Effect of sugars and protein sources on expression of genes involved in exopolysaccharide production by Streptococcus thermophiles ASCC1275

Abstract

Steptococcus thermophilus ASCC1275 (ST1275), a dairy starter bacterium, is known to produce high amount of exopolysaccharide (EPS). The genomic study of ST1275 revealed the presence of a novel EPS gene cluster containing 2 pair of chain length determining genes epsC – epsD, along with an efficient proteolytic and stress response system. Due to the complex and unique nature of the EPS gene cluster in this bacterium, it will be interesting to unravel the EPS biosynthetic pathway under different conditions. In this study, we investigated the influence of various sugars (glucose, sucrose and lactose) and protein sources (lactalbumin hydrolysate, casein hydrolysate, whey protein isolate) on the growth, EPS production and EPS gene expression of ST1275 in M17 media and in reconstituted skim milk (RSM), respectively. The EPS production by ST1275 was studied at different time intervals from 0 to 48 h. When grown in M17 supplemented with different sugars, ST1275 produced significantly high amount of EPS (630 mg/L) in sucrose (1%) supplemented M17 in 12 h at 37°C when compared with glucose and lactose supplemented M17. Interestingly, the pH was found to remain stable at 5.5 in lactose supplemented media from 12 h, when the pH dropped to 4.5 in the presence of other sugars. The lactic acid production was further validated using HPLC. In case of protein sources, EPS production was significantly increased when RSM was fortified with 0.5% whey protein isolate (826 mg/L) and casein hydrolysate (740 mg/L) in 12 h at 37°C in pH uncontrolled fermentation. The gene expression studies were also performed using q-PCR to investigate the regulatory genes involved in EPS production. It was observed that the expression of genes that resulted in amino sugar synthesis like galactokinase (GK), glutamine-fructose-6-phosphate transaminase, and UDP-glucose pyrophosphorylase have significantly increased in the presence of sucrose and WPI. The upregulation of galactokinase, a key enzyme involved in Leloir pathway, highlights the involvement of this pathway for EPS production in ST1275.