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M proteins of group G streptococci isolated from bacteremic human infections
Abstract:
We studied seven strains of group G streptococci isolated from clinically severe bacteremic infections in six intravenous drug abusers. These group G strains multiplied luxuriantly in fresh human blood. On electron microscopy, they exhibited surface fibrillae similar to those observed in M-protein-rich group A streptococci, but they were not serologically M typable with a battery of 39 M antisera. Rabbit antisera raised against two of the group G strains (1618 and 1750) opsonized the homologous but not the heterologous isolates and exhibited type-specific Ouchterlony immunoprecipitin reactions. Moreover, antisera raised against peptic extracts of strain 1750 also promoted phagocytic killing of that strain. Anti-1750 reacted in high titer in an enzyme-linked immunosorbent assay against peptic extracts of the homologous strain; these antibodies were removed by absorption with 1750 cells but not by absorption with heterologous strains. These studies represent, to our knowledge, the first analysis of virulence factors of group G streptococci isolated from invasive human disease. The seven epidemiologically related blood isolates of group G streptococci possess distinct type-specific, antiphagocytic surface virulence factors analogous to the M proteins of group A streptococci.
Insights
Group G streptococci causing severe infections in drug users have surface virulence factors similar to M proteins. These factors are type-specific and help bacteria evade immune responses.
Area of Science:
- Microbiology
- Immunology
- Infectious Diseases
Background:
- Group G streptococci are increasingly implicated in severe human infections.
- Intravenous drug abusers represent a population at risk for invasive streptococcal disease.
- The virulence factors of these strains are not well understood.
Purpose of the Study:
- To analyze the virulence factors of group G streptococci isolated from severe bacteremic infections.
- To investigate the antiphagocytic properties of these streptococcal strains.
- To determine if these strains possess surface factors analogous to M proteins of group A streptococci.
Main Methods:
- Isolation and characterization of seven group G streptococcal strains from blood cultures.
- Electron microscopy to visualize surface structures.
- Serological typing using M antisera.
- Generation of rabbit antisera against specific strains and their extracts.
- Opsonophagocytosis assays and enzyme-linked immunosorbent assays (ELISA).
Main Results:
- Group G streptococci multiplied readily in human blood and exhibited surface fibrillae.
- Strains were not serologically typable with existing M antisera.
- Type-specific antisera opsonized homologous strains and inhibited phagocytosis.
- Antibodies against peptic extracts of one strain showed high specificity.
Conclusions:
- Group G streptococci from invasive infections possess distinct type-specific antiphagocytic surface virulence factors.
- These factors are analogous to the M proteins found in group A streptococci.
- This study provides the first analysis of virulence factors in disease-causing group G streptococci.