Symbiotic and Antagonistic Relationships in Mixed Cultures of Lactobacillus bulgaricus and Streptococcus thermophilus

Abstract

Symbiotic relationship in yogurt culture was studied using 10 strains of Lactobacillus bulgaricus and 7 strains of Streptococcus thermophilus. Seven of 17 strains were selected from the culture collection of our institute. Four strains were derived from Prof. R. NEGRI, International Dairy Federation (IDF) and 6 strains were isolated from commercial products. In pasteurized reconstituted skim milk (heated at 100°C for 20min), growth of all the strains of L. bulgaricus was confirmed to be stimulated by added formic acid. Test strains of S. thermophilus produced 1.5 to 7.7ppm formic acid after the incubation of 16 hours at 37°C (Table 1). Cultures of L. bulgaricus could accumulate free amino acids in the skim milk in a few hours of incubation, though the amount varied greatly with the strains used (Table 3). The addition of formate did not affect the proteolytic activity of L. bulgaricus (Fig. 3). Vitamin-free casamino acids (Difco) or supernatant of L. bulgaricus culture enhanced acid production and growth of S. thermophilus in the milk (Table 2, Fig. 4). Except strain 510, S. thermophilus strains formed long chains generally in the milk in several hours of incubation (Fig. 5A). Contrastively, S. thermophilus cultures tended to from shorter chains when the milk was fortified with vitamin-free casamino acids or the supernatant of L. bulgaricus culture. Similar tendency was found in mixed culture of S. thermophilus and L. bulgaricus, in which the cells of the former were mostly in pairs (Fig. 5B). Higher viable count of S. thermophilus, except strain 510, was obtained when this organism was incubated with L. bulgaricus in the milk than when it was incubated singly (Fig. 1B, 1D). However, direct microscopic observation revealed that the individual counts of S. thermophilus cells did not increase in the mixed culture (Table 4). These results show that the growth stimulation of S. thermophilus by L. bulgaricus could be explained by the increase of colony forming unit (i. e., formation of shorter chains). As mentioned above, symbiotic relationship was cofirmed between L. bulgaricus and S. thermophilus in most cases. However, S. thermophilus 510, our laboratory strain, showed an exceptional behaviour. Growth of this organism, whose cells were mostly in pairs even in single strain culture (Fig. 5C), was not enhanced at all by L. bulgaricus (Fig. 1A). Some strains of L. bulgaricus of relatively high proteolytic activity, such as strain AY (IDF), inhibited the growth of strain 510 (Fig. 1C). Possibility of the presence of inhibitory compound(s) (most likely peptides) produced against S. thermophilus 510 by L. bulgaricus was discussed.