Tiny they may be, but good bacteria are a mighty big business. Traditionally added to yogurt, these live micro-organisms known as probiotics have made their way into supplements, cheese, granola bars, ice cream, chocolate and even juice.
Probiotics are believed to be beneficial for digestive health and immune function. With increasing interest and research in the ingredient, more health claims have emerged in recent times: from helping pregnant women ward off obesity and maintaining psychological well-being, to treating a variety of conditions from bad cholesterol to the common cold.
Unfortunately, they're all just that - claims - owing to less-than-rigorous studies. "A lot of research has been done, a lot of work is being carried out, but officially none of the claims have been confirmed," says Nagendra Shah, a professor of food science and technology at Hong Kong University, who has been researching probiotics for the past 20 years.
Probiotics, Shah says, are ''live micro-organism s believed to actively enhance health by improving the balance of microﬂora in the gut".
It is the leading functional food ingredient worldwide, according to Ewa Hudson, global head of health and wellness research at research group Euromonitor International.
Global probiotics demand was worth US$27.9 billion in 2011, and is expected to reach US$44.9 billion in 2018, according to a report by market intelligence company Transparency Market Research.
The Asia-Pacific, led by China and Japan, is the market leader for probiotics and the most promising market with an expected year-on-year growth rate of 7 per cent from 2013 to 2018.
More probiotic products were launched in the Asia-Pacific than any other region between 2008 and 2012, according to Laura-Daisy Jones, a global food science analyst at market research firm Mintel.
But probiotics are not made equal - a fact that Shah says most consumers don't know. Probiotic health benefits are specific to the strain.
"It is important to note no strain will provide all proposed beneﬁts, not even strains of the same species, and not all strains of the same species will be effective against deﬁned health conditions," Shah says.
A number of genera of bacteria are used as probiotics, including Lactobacillus, Leuconostoc, Pediococcus, Biﬁdobacterium, and Enterococcus. Members of the genera Lactobacillus and Biﬁdobacterium have a long and safe history in the manufacture of dairy products and are also found as a part of gut microﬂora in humans.
The main species that are thought to have probiotic characteristics are L. acidophilus, Biﬁdobacterium spp., and L. casei, according to Shah. Through his research, he has found good evidence that probiotics may be effective against diarrhoea, help lower serum cholesterol, have anti-cancer effects to some extent, be effective against inflammatory bowel disease and help improve immune function.
But "there's a lot of exaggeration" when it comes to probiotic health claims, Shah says.
The US Food and Drug Administration has not approved any claims, and since December last year, all claims have been rejected in the European Union. In Hong Kong, the claims are not regulated, says Shah.
The bright side is that chances of side effects are slim. Shah says there is meagre evidence that probiotics may cause infection, especially in individuals whose immune system has been compromised.
Probiotic bacteria, he explains, first enter newborn babies as they pass through their mother's vaginas. The bacteria colonise the babies' guts and form a foundation for their developing microbiomes.
The microbiome is made up of trillions of bacteria, both good and bad, that live everywhere in and on the body, but the heart of activity is inside the gut. Probiotic bacteria perform many important tasks, such as producing vitamins, helping to absorb nutrients, training the immune system and warding off harmful pathogens.
Each person's microbiome is distinct and doesn't change significantly with age unless a person becomes ill, takes an antibiotic or makes major changes in diet. Disturbances in the microbiome have been linked with everything from asthma and allergies to obesity, celiac disease and diabetes.
"A healthy gut has a lot of probiotic bacteria," says Shah. "But when we take antibiotics for some other treatment, they kill all the good and bad bacteria. So, you have to replenish the good bacteria again."
There is no set dosage for probiotics because there are so many different strains. Typical recommendations range from one billion to 10 billion live organisms, or colony-forming, units several days per week.
Although there is reference to sour milk or fermented cultures as far back as the Bible, Russian immunologist Dr Elie Metchnikoff is regarded as the grandfather of probiotics.
In 1908 he made a landmark observation that the regular consumption of lactic acid bacteria in fermented dairy products, such as yogurt, was associated with enhanced health and longevity in Bulgarian peasant populations.
Apart from the launch of Yakult, the fermented milk drink with the bacterium strain Lactobacillus casei Shirota, in Japan in the 1930s, there were few studies on probiotics. It wasn't until 2001, when a consensus definition of probiotics was adopted by the United Nations and World Health Organisation, that research interest in the ingredient was sparked.
Now, probiotics research is on fire, with scientists jostling to find new applications.
A recent study published in Gastroenterology and sponsored by the research arm of dairy company Danone showed the first evidence that probiotics can affect human brain function.
The researchers, from the University of California Los Angeles, say there's the potential to find ways to manipulate the intestinal contents to treat chronic pain conditions or other brain-related diseases, including potentially Parkinson's, Alzheimer's and autism.
Another study, carried out on macaques, suggests probiotic supplementation could help enhance gastrointestinal immune function and decrease inflammation in intestinal walls in antiretroviral-treated HIV patients.
Last year, researchers from McGill University in Montreal found that two daily doses of a probiotic lowered key cholesterol-bearing molecules in the blood as well as "bad" and total cholesterol.
A study in 2009 found that one year after giving birth, women were less likely to become severely obese if they had been given probiotics from the first trimester of pregnancy.
There's certainly much potential for probiotics - if their benefits can be backed by well-defined research studies. Dr Gregor Reid, director of the Canadian R&D Centre for Probiotics proposed, in Nature last year, a system in the US and Europe in which probiotics must all pass a set of minimum requirements before they can be called a probiotic.
But even if health claims are not approved, Shah says probiotics will still keep your gut healthy. "A cup of yogurt has got all the goodies - minerals, calcium, nutrients - and some good bacteria," he says. "You can't deny that fact."