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19 March 2015

Researchers have a gut feeling – could chronic pain be caused by bacteria?

This month, researchers are gathering in Cambridge to try and work out why we hurt. Michael Brooks weighs up one suggestion.

By Michael Brooks

This month the Wellcome Trust is gathering medical researchers in Cambridge to find out why we hurt. The meeting will consider ways of understanding the continuous, debilitating disturbance known as chronic pain.

Pain is a fascinating phenomenon. In essence, it’s a signal sent to the brain in order to divert your attention, so that you modify your behaviour. The acute pain of a burn or scald, for instance, forces you to pull away instinctively from the heat – there is no thinking involved. But chronic pain, most often associated with disease, is a similar phenomenon. It can be utterly distracting and is often catastrophic for an afflicted individual’s productivity.

In 2012, researchers from Johns Hopkins University estimated the cost of chronic pain to the US economy as $635bn annually – almost as much as the combined economic cost of cancer, heart disease and diabetes. In Europe, estimates suggest that chronic pain conditions wipe 3 per cent off the continent’s gross domestic product.

One of the most interesting aspects of chronic pain is the role the bacteria in your gut seem to play in some pain signalling. Despite what you may sometimes feel, you are never alone, and your hundred trillion bacterial lodgers know how to make their presence felt. We are accumulating heaps of evidence suggesting that certain forms of pain are the result of signals emitted by the bacteria living in your gut.

In Cambridge, Emeran Mayer, a bacteria specialist at the University of California Los Angeles, is presenting results that suggest bacterial influence. Mayer is the first to point out that most of the research has been done on rats and has yet to be translated properly to human beings. How closely our physiological and biochemical make-up resembles that of rodents is not yet clear. But if rats do provide a good analogue for people, we could learn some very useful information.

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There is good evidence, for instance, of two-way signalling between the brain and the bacteria found in the gastrointestinal tract. The autonomic nervous system, which regulates the activity of the lungs, the heart and other organs – including the gut – sends out signals that are read by and affect microbes, altering their activities. The microbes talk back using a system called quorum sensing.

Quorum sensing is the emission and reception of chemical signals designed to tell other bacteria what is going on, and how many of them are around. But those same receptors can intercept the host body’s own signals – and respond with subterfuge signals that trick the host into altering its behaviour.

They might want you to stop bombarding them with fats, or foods that are making their environment too alkaline or too acidic for comfort. They might be crying out for more fibre: cutting carbohydrates, for instance, can starve a significant proportion of your gut bacteria of their primary food source.

However unsettling it might seem to be so strongly influenced by your internal bacteria, it is becoming clear that the alliance between you and your bacteria can become harmful if not given all it needs to thrive. Micro-organisms influence the development of many of the brain’s emotion-processing apparatus, its stress response and pain-processing pathways. Links have been suggested between our gut microbiome and Parkinson’s disease, autism, irritable bowel syndrome, anxiety and depression-related illnesses.

We don’t yet know anything for sure; nonetheless, there is good reason to think that, if you are suffering chronic pain, you might in fact be hearing the cries of a hundred trillion nagging bacteria – and their complaints are well worth your attention. 

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