Rapid isolation of exopolysaccharide‐producing Streptococcus thermophilus based on molecular marker screening

BACKGROUND As a natural food additive, exopolysaccharide (EPS) produced by Streptococcus thermophilus can improve product viscosity and texture. The protein EpsA is a putative pathway‐specific transcriptional regulator for EPS biosynthesis in S. thermophilus. RESULTS According to comparative analysis of EPS biosynthetic gene clusters, a conserved region of epsA (609 bp) was employed to design primer pair epsA‐F/R as a molecular marker for the isolation of EPS‐producing (EPS+) S. thermophilus. Two EPS+ S. thermophiles strains, AR333 and S‐3, were band‐positive, whereas Lactococcus lactis NZ9000 (non‐EPS‐producing, EPS−), Lactobacillus casei LC2W (EPS+) and L. plantarum AR113 (EPS+) were negative by polymerase chain reaction (PCR) amplicon bands using the epsA probe. This indicated good specificity of the epsA probe to EPS+ S. thermophilus. Moreover, based on PCR screening with the epsA probe, 23 positive strains were isolated and identified as S. thermophilus from our microbial library and natural fermented milk with 141.3–309.2 mg L−1 of EPS production, demonstrating the validity of our molecular marker screening method. CONCLUSION The designed molecular marker of epsA can rapidly screen EPS+ S. thermophilus, which has potential application in the dairy and other food industries.