Microbial interactions between Lactobacillus bulgaricus and Streptococcus thermophilus in milk

In this paper,we investigate the growth of streptococcus thermophilus (ist) in milk in the presence (co-culture) or absence (monoculture) of lactobacillus bulgaricus (ilb). the strains used in this study are isolated from a concentrate (istlb) cultured in a fresh milk from a freeze-dried mixture provided by the dairy: cooperative laitière du maroc oriental (colaimo). the obtained results of the acidification of fresh milk are preferable than the reconstituted milk for both ist and ilb strains. i.e.,the ist strain acidified milk better than ilb strain. a synergistic effect (proto-cooperation) is demonstrated in the two culture media. the acidity produced in the fresh milk by the mixed culture (ist + ilb) is 101° d ±3.2,while each of the two pure culture strains produced only 92° d ±2.7. in the reconstituted milk,the same results are observed but with less acidity,which are 63° d ±1.2 and 52° d ±1.9 respectively for mixed and pure culture strains. these differences are significant at 99% threshold according to the student test. the filtrate of the ilb strain culture stimulates the lactic acid production of ist strain. no synergistic effect is observed on the proteolytic activity of the two strains. the ist strain seems to have an important proteolytic capacity than ilb strain. © 2017,university of mohammed premier oujda morocco.