An Inventory of CiaR-Dependent Small Regulatory RNAs in Streptococci

The involvement of CiaR and the CiaR-regulated serine protease HtrA in thermal adaptation of Streptococcus pneumoniae

The in vivo temperature can vary according to the host tissue and the response to infection. Streptococcus pneumoniae has evolved mechanisms to survive these temperature differences, but neither the consequences of different temperatures for pneumococcal phenotype nor the genetic basis of thermal adaptation are known in detail. In our previous study [16], we found that CiaR, which is a part of two-component regulatory system CiaRH, as well as 17 genes known to be controlled by CiaRH, were identified to be differentially expressed with temperature. One of the CiaRH-regulated genes shown to be differentially regulated by temperature is for the high-temperature requirement protein (HtrA), coded by SPD_2068 (htrA). In this study, we hypothesized that the CiaRH system plays an important role in pneumococcal thermal adaptation through its control over htrA. This hypothesis was evaluated by testing strains mutated or overexpressing ciaR and/or htrA, in in vitro and in vivo assays. The results showed that in the absence of ciaR, the growth, haemolytic activity, amount of capsule and biofilm formation were considerably diminished at 40 °C only, while the cell size and virulence were affected at both 34 and 40 °C. The overexpression of htrA in the ∆ciaR background reconstituted the growth at all temperatures, and the haemolytic activity, biofilm formation and virulence of ∆ciaR partially at 40 °C. We also showed that overexpression of htrA in the wild-type promoted pneumococcal virulence at 40 °C, while the increase of capsule was observed at 34 °C, suggesting that the role of htrA changes at different temperatures. Our data suggest that CiaR and HtrA play an important role in pneumococcal thermal adaptation.

Biofilm Formation in Streptococcus agalactiae Is Inhibited by a Small Regulatory RNA Regulated by the Two-Component System CiaRH

Regulatory small RNAs (sRNAs) are involved in the adaptation of bacteria to their environment. CiaR-dependent sRNAs (csRNAs) are controlled by the regulatory two-component system (TCS) CiaRH, which is widely conserved in streptococci. Except for Streptococcus pneumoniae and Streptococcus sanguinis, the targets of these csRNAs have not yet been investigated. Streptococcus agalactiae, the leading cause of neonatal infections, has four conserved csRNA genes, namely, srn015, srn024, srn070, and srn085. Here, we demonstrate the importance of the direct repeat TTTAAG-N5-TTTAAG in the regulation of these csRNAs by CiaRH. A 24-nucleotide Srn024-sap RNA base-pairing region is predicted in silico. The sap gene encodes a LPXTG-cell wall-anchored pullulanase. This protein cleaves α-glucan polysaccharides such as pullulan and glycogen present in the environment to release glucose and is involved in adhesion to human cervical epithelial cells. Inactivation of S. agalactiae pullulanase (SAP) leads to no bacterial growth in a medium with only pullulan as a carbon source and reduced biofilm formation, while deletion of ciaRH and srn024 genes significantly increases bacterial growth and biofilm formation. Using a new translational fusion vector, we demonstrated that Srn024 is involved in the posttranscriptional regulation of sap expression. Complementary base pair exchanges in S. agalactiae suggest that Srn024 interacts directly with sap mRNA and that disruption of this RNA pairing is sufficient to yield the biofilm phenotype of Srn024 deletion. These results suggest the involvement of Srn024 in the adaptation of S. agalactiae to environmental changes and biofilm formation, likely through the regulation of the sap gene. IMPORTANCE Although Streptococcus agalactiae is a commensal bacterium of the human digestive and genitourinary tracts, it is also an opportunistic pathogen for humans and other animals. As the main cause of neonatal infections, it is responsible for pneumonia, bacteremia, and meningitis. However, its adaptation to these different ecological niches is not fully understood. Bacterial regulatory networks are involved in this adaptation, and the regulatory TCSs (e.g., CiaRH), as well as the regulatory sRNAs, are part of it. This study is the first step to understand the role of csRNAs in the adaptation of S. agalactiae. This bacterium does not currently exhibit extensive antibiotic resistance. However, it is crucial to find alternatives before multidrug resistance emerges. Therefore, we propose that drugs targeting regulatory RNAs with Srn024-like activities would affect pathogens by reducing their abilities to form biofilm and to adapt to host niches.