Yogurt is milk that has been fermented using two lactic acid bacteria: Lactobacillus delbrueckii subsp bulgaricus (often known as Lactobacillus bulgaricus) and Streptococcus thermophilus. We know that because of the lactic acid-production of the bacteria it contains, yogurt has a positive effect on lactose digestion for people with poor tolerance to lactose.
The author of this article discusses numerous studies and puts forward another possible benefit of yogurt consumption. Yogurt may have a positive effect on the immune system. An in-vitro study followed by an in-vivo study of mice has shown, for example, that the Lactobacillus bulgaricus OLL1073R-1 strain produces an immunomodulating extra-cellular polysaccharide that is shown to considerably increase the number of gamma interferons and the activity of NK cells (Natural Killers, agents in cellular immunity).
In humans, a clinical trial has shown that daily consumption of yogurt containing this strain over 12 weeks reduces the frequency of colds in elderly people by a factor of 2.6. This effect is not found with other strains.
The benefits offered by the lactic acid bacteria in yogurt could be closely linked to the strain used in the fermentation of the product. As well as indicating the direction of new research, the article argues in favour of the benefits of consuming yogurt and fermented milk, particularly for elderly people.