The rgg0182 gene encodes a transcriptional regulator required for the full Streptococcus thermophilus LMG18311 thermal adaptation

Novel Genus of Phages Infecting Streptococcus thermophilus: Genomic and Morphological Characterization

Streptococcus thermophilus is a lactic acid bacterium commonly used for the manufacture of yogurt and specialty cheeses. Virulent phages represent a major risk for milk fermentation processes worldwide, as they can inactivate the added starter bacterial cells, leading to low-quality fermented dairy products. To date, four genetically distinct groups of phages infecting S. thermophilus have been described. Here, we describe a fifth group. Phages P738 and D4446 are virulent siphophages that infect a few industrial strains of S. thermophilus The genomes of phages P738 and D4446 were sequenced and found to contain 34,037 and 33,656 bp as well as 48 and 46 open reading frames, respectively. Comparative genomic analyses revealed that the two phages are closely related to each other but display very limited similarities to other S. thermophilus phages. In fact, these two novel S. thermophilus phages share similarities with streptococcal phages of nondairy origin, suggesting that they emerged recently in the dairy environment.IMPORTANCE Despite decades of research and adapted antiphage strategies such as CRISPR-Cas systems, virulent phages are still a persistent risk for the milk fermentation industry worldwide, as they can cause manufacturing failures and alter product quality. Phages P738 and D4446 are novel virulent phages that infect the food-grade Gram-positive bacterial species Streptococcus thermophilus These two related viruses represent a fifth group of S. thermophilus phages, as they are significantly distinct from other known S. thermophilus phages. Both phages share similarities with phages infecting nondairy streptococci, suggesting their recent emergence and probable coexistence in dairy environments. These findings highlight the necessity of phage surveillance programs as the phage population evolves in response to the application of antiphage strategies.

Rgg-associated SHP signaling peptides mediate cross-talk in Streptococci

We described a quorum-sensing mechanism in the streptococci genus involving a short hydrophobic peptide (SHP), which acts as a pheromone, and a transcriptional regulator belonging to the Rgg family. The shp/rgg genes, found in nearly all streptococcal genomes and in several copies in some, have been classified into three groups. We used a genetic approach to evaluate the functionality of the SHP/Rgg quorum-sensing mechanism, encoded by three selected shp/rgg loci, in pathogenic and non-pathogenic streptococci. We characterized the mature form of each SHP pheromone by mass-spectrometry. We produced synthetic peptides corresponding to these mature forms, and used them to study functional complementation and cross-talk between these different SHP/Rgg systems. We demonstrate that a SHP pheromone of one system can influence the activity of a different system. Interestingly, this does not seem to be dependent on the SHP/Rgg group and cross-talk between pathogenic and non-pathogenic streptococci is observed.

Thermo-regulated adhesion of the Streptococcus thermophilus Δrgg0182 strain

The physicochemical determinants governing the temperature-dependent adhesion of Streptococcus thermophilus to abiotic surfaces are identified under physiological condition for cells either lacking or not the Rgg0182 transcriptional regulator involved in their thermal adaptation. For that purpose, the wild type LMG18311 strain and Δrgg0182 mutant were imaged using highly resolved atomic force microscopy (AFM) at various cell growth temperatures (42 to 55 °C). The corresponding hydrophobic/hydrophilic balance of the cells was quantitatively addressed via the measurement by chemical force microcopy of their adhesion to a reference hydrophobic surface. Analysis of force-separation distance curves further allowed us to discriminate cell surfaces according to the presence or absence of biopolymers. These results were interpreted in relation to the measured adhesion of the Δrgg0182 mutant onto the hydrophobic wall of microwells in the temperature range from 46 to 52 °C. It is evidenced that the viscoelastic Δrgg0182 cell envelop behaves as a thermo-responsive film whose hydrophobicity increases with increasing temperature, thereby favoring cell attachment to hydrophobic surfaces. Regardless cell growth temperature, wild-type cells do not attach to hydrophobic surfaces and the presence of the Rgg0182 transcriptional regulator is associated with the synthesis of hydrophilic cell surface biopolymers. Throughout, the impact of electrostatics on bioadhesion is ruled out upon examination of electrohydrodynamic cell properties at 50 °C.