VATIS UPDATE Article

A team from Chr. Hansen A/S, Denmark, has manipulated the metabolic properties of yogurt-producing bacteria to sweeten the yogurt naturally, while reducing sugar in the final product. Similar manipulations have also all but eliminated lactose, so that those with lactose intolerance can enjoy the yogurt. They have accomplished all of this using microbiological method that predate the era of genetic technologies. “The goal was to engineer the yogurt bacteria not to consume glucose, a fermentation product that is a particularly sweet form of sugar,” said Eric Johansen, at Chr. Hansen A/S.

In certain countries, including Denmark, yogurt is defined as containing live cultures of Streptococcus thermophilus and Lactobacillus delbrueckii subsp bulgaricus.

Normally, when grown in milk, the two bacterial species break down lactose, a disaccharide, into its monosaccharide components, glucose, and galactose. They consume the glucose and secrete the galactose. “We wanted to change them so that they would eat the galactose and spit out the glucose. That required a number of changes in metabolism,” said Johansen.

In the first step, the investigators grew S. thermophilus on a medium where galactose was the sole food source. Thus, individual bacteria had to consume galactose in order to grow. A few mutants were capable of doing so, and the investigators cultured these. The next steps were to modify the bacteria so that they would no longer consume glucose, and would no longer even transport glucose into the cell. To this end, Johansen’s team grew the bacteria in a medium containing a glucose analog called 2-deoxyglucose, which is toxic to cells. The few mutants that survived in this medium lacked the ability to metabolize glucose.