Are gut bacteria a cause of depression?

Question

Media from Belgium put out a far reaching article a couple of days ago claiming

Depression does not start in the brain, but in the intestines

Source: "Depressie ontstaat niet in het brein, maar in de darmen": zo groot is de invloed van uw darmen op uw leven

The article where the claim is presented is an interview with Michael Mosley in response to him writing a book called Clever Gut Diet.

A lot of pages on the internet seem to make similar claims. Did some research myself, so might self answer at some point, but I wasn't able to find the original research which kicked of this idea, so I feel somewhat uncomfortable quoting (higher quality) popular science in an answer.

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Some excerpts from the interview to make it more accessible for non-Dutch speakers and help in researching the actual claim:

The brain in our head is one lump of brain cells. The brain in our intestines contains about as many brain cells as a cat's brain and lies like a thin layer along the entire length of the intestine. It consists of exactly the same brain cells as our other brain and produces the same neurotransmitters. It communicates with our brain via the nervus vagus, the nerve that connects the brain to the intestines and one of the most important nerves we have. It is also super fast, because our intestines and brains need to be able to communicate very quickly with each other. The bacteria in our intestines can hack that system and change the signals. That way they can give our main brain instructions.

[...]

A psychiatrist told me that people with depression often suffer from severe constipation. This has been known for a long time, but no one has ever come up with the idea to remedy the constipation and see if the depression then improved. And that turns out to be the case. This is not because people feel better because they are no longer constipated, but because the constipation has a direct effect on the brain. We have long thought that depression would originate in the brain, but there is increasing evidence that it happens in the intestines. You have small inflammations in your body all the time and the intestinal bacteria play an important role in this.

Answer 1

I can't find a scientific article with this claim or anything like it made explicit, but a quick search of the scientific literature turns up research that suggests this may be the case.

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Melancholic microbes: a link between gut microbiota and depression? investigates possible links between bacteria in the guts and depression. The title ends in a question mark, because the paper shows an interesting correlation, but stops short of proving causality.

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The interplay between the intestinal microbiota and the brain investigates the chemical and neural communication pathways between the gut bacteria and the brain. They "review how this extended communication system might influence a broad spectrum of diseases, including ... psychiatric disorders... [emphasis mine]" Once again, scientists are stopping short of making conclusive statements.

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A Possible Link between Food and Mood: Dietary Impact on Gut Microbiota and Behavior in BALB/c Mice, shows that a diet can affect mood in mice. This suggests that the same is true for people. They also showed that the different diets had an impact on the mice's gut bacteria.

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Correlation between the human fecal microbiota and depression found that people with depression are more likely to have certain types of bacteria in their guts.

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Gut brain axis: diet microbiota interactions and implications for modulation of anxiety and depression reviews the literature about the interaction between the guts and anxiety and depression. Their final section contains the following passage:

We now know that microbiota influence behaviour, in particular, stress-related behaviours such as anxiety and depression. Attention to the importance of microbiota and behaviour is rapidly expanding. Clinically, the link between obesity and anxiety is suggested [43], however studies have not yet considered the role of microbiota in this overlap. Understanding the link between mood and metabolism is a necessary direction for research studies to pursue.

This further suggests that we know there is some kind of link, but scientists are being cautious about making firm statements about what that link is in peer reviewed papers.

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Disclaimer I did not do a thorough literature search reading all of the latest literature; those can take hundreds of hours. It is possible that a paper I did not read has a definite answer to this question.

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Summary: I found a decent amount of research suggesting that this might be the case. This same research does not rule out other possibilities, depression may cause changes in the microbiome instead of the other way around. The review article seemed hopeful that future science will find a more conclusive answer.