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Surface-expressed mig protein protects Streptococcus dysgalactiae against phagocytosis by bovine neutrophils
X M Song1, J Perez-Casal, A Bolton
1Veterinary Infectious Disease Organization, Saskatoon, Saskatchewan, Canada.
Abstract:
The mig gene of Streptococcus dysgalactiae, a major bovine mastitis pathogen, encodes two plasma protein-binding receptors, alpha2-macroglobulin (alpha2-M) and immunoglobulin G (IgG). In this study, the mig gene from one S. dysgalactiae isolate was cloned and expressed in Escherichia coli. The IgG receptor region encoded by mig was conserved in 16 S. dysgalactiae strains. An isogenic mig mutant was constructed by allele replacement mutagenesis of the wild-type gene in S. dysgalactiae. The IgG-binding activity was lost in the mig mutant strain, whereas the alpha2-M receptor activity was still expressed but was detected only in the culture supernatant. In flow cytometry phagocytosis and bacterial-colony-counting bactericidal assays, the wild-type strain was found to be significantly more resistant to phagocytosis and killing by bovine neutrophils (PMNs) than the mig mutant strain when bacteria were preincubated with bovine serum. We therefore speculate that the Mig protein of S. dysgalactiae plays a role in virulence of the bacteria by binding to the plasma protein alpha2-M or IgG and thus preventing phagocytosis by bovine PMNs.
Insights
Streptococcus dysgalactiae
Area of Science:
- Veterinary Microbiology
- Bacterial Pathogenesis
- Immunology
Background:
- Streptococcus dysgalactiae is a significant cause of bovine mastitis.
- The mig gene in S. dysgalactiae encodes proteins that bind host plasma proteins.
- Understanding these interactions is crucial for controlling bovine mastitis.
Purpose of the Study:
- To investigate the function of the mig gene in S. dysgalactiae virulence.
- To characterize the plasma protein-binding activities of the Mig protein.
- To determine the role of Mig in resistance to bovine neutrophil phagocytosis.
Main Methods:
- Cloning and expression of the mig gene in Escherichia coli.
- Construction of an isogenic mig mutant using allele replacement mutagenesis.
- Assays for alpha2-macroglobulin (alpha2-M) and immunoglobulin G (IgG) binding.
- Flow cytometry and bactericidal assays to assess resistance to bovine neutrophils (PMNs).
Main Results:
- The mig gene's IgG receptor region is conserved across S. dysgalactiae strains.
- The mig mutant lost IgG-binding activity but retained alpha2-M binding in supernatant.
- The wild-type strain showed significantly greater resistance to PMN phagocytosis and killing compared to the mig mutant when preincubated with serum.
Conclusions:
- The Mig protein of S. dysgalactiae likely contributes to virulence.
- Mig may prevent phagocytosis by bovine neutrophils by binding alpha2-M or IgG.
- This interaction is a potential target for controlling S. dysgalactiae infections in cattle.