electricity.

Until now, researchers do not know much about cable bacteria, as they usually live in marine, freshwater and salt-marsh sediments. Here, they often use unusual components like sulfur or sulfur complexes to gain energy and grow. Such conditions are pretty difficult to imitate. So currently, researchers are struggling to grow them in a lab.

However, recent studies managed to find out what cable bacteria look like and how cable bacteria conduct electricity.

What do cable bacteria look like? 

For this, researchers took water samples from different locations. They then visualised the cable bacteria with different spectroscopic techniques. With these machines, researchers can magnify the tiniest things and get high-resolution pictures of bacterial cells.

Within these pictures, they actually saw that one “cable bacterium” is a multi-cellular microorganism. In the pictures below, you can see long chains or cables. Interestingly, these are completely surrounded by isolating walls. And within these cables, you can see single compartments, which are the bacterial cells.

These cables or chains, researchers call filaments and they can get up to 7 cm long.

What are cable bacteria made of?

Researchers are still trying to classify the family of cable bacteria. However, they know so far that cable bacteria are rod-shaped Gram-negative bacteria. This means these bacteria have an inner and an outer membrane as a surrounding and protecting wall.

Generally, between the inner and outer membrane is a liquid space that is the periplasm. This space kind of works as a flexible and soft buffer zone between the two membranes.

However, in cable bacteria, one bacterial cell is surrounded by an inner membrane to protect its cellular content. Then these cells arrange into a chain and share one common liquid periplasm. Next, one common outer membrane surrounds and stabilizes this bacterial-cell-periplasm chain. So, one outer membrane isolates the whole bacterial chain from the outside.

The whole chain is further held together by so-called fibres. You can see these fibres as those long dark grey lines in the pictures above. These fibres go along the cells through the whole periplasm of the cable filament. Researchers think, that these fibres give the cable its structure, stabilise the cable and protect it from breaking. 

As you can see, this whole structure indeed looks like a cable and insulate the bacteria on the inside from the outside. That’s pretty much where these bacteria got their name from. 

How do cable bacteria transport electrons and conduct electricity?

And why would they even create an electric current?

The short answer is to survive. As always, all organisms just want and need to survive.

So, let’s look at this in more detail.

Cable bacteria live in water sediments. These waters are full of oxygen in the top layers close to the surface. And they are full of sulfur in the deeper layers.

And cable bacteria use this oxygen-sulfur gradient to conduct electricity.

Cable bacteria now actually sense the oxygen in the water and align perpendicular to the waterfront. This means one end of the cable is in the oxygen-rich area at the top and the other end in the sulfur-rich area towards the bottom. Similar to the model below.

Bacteria breathe sulfur and conduct electricity

Now the filament works as a half-cell.

The bacteria in the deeper layers “breathe” the sulfur. This produces electrons (e^–) and protons (H⁺) as an anodic half-reaction. These electrons then travel along the cable within the liquid periplasm to the upper part of the cable. Here, the bacteria consume these electrons to reduce oxygen in a cathodic half-reaction.

These oxidation and reduction processes are part of each cell’s metabolism. So, this is what keeps every cell alive.

However, usually, both reactions happen within the same cell so that one cell gains energy from both reactions at the same time. Interestingly, cable bacteria found a way to uncouple the oxidation and reduction reactions.

Hence, oxidation and reduction mechanisms happen in different cells – one reaction in cells at the bottom of the cable, the other reaction in cells at the top of the cable. Yet, they somehow learned to do both reactions within one system – the cable. And this is the mind-blowing part.

What does it mean for the environment?

This unusual metabolism of cable bacteria has some interesting effects on their surroundings. These reactions produce protons in the deeper water layer and reduce them in the surface layer. This leads to a drop in pH in the deeper layer and an increase in the pH near the surface. This can mineralise and demineralise metal complexes. 

Researchers suggest that these (de-)mineralisation processes also have an impact on the geochemistry within the local environment in the water. However, how exactly the metabolism of cable bacteria influences the environment and maybe other species is still not well understood.

Since researchers only recently discovered cable bacteria, it is understandable that there are still many questions unanswered. But I am convinced that we will hear many more interesting facts about them and who knows, maybe one day bacteria will fuel some kind of seawater-based batteries…

Takeaway from this week’s articles:

• cable bacteria are filaments of a multitude of bacterial cells
• they can produce and consume electrons and transport them from one end to the other
• this electric current shapes the local environment in water sediments

The post Cable bacteria – unusual bacteria conduct electricity appeared first on Bacterialworld.
Bacterialworld - A blog about bacteria: from scientific studies to vivid stories about the fascinating bacterial world