Think of bacteria and you may think of unhealthy connotations, like a nasty infection, but they can actually be quite useful for our health

It’s a staggering and, at first sight, rather unpleasant fact: one hundred trillion bacteria – that’s 300 times the number of stars in the Milky Way – call your body home. Some are potentially deadly.

For example, eight per cent of 78 GPs tested in Ireland carried the superbug methicillinresistant Staphylococcus aureus (MRSA) in their noses. And about one in 20 people harbour another superbug – Clostridium difficile (C. difficile) – in their guts. This ‘colonisation’ does not cause health problems, but could, potentially, infect vulnerable people.

On the whole, however, we live in harmony with our bacterial lodgers – technically called the ‘microbiome’ or ‘microbiota’. The bacteria get somewhere to live and a steady food supply. In return, they break down foods that we cannot digest, boosting the energy released from our diets by about 10 per cent.

Our friendly’ bacteria also make vitamin K (essential for normal blood clotting), shape our immune responses, prevent more dangerous bacteria from moving in, reduce the risk of colorectal cancer, and even help us keep trim. In other words, the bacteria in your gut are almost an extra organ – and prebiotic and probiotic foods and supplements give this ‘organ’ a boost.

Diversity and disease

It’s difficult to grasp the huge variety of bacteria that we carry around. There are more than 30,000 different strains of bacteria in the lower gastrointestinal tract alone. In comparison, an acre of undisturbed tropical rainforest might contain 15,000 species. Our mouths contain between 500 and 700 bacterial species, while a 10-second kiss transmits up to 80 million bacteria.

Nevertheless, people in industrialised nations have a more restricted microbiota than those from less developed nations, partly because we eat more meat (we need more complex bacterial communities to break down plants), spend more time indoors, use more antibacterial soaps and cleaners and take more antibiotics. Research links poor microbial diversity in our guts to at least 25 diseases, including asthma, psoriasis, cardiovascular disease and obesity.

For example, researchers looked at gut bacteria from 123 non-obese and 169 obese Danish people. The 23 per cent who had limited ‘bacterial richness’ were more likely to be overweight, show insulin resistance, have dangerous levels of fats in their blood and gained more weight over time. Changes in the microbiota during stress or a viral infection might trigger type 2 diabetes in people with insulin resistance.

Only a few species of bacteria seem to account for the difference between obese and non-obese people. Indeed, while some obese people host a diverse microbiota, it is the specific bacteria that matter.People with greater numbers of a major group of bacteria called Bacteroidetes tend to be leaner. Those with higher levels of another group – Firmicutes – tend to be more obese. Meanwhile, Clostridia, another common group of gut bacteria, seem to induce immune responses that stop food allergens (allergic triggers) from entering the blood, suggests a new study.

“Environmental stimuli such as antibiotic overuse, high-fat diets, caesarean birth, removal of common pathogens and even formula feeding have affected the microbiota with which we’ve co-evolved,” said Cathryn Nagler, Bunning Food Allergy Professor at the University of Chicago. “Our results suggest this could contribute to the increasing susceptibility to food allergies. We’ve identified a bacterial population that protects against food allergen sensitisation.” Nagler’s team is now developing a probiotic for food allergies. The growing appreciation of the microbiota’s importance for health and wellbeing has prompted the development of numerous probiotics and prebiotics.

The British Dietetic Association (BDA) notes that probiotics are generally safe for healthy people of all ages. However, people with immune problems should see a doctor or dietitian before taking probiotics. It’s important to use the right probiotic. Broadly speaking, the body is host to five distinct bacterial communities: skin, nose, mouth, lower gastrointestinal tract, and vagina.

Each of these communities has several bacterial ‘neighbourhoods’: microbes on the cheek lining differ markedly from those in dental plaque, for example. So, a probiotic for the gut may not help bolster the health of the vagina. Even two bacteria from the same community can have very different biological actions.

Probiotic benefits

Antibiotics kill friendly as well as harmful bacteria. This collateral damage makes it easier for pathogens (i.e. anything that can cause a disease) – including those resistant to antibiotics – to become more prominent. For example, in healthy people, beneficial bacteria control the numbers of C. difficile. However, C. difficile levels can increase rapidly in people taking antibiotics, causing severe diarrhoea, bleeding and even death.

Probiotics reduce the risk of antibiotic-associated diarrhoea (AAD). In one study, for example, probiotic containing Lactobacillus acidophilus and Lactobacillus casei reduced the number of people who developed AAD from 44.1 per cent with placebo to 15.5 per cent. Among those who developed AAD, symptoms lasted, on average, 2.8 days with probiotics compared to 6.4 days with placebo.

The combination of probiotics also reduced the risk of AAD associated with C. difficile from 23.8 per cent to 1.2 per cent. According to the BDA, patients should start probiotics as soon as they begin antibiotics and continue for at least a week after the end of the course. Probiotics also prevent about one in seven cases of travellers’ diarrhoea, according to a review of 12 studies.

The BDA suggests that taking Saccharomyces boulardii or a mixture of L. acidophilus and Bifidobacterium bifidum while away might help stop an upset stomach from spoiling the holiday. The microbiota could even offer a rich source of new antibacterials to tackle resistance. Lactobacilli help keep healthy vaginas slightly acidic, which defends against harmful bacteria, and some produce potent antibiotics.

Researchers at the University of California, San Francisco (UCSF) recently purified a new type of antibiotic, lactocillin, from a common member (Lactobacillus gasseri) of the vaginal microbiota. Lactocillin is active against a range of disease-causing vaginal bacteria, but spares friendly species in the vagina.

Prebiotics

Stomach acid can kill some probiotics. So, prebiotics could be an additional option – these are carbohydrates that only gut bacteria feed on. The BDA says that dietary sources of prebiotics include onions, garlic, asparagus, artichoke, chicory and banana. Some people may wish to take a prebiotic supplement such as fructooligosaccharides. Furthermore, ‘friendly’ gut bacteria feed on soluble fibre. So, a diet high in fibre and whole grains helps boosts levels of friendly bacteria.

Further reading

• The BDA produces an excellent leaflet
• The Superorganism Revolution by Robert Dorit is published in American Scientist: