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Background: Hyaluronic acid (HA) is a critical component of the human integumentary system. Hyaluronate lyase (Hyl), degrading hyaluronic acid, is a virulence factor of some streptococci such as Streptococcus agalactiae (GBS), Streptococcus pyogenes (GAS), Streptococcus pneumoniae. The present study evaluated the role of the hyaluronate lyase in the pathogenicity of Streptococcus dysgalactiae subsp. equisimilis (SDSE).

Methods: SDSE, ?hylD SDSE (the deletion of hylD gene encoding Hyl strain), GBS, and GAS were used in the study. The impacts of hylD deletion on the pathogenicity of SDSE were assessed via mouse models of SDSE infection. Gene expressions were quantified using RT-qPCR. The HA degradation and growth levels, and Hyl enzymatic activity of bacteria were also assessed.

Results: The mortality rate of mice infected with ?hylD SDSE was significantly lower than that of those infected with wild-type (WT) SDSE (13% versus 67%, respectively; p = 0.002). Transcripts of hylD and ugl that encodes unsaturated glucuronyl hydrolase (UGL), catalyzing the degradation of HA-derived oligosaccharides, from WT SDSE collected from the mouse peritoneal cavity were significantly upregulated after 6 hours; upregulated expression of ugl was not observed in ?hylD SDSE. Serum IL-6 concentrations were significantly higher in WT SDSE-infected mice than in ?hylD SDSE-infected mice (means of log₁₀ IL6 (pg/ml) were 2.02 versus 0.25, respectively; p = 0.026); WT SDSE cells, but not ?hylD SDSE cells were observed at the infected wounds with intensive infiltration of neutrophils on day 2 post-infection, whereas fewer neutrophils were infiltrated around the wounds infected with ?hylD SDSE. HA degradation levels of SDSE strains were significantly higher than that of GBS and GAS strains at the same concentration. WT SDSE grew and divided more rapidly than other streptococci, and the expression levels of genes encoding Hyl and UGL in SDSE were significantly higher than those in GBS in the presence of 0.4 mg/ml of HA, which is similar to HA concentration in human skin, as a sole carbon source. The enzymatic activity of Hyl in SDSE was significantly higher than that in GBS at pH = 6.0 (p < 0.01), conditions detected in the skin of both elderly individuals and diabetes mellitus patients. In brief, our study suggested that Hyl and UGL of SDSE play important roles in nutrient acquisition from hosts, followed by the bacterial pathogenicity damaging host tissues.