Yogurt has long been recognized as a nutritious, natural, and safe component of a healthy diet and is at the basis of the concept of probiotics. A well documented health benefit of the consumption of yogurt containing live L. bulgaricus and Streptococcus thermophilus is an attenuation of lactose intolerance. In addition, immune modulation and diarrhea-alleviating effects have been reported, and both L. bulgaricus and S. thermophilus have been implicated in these effects. During yogurt fermentations protocooperation between these two bacteria results in an accelerated acidification, but the mechanisms involved are not completely understood.
Among the lactic acid bacteria, L. bulgaricus belongs to the acidophilus complex, a group of lactobacilli related to Lactobacillus acidophilus, Lactobacillus johnsonii, and Lactobacillus gasseri, which have been used as probiotic cultures. Although within this group L. bulgaricus is considered unique because of its atypical GC content, until recently the lack of tools for genetic manipulation has severely hampered a more detailed analysis of this organism