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Could Probiotic Yoghurts Cure Depression?

Evidence from recent medical research studies suggests an answer of yes, quite possibly. It therefore begs the question; can we maintain a healthy life without microbes?

We spend most of our time attempting to destroy all forms of bacteria, fungi, and viruses, in fear that they will make us sick. Our handwashing is extreme, our clothes cleaning processes blast fabrics with detergent and antiseptic rinses, our lives smell more of artificial products than of human beings. Certainly, there is a place for such zeal, hospitals, and communal spaces being but a couple, but are we lowering our resistance to the more detrimental strains of microbe in our efforts to maintain a sterile world?

Without exposure to harmful lifeforms, our immune systems cannot produce antibodies against a more virulent attack. Some strains of positive microbes actually battle against those dangerous ones in a constant push for more space inside us. This symbiotic relationship is not unique in nature. Many species coexist and thrive as a result of joining forces. Trees will often share nutrients through roots and rhizome networks underground, keeping stumps and sickly vegetation alive. The Portuguese man of war jellyfish is actually a collection of specialised creatures, each with specific roles, from feeding fronds to stinging cells to reproductive organs.

The bad guys…

On the other hand, microbial parasitism is what drives our fascination with cleanliness. Take parasitic infection in the natural world. From microscopic viruses to flatworms and wasps, there are so many incidents of species taking command of biological and neurological components of other animals or insects, that it’s hard to keep track. Even our cute little ladybirds can fall prey to certain wasp larvae.

Some possessions use the host merely as a food source or a place to incubate until the next stage of their life cycle, but the truly scary ones are those that can alter the behaviour of the carrying creature.

Take, for example, the jewel wasp, Ampulex Compressa. It lays an egg on the underside of a cockroach. When the larvae emerge, they burrow into the flesh of the roach’s abdomen before guiding its stinger up into the brain until it locates the precise region responsible for movement. At this point, it releases a cocktail of chemical neurotransmitters which have a psychotropic effect.

Frederic Libersat of Ben Gurion University and his colleagues working on studies involving jewel wasps believe that these drugs dampen the cockroach’s response to danger. By doing so, it encourages them to stand out in the open increasing the chances of being eaten by larger prey. Not that anyone would pity the cockroach’s demise.

There are other similar neurological parasites, including baculovirus which infects caterpillars, parasitic fungi which enslave ants, flatworms directing killifish to swim to the surface so that they are eaten by birds, and many more besides.

Of course, these are examples of negative relationships with microbes, but for every bad germ, there are as many good ones. Without yeast, our bread would be flat, our alcohol distilled and never brewed, and we’d have no Marmite for our toast in the morning.

The good guys…

Symbiosis is a critical part of our existence, and scientists are only now catching up with how microbes play a delicate and personal role in the human condition, some 350 years after Leeuwenhoek first discovered them.

There is plenty of evidence to support the theory that the crucial power generators in our cells, Mitochrondia, evolved from certain bacteria millions of years ago. Without these cell components, we would not have the chemical breakdown of glucose to form mini bursts of energy. These microscopic creatures enabled us to function as more than a few cells in the earliest stages of life.

Without gut bacteria, a large proportion of our food would go undigested. As a consequence, we would not have access to certain vitamins nor utilise the stored energy from food efficiently. These friendly bacteria crowd out pathogens in the alimentary canal, but also in other areas of our bodies too. They boost our immune system by keeping the number of pathogens low.

The gut-brain axis.

Since 2004, scientists have turned their attention to the links between gut health and neurological symptoms. Nobuyuki Sudo, at Kyushu University in Japan, found that mice lacking microbes showed abnormal responses to stress. To assure sterile conditions, these test mice were born through antiseptic caesarean section and raised in a specialised cleanroom. Under normal conditions, mice are coated in microbes, particularly in their guts. When the clean mice were fed as few as twelve bacteria, they exhibited stressful symptoms, while the control group showed no change at all.

Further tests revealed that the clean mice had less developed brains, fussed over inanimate objects and were agitated in a stressed way. Sudo and his team went on to feed a batch of clean, stressed mice a mixture of gut-positive bacteria, and within days their stress behaviour returned to normal parameters.

Sudos experimental results sparked new and more complex studies across the world. A team at the University of California published a study in The Journal of Psychosomatic Medicine, based on analysis of gut microbiomes from 40 different women. These ladies provided samples for microbe analysis immediately before viewing emotive images while undergoing an MRI scan.

The team found two distinct groups of bacterium present which correlated with the patterns of brain activity in the woman. The first was a group of bacteria called Prevotella. They occurred in 7 of the subjects. Those women showed greater connectivity within the sensory, attention and emotional response sectors of the brain. Their hippocampi were smaller than average and less active. This is the region related to emotional regulation, consciousness and the formation of long-term memories. When shown emotive pictures, they seemed to experience a disproportionate degree of distress and anxiety from the negative imagery.

The second group was the Bacterioids. These were common in the remaining 33 subjects of the study. Their active brain regions under the same conditions were related to problem-solving and information processing. Their hippocampi were much larger and highly active during exposure to negative images. Consequently, they were less likely to experience such deep distress and emotions.

A study conducted at the University College of Cork, Ireland, by Cryan and Dinan, seems to confirm this gut-brain linkage. They performed faecal transplants from a person with acute depression into mice. The animals went on to develop depressive patterns of behaviour soon after.

Together with their colleague, Catherine Stanton, Cryan and Dinan proposed the notion of Psychobiotics. The problem was that no one could pinpoint how the linkage worked. Despite the fact that microbes can produce almost every human neurotransmitter, they could not see a way for those chemicals to transfer from the microbes across the blood-brain barrier into the brain.

In 2017, Dinan and Cryan discovered a specialist cell in the gut lining called an enterochromaffin, which can detect neurotransmitters from microbes. When triggered, it sends a pulse up to the brain via the vagus nerve, mirroring the signals there. This supposition is supported by the fact that when the vagus nerve is severed in mice, no more psychobiotic events took place.

Although the team claims that they can extrapolate their findings to explain poorer brain development in foetuses, it is harder to prove. The complex wiring and plasticity of an ever-changing mass of interconnected neurons may, or may not be affected by the psychobiotic influences of bacteria present during gestation. It is likely that once born, however, those stimuli will inevitably impact the child.

The exciting part of this will follow from this point. It is known for a fact that depression can cause your amygdala to swell and become more active. This area is responsible for the fight or flight response. If this too is affected by gut fauna, then a potential cure may involve fewer drugs and a diet which increases the optimum environment for a helpful symbiotic relationship with bacteria. This would be particularly beneficial to pregnant women, since up to 15% experience depression during or after childbirth, where traditional drug therapy could jeopardise the foetal health.

Thus far, most studies rely on a subjective measure of mental and emotional health, but initial findings are promising. While chugging a couple of probiotic yoghurts could boost your wellbeing for a time, the best method is to increase the variety of friendly bacteria by eating a large range of vegetable matter. Having five portions of fruit and veg per day is only beneficial if they are varied. Eating five of the same per day is not the same as having 30 different types per week. The larger the selection, the greater the likelihood of ingesting a massive range of health-promoting microbes.

As for them taking over our bodies and turning us into their zombie-like flesh machines, I think they have more to offer us than we do for them. If bacterium can breakdown glucose to power virtually every cell in our bodies, then imagine what another few thousand years of evolution could do. We could become the biological sum of all our symbiotic partners. Cue the Star Trek references…

Sam Nash is the author of mind-altering fiction. Check out her latest news at or follow her on Facebook at

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