Abstract
In a previous study, incorporation of high exopolysaccharide (EPS) producing dairy starter bacterium Streptococcus thermophilus ASCC 1275 was found to improve functionality of low fat mozzarella cheese and yogurt. This bacterium in its eps gene cluster has a unique pair of chain length determining genes, epsC- epsD, when compared to other sequenced S. thermophilus strains. Hence, the aim of this study was to understand the regulatory mechanism of EPS production in this bacterium using transcriptomic analysis to provide opportunities to improve the yield of EPS. As sugars are considered as one of the major determinants of EPS production, after preliminary screening, we selected three sugars, glucose, sucrose and lactose to identify the EPS producing mechanism of this bacterium in M17 medium. Complete RNA-seq analysis was performed using Illumina HiSeq 2000 sequencing system on S. thermophilus 1275 grown in three different sugars at two-time points, 5 h (log phase) and 10 h (stationary phase) to recognize the genes involved in sugar uptake, UDP-sugar formation, EPS assembly and export of EPS outside the bacterial cell. S. thermophilus 1275 was found to produce high amount of EPS (∼430 mg/L) in sucrose (1%) supplemented M17 medium when compared to other two sugars. Differential gene expression analysis revealed the involvement of phosphoenolpyruvate phosphotransferase system (PEP-PTS) for glucose and sucrose uptake, and lacS gene for lactose uptake. The pathways for the formation of UDP-glucose and UDP-galactose were highly upregulated in all the three sugars. In the presence of sucrose, eps1C1D2C2D were found to be highly expressed which refers to high EPS production. Protein homology study suggested the presence of Wzx/Wzy-dependent EPS synthesis and transport pathway in this bacterium. KEGG pathway and COG functional enrichment analysis were also performed to support the result. This is the first report providing the transcriptomic insights into the EPS production mechanism of a common dairy bacterium, S. thermophilus.
Highlights
Lactic acid bacteria (LAB) have a long history of use to produce fermented foods (Holzapfel and Wood, 2014)
Exopolysaccharide produced by S. thermophilus is of interest in the food industry due to its ability to enhance the properties of fermented foods, like texture, mouthfeel, viscosity, and decrease the syneresis in yogurt (Purwandari et al, 2007)
Various strategies like fermentation condition optimization and genetic manipulation have been used in improving EPS production in numerous LAB stains including S. thermophilus (Degeest et al, 1999; Zisu and Shah, 2003; Ruffing and Chen, 2006; Shene et al, 2008; Zhang et al, 2011; Papagianni, 2012)
Summary
Lactic acid bacteria (LAB) have a long history of use to produce fermented foods (Holzapfel and Wood, 2014). Bulgaricus, Lactobacillus plantarum, and Lactobacillus acidophilus, which are widely used in the traditional and modern fermented food production, mainly yogurt, kefir and cheese (Bhaskaracharya and Shah, 2000; Wu et al, 2014; Yang et al, 2014) These long chain polymers produced by LAB are known to impart better quality, and sensory attributes to fermented foods in which they are grown (Lynch et al, 2018). The results from full genome sequence (Wu et al, 2014) showed the presence of a novel EPS gene cluster that contains two sets of chain length determining genes, epsC – epsD, in the genome of this bacterium This suggests our earlier findings about the ability of S. thermophilus 1275 to produce capsular and ropy EPS (Zisu and Shah, 2003). In this study we investigated the EPS production mechanism in S. thermophilus 1275 through a detailed RNA-seq analysis under the presence of sugars that can influence EPS production
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