You must know the Bacteria Guy.

He appears in those TV adverts for yogurt drinks which bang on about ‘friendly bacteria’. In case you have evaded this marketing masterpiece, the set-up is like this: attractive woman arrives at a dinner party and is disappointed to discover, sat next to her at table, a type specimen of geek. From his carefully combed hair through to his toothy grin and unfortunate sweater, he represents a part of our shared cultural iconography we immediately recognise as ‘nerd’, with a side helping of ‘jerk’.

In case we don’t get the message, we cut forward to a point between courses when his pretty neighbour asks him what he is into besides quantum physics. (Of course! He’s a scientist!) He goes on to say how ‘friendly bacteria’ in yogurt are really good for you. Finally we see his new friend telling her mates all about the friendly bacteria and so the news spreads. Amazing you think, public communication of science and selling yogurt at the same time. Genius.

In subsequent commercials Bacteria Guy introduces us to his friend Tarquin. ‘He’s big!’ BG says, followed by a pause to allow viewers to stitch up their sides after the burst of hilarity induced by this double entendre. Then BG reveals to us that this is bigness ‘in digestive health’. Tarquin turns out to look like a ferret, with the sort of sparse, wispy goatee which can transform a normal man into a gold-plated tosser. As he enters the room he lets loose a distinctly creepy smirk. This does not cause Bacteria Guy’s lady friends to run screaming. No, they seem unaccountably aroused.

I have to declare an interest here. As someone who is meant to know about microbiology, I keep getting asked about these friendly or ‘probiotic’ bacteria. I tend to smile politely and back off. This fascination with friendly bacteria amazes me though, because so many of these same people are normally sceptical about science. They are the sort who are worried about GM food, wonder about the link between the MMR vaccine and autism, or reckon that environmental scientists telling them to cut back on car use are in the pay of the Green Lobby (that well-known astonishingly wealthy group which influences politics using its aforementioned astonishing wealth – unlike, say, the petrochemical industry). However, put these sceptics in front of adverts claiming wondrous properties for dairy products and they think, ‘Hmmm, there must be something in that.’ So here is the situation, as I understand it.

Let’s start out with bacteria in our digestive system: it’s full of them. Our guts are populated with bacteria of many species (beyond the famous E. coli) including those which you find in these ‘probiotic’ yogurt drinks. These denizens in the dark do us several services. One of these can be summed up by the term ‘competitive exclusion’. The idea is that if our guts are full of the friendlies they can bar the way for less pleasant, and dangerous, invaders. Is this what people mean when they are talking about ‘digestive health’?

Well, if they do, then it’s not at all unreasonable. In fact, there are a whole bunch of studies which show that probiotics can prevent diarrhoea after antibiotic treatment (for example, de Roos and Katan, 2000). When we take antibiotics, we kill off a proportion of our gut flora and the resulting disruption is the reason why diarrhoea is a common side-effect of antibiotic treatment. Under these circumstances, ‘topping up your friendly bacteria’ seems a sensible idea and the science bears it out. Benefits have also been shown in children, especially in preventing rotavirus infection.

But what about normal healthy adults like Bacteria Guy and his friends? Presumably they already have plenty of bacteria in their guts and these must be pretty friendly or they would be on the loo all the time and blissfully unable to make annoying commercials. Well, there have been surprisingly few studies on this, but one recent offering (Pereg et al 2005) took army recruits and gave them a yogurt drink every morning. Only half of the recruits, however, were given a drink containing probiotic bacteria. The researchers then monitored the recruits to see if there was any evidence of protection from diarrhoea. The result? No significant effect. On this occasion the friendly bacteria were evidently in a bad mood.

Of course, this is only one definition of digestive health. It would be reasonable to include constipation and other relatively mild afflictions. What is the evidence on these? Well, frankly, confused. This is not surprising: clear biomedical markers for efficacy against these milder conditions are not easy to define, so it’s hard to draw objective conclusions from experiments involving subjective symptoms. Of course, there are several different bacteria which are alleged to have probiotic qualities, and some of them may be useful in certain contexts. The fact remains that we lack clear evidence of clinical efficacy in healthy adults, at the dosages of bacteria administered by these products. This is important; it is hard to know how many of the bacteria survive the acid bath of our stomachs. This means that it is difficult to be sure everyone receives the same dosage.

And this is one of the biggest problems with these products which, because they are classified as novel foods rather than drugs, are subject to different standards of proof. It is hard to see how any bacteria you swallow, even if they make their way through the stomach, would have any effect upon the resident flora which, after all, will be exercising their rights to competitive exclusion.

Of course there is another route to the place these bacteria live, one which would enable us to administer known amounts of bacteria directly to the site of action. But I don’t think yogurt enemas are going to take off somehow. Although it might explain Tarquin’s dodgy little smirk.

References and further reading online

de Roos and Kattan, "Effects of probiotic bacteria on diarrhea, lipid metabolism, and carcinogenesis: a review of papers published between 1988 and 1998". American Journal of Clinical Nutrition, 2000.

Pereg et al., "The effect of fermented yogurt on the prevention of diarrhea in a healthy adult population". American Journal of Infection Control, 2005.

de Roos and Kattan, "Towards evidence-based health claims for foods". Science, 2003.

BBC news story about calls for tougher controls on claims made about probiotics.