Streptococcus pyogenes, also known as group A streptococcus (GAS), is responsible for 517,000 deaths per year worldwide, 163,000 of which are due to invasive infections, and the remainder to post-infectious sequelae. In 2022, there has been a resurgence of mild and fatal GAS infections in Europe. In clustered cases of GAS infections, some patients may have severe invasive infections while others have mild infections.
The team investigated the possible causes of the non-invasive strain-invasive strain switch. To this end, they determined the genomic sequence of ten isolates from five pairs, originating from five clusters of infection, and each composed of an invasive and a carriage strain. Of these pairs, one had a single nucleotide difference in the gene encoding the histidine kinase sensor, CovS, of the two-component regulatory system, CovRS. CovRS controls the expression of 15% of GAS genome, with phosphorylated CovR repressing the vast majority of these genes, including many virulence genes. CovS controls the activity of CovR by phosphorylating or dephosphorylating it according to environmental signals.
The presence of the antimicrobial peptide LL-37 or Mg2+ promotes the phosphorylation and dephosphorylation of CovR, respectively. Clinical isolates with mutations in CovS have been previously described. Invasive strains harbor non-functional CovS proteins leading to lack of CovR phosphorylation and derepression of many virulence factors. In contrast, in this work, the mutated strain isolated is the carriage strain. The mutation results in the replacement of the tyrosine at position 39 by a histidine, CovS-Y39H. This residue is the first amino acid after the transmembrane domain; it is therefore at one end of the surface-exposed domain, the domain that receives environmental signals Mg2+ and LL-37.
Figure legend: The clinical isolate producing the CovS-Y39H mutated protein produces much more phosphorylated CovR than the wild-type strain and thereby represses the expression of most virulence genes. This isolate, which is a carrier strain, is less virulent than the wild-type strain.
The authors analyzed the consequences of this mutation on the regulation exerted by CovRS by determining the transcriptomic profiles of the strains. The CovS-Y39H mutation affects the expression of the CovR regulon genes in a unique way. Genes usually overexpressed in covS mutant strains are underexpressed and vice versa. These results suggest that the CovS-Y39H mutation increases the proportion of phosphorylated CovR. In addition, gene expression in the strain with the CovS-Y39H mutation is virtually unresponsive to the addition of environmental compounds, either LL-37 or Mg2+. The mechanism of CovS phosphorylation-dephosphorylation of CovR indicates a strong decrease in the phosphatase activity of the CovS-Y39H mutated protein. The accumulation of two major virulence factors, the cysteine protease SpeB and the streptolysin Slo, was measured and shown to be modified similarly to the transcription of their genes.
The authors were then interested in the physiological consequences of this change in the regulation of gene expression in the mutant strain. This strain survives poorly compared to its wild-type counterpart in murine macrophages. Finally, in two mouse models of infection, by inhalation and by intravenous injection, models validated by the Université Paris Cité ethics committee (CEEA 34), the mutant strain was less virulent than the wild-type clinical isolate from the same cluster.
This study suggests that the CovS-Y39H mutation compromises the phosphatase activity of CovS and that this leads to a non-invasive strain. Furthermore, it confirms the importance of derepression of the expression of most virulence genes, via mutations impacting the phosphorylation of the CovR regulator, to promote invasive GAS infections.
Reference
A novel CovS variant harbored by a colonization strain reduces Streptococcus pyogenes virulence. Plainvert C, Rosinski-Chupin I, Weckel A , Lambert C, Touak G, Elisabeth Sauvage E, Poyart C, Glaser P, Agnès Fouet A. 2023 J. Bacteriol. Mar 15;e0003923. doi: 10.1128/jb.00039-23. Online ahead of print.. PMID: 36920220.
