The team from a Danish food ingredients company 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 accomplished all of this by using microbiological methods 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 corresponding author Eric Johansen.
Normally, when grown in milk, the two bacterial species – Streptococcus thermophilus and Lactobacillus delbrueckii subsp bulgaricus break down lactose, a disaccharide, into its monosaccharide components, glucose, and galactose. They consume the glucose and secrete the galactose.
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. A second round of selection, with higher levels of 2-deoxyglucose, resulted in survival of mutants lacking the glucose transport mechanism.
Johansen et al. also used 2-deoxyglucose to isolate mutants of Lactobacillus bulgaricus, to select for mutants that were unable to transport glucose into the cell. This prevented them from consuming the glucose produced by S. thermophilus.
Now they made yogurt with the modified bacteria. The results: they were able to reduce added sucrose by 20 percent while maintaining the desired sweetness.
The research appears in journal Applied and Environmental Microbiology.