While the study, carried out at the University of Reading, was not designed to look at the benefit of probiotic foods to health, the efficacy of probiotic bacteria is known to depend on their survival through both the acidic conditions of the stomach and the digestive tract.
The new results suggests that, in general, most well known products on the market matched their claims in content and quality but it underlines the vulnerable stability of probiotic bacteria and the continual need for work to improve their resistance to digestive conditions.
The researchers tested 35 strains of bacteria found in 11 different products on the UK market, including juices, fermented drinks like Yakult and dry preparations in capsules and tablets.
The strains included 19 isolates of Lactobacillus spp, five isolates of Bifidobacterium spp, one isolate of Lactococcus lactis sp and five isolates of Enterococcus sp.
They used laboratory models of the human gut to imitate the conditions of the stomach, upper intestine and lower intestine. The bacteria tested were grown and their numbers standardised before each experiment began.
Overall L.plantarum, found in Skane dairy's Proviva fruit drink, showed the best ability to colonise the three compartments of the lower digestive tract, said the researchers. This was followed by L. pentosus, L. casei shirota - the Yakult bacteria - and L. reuterii.
"Generally strains coming from human sources such as L plantarum and L reuterii were more able to survive in the lower digestive tract than dairy strains," they write.
The study also suggests that Lactobacillus spp were resistant to the gastric environment but were sensitive to upper intestinal content. Conversely Bifidobacterium spp were more likely to be affected by stomach digesta but survived well in the upper intestine compartment.
"This research is by no means conclusive but can be seen as a preliminarystep and adding to the body of evidence surrounding probiotics," said a spokesperson from FSA.
"It is fair to say that the large intestine is considered the most common target site for probiotic intake, however there are reports of probiotics having an effect in other parts of the gut as well," he added.
The researchers noted that studying strains individually in standardised conditions may not have been optimum for their growth.
Also interaction of strains when combined in one product may enhance survival and the biological effect, and this was not accounted for in the study.
However the survival of probiotic strains through the digestive tract does not necessarily lead to a beneficial response from the host, warned the authors.
"A thorough in vivo investigation of the probiotic strains included in this study as well as more information on the impact of probiotics on the immune system would be requirede to achieve a comprehensive overview of the efficacy of probiotics in humans," concluded the researchers.
The study is published on the Food Standards Agency website.