The study, carried out at the University of Ghent in Belgium, found that in the nine products tested, only three contained the bacteria indicated on the label and five contained sufficient bacteria for a probiotic effect.
"This is of concern as clinical efficacy is dependent on strain specificity and organism numbers," write the researchers.
Probiotics are thought to protect the immune system and benefit digestive health. They have also been researched for potential anti-carcinogenic properties and the management of inflammatory bowel disease and urogenital infections.
Specific strains are well documented for their successful treatment of infectious diarrhoea and antibiotic associated conditions.
The three products that met label claims included two BioPro products - the Reuteri straws and chew-tablets containing Lactobacillus reuteri - and Infantiforte, which contains Bifidobacterium infantis.
However the research also identified the potential pathogen Enterococcus faecium and Saccharomyces cerevisiae in two of the products, adding to concerns over mislabelling.
"Current regulatory requirements do not address this discrepancy. As such, we recommend that commercially available probiotic products be screened annually, and the results of such quality control measures be made available to the Medicines Control Council (MCC)," write the authors.
The study authors pointed out that five new products have been launched onto the South African market in the last two years as the ingredient becomes increasingly popular. The probiotic industry in South Africa is estimated to be worth R45 million (€5.4m) per year and over 11 million doses of probiotics are taken annually.
Until recently, probiotic products were registered as food supplements with the Department of Health. But the Medicines Control Council (MCC) has recently set up a Complementary Medicines Committee charged with overseeing clinical evidence to support products claiming to have medical benefit.
"Unfortunately, assessment of these products is limited by the lack of independent technical expertise available in South Africa and the expense of setting up the infrastructure to do such testing," Dr Gene Elliott, a South African medical microbiologist, and author of the article, told a HealthDayNews report.
He claims that manufacturers' claims are therefore difficult to validate and the new regulatory body has no capacity to carry out post marketing testing.
"Standardising such evaluation with a validated method would provide a means to assess and compare products, confirm their contents and monitor the effect of storage on their shelf life," he said.
Evaluations of products in the US and Europe have also shown poor correlation between label claims and actual contents.
Despite this, the total European probiotics market comprising the four main application areas - dairy, animal feed, supplements and infant nutrition - was estimated to be worth $40.3 million (€34.6m) in 2003, with the US market valued at $143.9 million. The ingredient will more than triple in value by 2010, forecast analysts Frost & Sullivan.