Probiotics - beneficial bacteria that are naturally found in the human gut - have become a household term as more and more food and beverage products containing them appear on the global market. The success of the category is largely put down to the marketing efforts of Yakult, which is considered to be the world's original probiotic company, and is today worth $2.3bn. Its namesake beverage is currently sold in 31 countries and regions around the world, and the firm claims its product is consumed by 25 million people every day. This year marks its launch in the US, where it is currently sold only in California. Science According to Lauren Weidelman, the firm's corporate communications manager, a major key to Yakult's success is the extensive science behind its probiotic strain. Lactobacillus casei Shirota, which is exclusive to Yakult, was cultured in 1930 by Japanese microbiologist Dr Minoru Shirota. In 1935, Dr Shirota incorporated the strain - which was at the time touted as being "stronger than most" - into a yogurt beverage: Yakult. The company says its product contains "one of the most studied probiotic strains" which help stimulate the immune system. A study published in 1994 (Tanaka et al.) found that ingestion of Lactobacillus casei Shirota increases beneficial bacteria (bifidobacteria) and decreases harmful bacteria (enterobacteriaceae). Another study (Koebnick et al., 2003) found that Yakult improves stool consistency and constipation. However, despite the scientific backing, Yakult is holding back on making health claims for its product. These are "on hold" until clinical result trials are published, said Dr Rob Unal, Yakult USA's science manager. Bacteria in the billions Another major reason for the success of the product is the high level of bacteria found in each serving, according to Weidelman. In order for probiotic bacteria to have beneficial effects, they need to be alive and ingested in high quantities, generally thought to be between five and ten billion bacteria daily. "A lot of the probiotic products on the market don't have any live bacteria in them," said Dr Unal. Yakult is said to contain more than eight billion probiotic bacteria in each serving. In addition, studies (Kobayashi et al., 1974; Tanaka et al., 1994) have shown that Lactobacillus casei Shirota reach the intestine alive. According to Weidelman, when food and drink manufacturers are looking to use probiotics in their products, the two key questions they need to ask are 'what kind?' and 'how many?' "Look for the genus (e.g. Lactobacillus casei) and strain (e.g. Shirota) name, and look for the number of probiotic strains per serving," she said. Price Another key selling point for Yakult is that it is at a price "that everyone can afford", said Weidelman. The suggested retail price in the US for a pack of five 2.7 ounce servings is $2.99. Probiotic market The probiotics market in the US has grown exponentially in the past few years and is predicted to have significant room to grow for those companies that can effectively communicate the benefits of the ingredient to consumers. According to Datamonitor, there were only 40 probiotic and prebiotic products on the US market in 2005. In the last two years, more than 250 new products were launched. A report published last year by Frost & Sullivan valued the North American probiotics market at $698m in 2006, and said it is expected to reach $1.7bn by 2013, at a compound annual growth rate (CAGR) of 13.7 percent. The fastest growing sector is probiotic beverages, with a CAGR of 24.6 percent. Lauren Weidelman and Dr Rob Unal were addressing attendees at the Expo West trade show in Anaheim, California earlier this month.