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Social bacteria – the good, the bad and the biofilms

13 Aug, 2013

Sometimes they swim alone, other times they like to stick together, Lynne Cairns explains the social life of bacteria. Her research, part of a Wellcome-funded PhD at the University of Dundee, focusses on the way bacteria switch between swimming alone or living in social communities called biofilms.Bacillcus subtillis complex colony

You may consider yourself human, but to a microbiologist you’re a microbial super-organism! In fact there are 10 times as many microbes living on – and in – the human body as there are human cells.

For the most part we live happily alongside these microorganisms; indeed they are essential for our health. We also use microbes in many different industrial processes, from food preparation, to antibiotic production. However, some microbes can cause infections and are associated with diseases such as meningitis, cystic fibrosis and pneumonia to name but a few. So how can these microscopic organisms impact our daily lives?

The answer lies (partly) in the lifestyle choices a microbe makes. As introverts, bacteria (one type of microbe) live alone swimming through liquid, be it your blood or a fast flowing river, powered by a powerful rotary motor called the flagellum. The flagellum is able to convert chemical energy into mechanical energy, propelling the bacterial cell forward. However, a life-changing decision awaits these bacteria. When the solitary microbes reach a surface they must decide between continuing to move alone or settling down. If they choose to stick to the surface, these microbes will become part of a huge, social group of bacterial cells encased in a sticky matrix of proteins, sugars and DNA that protects them. This community of bacteria is called a biofilm. Similar to ants living as part of a colony, bacteria in the biofilm are able to work together to perform tasks together that they could not accomplish alone.

You might not have come across the term “biofilm” before, but you’re sure to know about dental plaque that causes cavities and gum disease. Dental plaque is an example of a biofilm.

Many bacteria live within the confines of a biofilm as a means of protection from the external environment. This “power in numbers” approach means that anti-microbial drugs are often less able to penetrate the biofilm and kill cells residing within it. This makes biofilms a problem in clinical settings since they are harder to beat and so are associated with recurring infections.

Biofilms are not all bad news though. We can also harness the power of biofilms to protect plants from infections and to clean up environmental pollutants (for example, in sewage or in oil spills where bacteria can use toxic waste as a food source).  If we can understand more about how bacteria make this lifestyle switch, we might find new ways of promoting or preventing biofilm formation in different situations.

Bacillus subtillis - credit: Lynne Cairns

Microscopy images of Bacillus subtilis. Left hand side image is Bacillus subtilis cells. Right hand side shows the flagella stained with a fluorescent dye, shown in green.

Our lab has looked at how swimming cells sense a surface and make the decision to produce their sticky matrix and build such a community. We predicted that the first step in building a biofilm might be the physical restriction of the fast-turning rotary motor (flagellum) when the swimming cell reaches a surface. In order to test this, we used the bacterium Bacillus subtilis and engaged a protein that functions as a flagellar “clutch”.

Similar to the way the clutch in a car separates the engine from the wheels, the bacterial clutch is able to stop the movement of the cell by disconnecting the flagellum from its power source. Controlling the activation of this allowed us to stop the motor that powers the flagellum, preventing the bacteria from swimming.

Having mimicked the effect of bacteria meeting a surface, we tested how this impacted a specific signalling pathway, called DegS-DegU, which is known to play a role in many different bacterial processes, including biofilm formation. By analysing the expression of different genes, we were able to show that by stopping rotation of the flagellum, the DegS-DegU pathway was activated and expression of one of the essential building blocks of the biofilm increased.

Our experiments show that not only can bacteria use their flagella for motility, but they can also use them to sample and sense their external environments. By sensing when they are at a surface, bacteria can then switch on genes needed to build a biofilm.

By understanding how bacteria live together we can not only find new ways of preventing their accumulation and reducing their disease-causing potential, but also learn more about how we might be able to harness their power for our own benefit. We hope that our work will be an important step forward in this field.

Lynne Cairns is a student on the Wellcome Trust Molecular and Cellular Biology PhD programme in the Division of Molecular Microbiology, College of Life Sciences at the University of Dundee.

Find out more about this work: 
Belas, R. (2013). When the swimming gets tough, the tough form a biofilm. Molecular Microbiology. doi: 10.1111/mmi.12354
Cairns, L. S., Marlow, V. L., Bissett, E., Ostrowski, A. & Stanley-Wall, N. R. (2013). A mechanical signal transmitted by the flagellum controls signalling in Bacillus subtilis. Molecular Microbiology. doi: 10.1111/mmi.12342

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