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Behind the print: Bacterial shapes

Behind the print: Bacterial shapes

Dominant. Evolutionarily successful. Wide-spread. Essential.

What organisms come to mind when thinking about these words? You might say sharks, or large looming trees, or human beings. Bacteria probably weren’t the first thing you thought of. But actually, none of the above would have originated without bacteria, nor managed to stay alive.

Bacteria were the first living organisms on Earth, ruling it for a billion years before single-celled eukaryotes evolved, and eventually multicellular life forms 2 billion years later.

We often think of bacteria as primitive and simple, but the fact that they are so ancient yet so successful would suggest they are the opposite. As the most abundant life form, they continue to thrive alongside more complex organisms living everywhere from inside animals and plants, to toxic waste, to arctic ice. You don’t have to look any further than your own body to find these crafty organisms – a ratio of 1.3:1 bacterial to human cells in the human body shows just how prevalent they are.

Despite making up about half of any human being’s total body mass, bacteria often have a bad rep. Many diseases and infections are caused by harmful bacteria, such as food poisoning by Salmonella, Campylobacter or E. coli, or lethal diseases like the Black Death with Yersinia pestis being the culprit. These bacteria have had a historically huge impact on humans and so take a lot of focus, but actually fewer than 1% of bacteria cause disease!

On the other hand, many bacteria are so useful that other organisms have coevolved with them into obligate symbiotic relationships. This dependence spans a wide range of lifestyles:
• the microbial gut flora in many large animals
• the digestive chamber filled with bacteria that help ruminants break down tough plant fibre
• growth-promoting bacteria in and on plants
• fluorescing bacteria in luminous animals like the bobtail squid
• essential amino acid-producing bacteria in the gut of aphids
• production of chemical defence compounds in insects
• even some tiny bacteria living within another bacterium that lives in a host animal, together providing their host with nutrients

Due to the small scale of bacterial sizes (the smallest being 200nm and the largest being 0.3mm), they are harder to observe, quantify and study than your average animal or plant study species. The vast majority can only be seen under a microscope, and so their shapes are important in classifying them. The bacterial shapes print illustrates an array of the standard bacterial shape groupings:
• Bacillus, an oblong oval often called a rod; Salmonella is a well-known bacterium with this shape. Interestingly, bacillus is the term used for this shape which is common across the microbial kingdom, but it is also the name of a specific genus of bacteria that among other things are characterised by their rod shape. Most bacteria with the bacillus shape appear as single rods, but they also exist in pairs (known as diplobacilli), chains (streptobacilli) and palisades.
• Coccus, the spherical bacteria. Bacteria of this shape are often the tiniest, and tend to aggregate in different patterns that can be used for further classifications. Diplococci occur in pairs, tetrads are groups of four cells, sarcina is 8 cells in a cube-like shape, streptococci form chains and staphylococci form grape-like clusters. Both harmful and useful bacteria can have the coccus shape.
• Spirillum has a corkscrew shape. These can range from a gentle curve to a tight spiral. Borrelia, which causes Lyme’s disease, is an example of a spirillum-shaped bacterium.
• Vibrio is a curved rod shape, differing from spirilla in that they have less than one complete twist. Vibrio cholera causes cholera in humans, and it has been discovered that this particular bacterium is able to morph into a spirillum shape to enter the mucus lining of the human gut.

The first person to realise that bacteria have different shapes was a Dutch microscope builder named Antony van Leeuwenhoek. He decided to have a look at the white material he flossed from between his teeth, and with a microscope he was amazed to find little moving organisms with different shapes. Since then, science has come a long way, and bacteria are classified mainly by DNA sequencing today, but the shapes still play a large role.

Not only is the shape of a bacterium important to the scientist studying it, the shape is also essential to the bacterium itself. Just like most other characteristics of a living organism, the bacterial shape morphology has an evolutionary history. The shape of a bacterium contributes to its ability to find and absorb nutrients, its movements and the escape from predators. The bacillus rod shape is now believed to be the ancestral form of bacteria, rather than the spherical shape of coccus, which turns out is an evolutionary endpoint rather than an origin.

In addition to the basic shapes, some bacteria flaunt exciting looks like star or rectangular shapes. Scientists have also realised that many bacteria tend to change things up and actually alter their shape! Legionella pneumophila has at least 8 different shapes through its development. This is an exciting idea for medicine; bacterial shape can actually influence pathogenicity, and by disrupting the shape we may find a range of new solutions and treatments.

There are many unknowns as to why bacteria are shaped the way they are, and it has only recently become a topic of scientific interest. Despite bacterial shapes being the result of many selective pressures, it is difficult to predict the shape based on for instance environment. A little more is known about the how of shapes, but there are many questions left to answer. For instance, Eubacteria create their shapes using peptidoglycan in the cell wall, but Archaea (that together with Eubacteria make up the microbial group commonly referred to as bacteria) do not contain peptidoglycans yet display a range of shapes very similar to Eubacteria!

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