Abstract
The group B pathogen Streptococcus agalactiae commonly populates the human gut and urogenital tract, and is a major cause of infection-based mortality in neonatal infants and in elderly or immunocompromised adults. Nuclease A (GBS_NucA), a secreted DNA/RNA nuclease, serves as a virulence factor for S. agalactiae, facilitating bacterial evasion of the human innate immune response. GBS_NucA efficiently degrades the DNA matrix component of neutrophil extracellular traps (NETs), which attempt to kill and clear invading bacteria during the early stages of infection. In order to better understand the mechanisms of DNA substrate binding and catalysis of GBS_NucA, the high-resolution structure of a catalytically inactive mutant (H148G) was solved by X-ray crystallography. Several mutants on the surface of GBS_NucA which might influence DNA substrate binding and catalysis were generated and evaluated using an imidazole chemical rescue technique. While several of these mutants severely inhibited nuclease activity, two mutants (K146R and Q183A) exhibited significantly increased activity. These structural and biochemical studies have greatly increased our understanding of the mechanism of action of GBS_NucA in bacterial virulence and may serve as a foundation for the structure-based drug design of antibacterial compounds targeted to S. agalactiae.
Highlights
Group B Streptococcus agalactiae (GBS) is a Gram-positive encapsulated bacterium that commonly colonizes the human urogenital tract, asymptomatically in healthy adults (McKenna & Iams, 1998)
S. agalactiae is the causative agent of nonfocused bacteraemia, endocarditis and bone or joint infections (Tazi et al, 2011)
S. agalactiae invasive infection is normally treated with antibiotics to ensure a favorable outcome, severe infections can lead to sepsis, meningitis and death, especially in neonates with immature immune function (Oh, 2013)
Summary
Group B Streptococcus agalactiae (GBS) is a Gram-positive encapsulated bacterium that commonly colonizes the human urogenital tract, asymptomatically in healthy adults (McKenna & Iams, 1998). GBS is a major cause of opportunistic infection in neonates (Doran & Nizet, 2004), the immunocompromised (Persson et al, 2004) and in elderly adults (Maisey et al, 2008). S. agalactiae is the causative agent of nonfocused bacteraemia, endocarditis and bone or joint infections (Tazi et al, 2011). S. agalactiae invasive infection is normally treated with antibiotics to ensure a favorable outcome, severe infections can lead to sepsis, meningitis and death, especially in neonates with immature immune function (Oh, 2013). GBS has evolved a formidable array of virulence factors to aid invasion, establishment of infection and evasion of the host’s immune system (Doran & Nizet, 2004). One key component of the mammalian innate immune response is the generation of neutrophil extracellular traps (NETs), which are extracellular extrusions of a dense, fibrous matrix comprised of DNA and antimicrobial proteins (Brinkmann et al, 2004)
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