The eps locus of Streptococcus thermophilus IP6756 is not involved in exopolysaccharide production

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. © 2021 Society of Chemical Industry.

Intraspecific and interspecific interactions among proteins regulating exopolysaccharide synthesis in Streptococcus thermophilus, Streptococcus iniae, and Lactococcus lactis subsp. cremoris and the assessment of potential lateral gene transfer

Using the yeast two-hybrid system, intraspecific protein interactions were detected in Streptococcus iniae and Lactococcus lactis subsp. cremoris between the transmembrane activation protein (CpsC and EpsA, respectively) and the protein tyrosine kinase (CpsD and EpsB, respectively), between two protein tyrosine kinases, and between the protein tyrosine kinase and the phosphotyrosine phosphatase (CpsB and EpsC, respectively). For each of these intraspecific interactions, interspecific interactions were also detected when one protein was from S. iniae and the other was from Streptococcus thermophilus . Interactions were also observed between two protein tyrosine kinases when one protein was from either of the Streptococcus species and the other from L. lactis subsp. cremoris. The results and sequence comparisons performed in this study support the conclusion that interactions among the components of the tyrosine kinase - phosphatase regulatory system are conserved in the order Lactobacillales and that interspecific genetic exchanges of the genes that encode these proteins have the potential to form functional recombinants. A better understanding of intraspecific and interspecific protein interactions involved in regulating exopolysaccharide biosynthesis may facilitate construction of improved strains for industrial uses as well as identification of factors needed to form functional regulatory complexes in naturally occurring recombinants.

Site-specific accretion of an integrative conjugative element together with a related genomic island leads to cis mobilization and gene capture

Genomic islands, flanked by attachment sites, devoid of conjugation and recombination modules and related to the integrative and conjugative element (ICE) ICESt3, were previously found in Streptococcus thermophilus. Here, we show that ICESt3 transfers to a recipient harbouring a similar engineered genomic island, CIMEL₃catR₃, and integrates by site-specific recombination into its attachment sites, leading to their accretion. The resulting composite island can excise, showing that ICESt3 mobilizes CIMEL₃catR₃, in cis. ICESt3, CIMEL₃catR₃, and the whole composite element can transfer from the strain harbouring the composite structure. The ICESt3 transfer to a recipient bearing CIMEL₃catR₃, can also lead to retromobilization, i.e. its capture by the donor. This is the first demonstration of specific conjugative mobilization of a genomic island in cis and the first report of ICE-mediated retromobilization. CIMEL₃catR₃, would be the prototype of a novel class of non-autonomous mobile elements (CIMEs: CIs mobilizable elements), which hijack the recombination and conjugation machinery of related ICEs to excise, transfer and integrate. Few genome analyses have shown that CIMEs could be widespread and have revealed internal repeats that could result from accretions in numerous genomic islands, suggesting that accretion and cis mobilization have a key role in evolution of genomic islands.

Conjugative transfer of the integrative conjugative elements ICESt1 and ICESt3 from Streptococcus thermophilus

Integrative and conjugative elements (ICEs), also called conjugative transposons, are genomic islands that excise, self-transfer by conjugation, and integrate in the genome of the recipient bacterium. The current investigation shows the intraspecies conjugative transfer of the first described ICEs in Streptococcus thermophilus, ICESt1 and ICESt3. Mitomycin C, a DNA-damaging agent, derepresses ICESt3 conjugative transfer almost 25-fold. The ICESt3 host range was determined using various members of the Firmicutes as recipients. Whereas numerous ICESt3 transconjugants of Streptococcus pyogenes and Enterococcus faecalis were recovered, only one transconjugant of Lactococcus lactis was obtained. The newly incoming ICEs, except the one from L. lactis, are site-specifically integrated into the 3' end of the fda gene and are still able to excise in these transconjugants. Furthermore, ICESt3 was retransferred from E. faecalis to S. thermophilus. Recombinant plasmids carrying different parts of the ICESt1 recombination module were used to show that the integrase gene is required for the site-specific integration and excision of the ICEs, whereas the excisionase gene is required for the site-specific excision only.

The constant gene orf14.9, which belongs to the variable eps (exopolysaccharide) cluster, is involved in the cell growth of Streptococcus thermophilus

In Streptococcus thermophilus, the eps clusters involved in exopolysaccharide (EPS) biosynthesis are very polymorphic, nevertheless they all contain a highly conserved sequence corresponding to that of orf14.9. This open reading frame (ORF) is transcribed in a reverse direction with respect to eps genes. Amino acid sequence analysis showed a possible transmembrane location of the putative Orf14.9 protein but did not permit a proposed function. Insertional mutants of orf14.9 were obtained in strains NST2280 and A054 of S. thermophilus. EPS yields of these mutants are similar to those of their respective wild strains, suggesting that orf14.9 does not modify the quantity of produced EPS. Growth parameter determination for wild strains and their respective mutants showed that orf14.9 is involved in the cell growth of S. thermophilus.Key words: Streptococcus thermophilus, eps cluster, orf14.9, sequence analysis, exopolysaccharide biosynthesis, cell growth.