Streptococcus agalactiae (GBS)

Streptococci Characters of Streptococci Gram positive cocci 1µm in diameter Chains or pairs Usually capsulated Non motile Non spore forming Fastidious Catalase negative (Staphylococci are catalase positive)

Streptococcus agalactiae (GBS) Physiology and Structure Gram (+) streptococci Facultative anaerobe β –hemolytic (1-2% are nonhemolytic) Classified by B antigen

Streptococcus agalactiae

Streptococcus agalactiae Epidemiology Site of colonization: Lower gastrointestinal tract Genitourinary tract 10% to 30% of pregnant women are carriers. 60% of infants born to colonized mothers become infected with mothers’ organisms. This can lead to pneumonia, bacteremia, and meningitis

Streptococcus agalactiae Epidemiology cont. Infections in men and non-pregnant women: Primarily skin and soft-tissue Bacteremia Urosepsis (UTI with bacteremia) Pneumonia Conditions that predispose disease in adults: Diabetes mellitus Cancer Alcoholism

Pathogenesis Fig. 1. Stages in the molecular and cellular pathogenesis of neonatal group B Streptococcal (GBS) infection. Doran, Kelly S. & Nizet, Victor. Molecular pathogenesis of neonatal group B streptococcal infection: no longer in its infancy. Molecular Microbiology  2004;54:23-31.

Virulence Factor GBS Surface Polysaccharide Capsule Antiphagocytic properties Capsule-deficient mutants diminished virulence in animal models Sialic acid residues on capsule inhibit the binding of opsonically-active C3 component of complement to the cell surface blocking activation of the alternative pathway Transplacental passage of type-specific anticapsular IgG antibody from mother to infant is an important protective factor against invasive disease http://medicine.ucsd.edu/NizetLab

Virulence Factor GBS β-hemolysin Cytotoxic to pulmonary epithelial and endothelial cells Pulmonary injury and alveolar protein exudate in early-onset pneumonia Activity is blocked by surfactant phospholipid Increased risk of premature, surfactant-deficient neonates for severe pneumonia Induces cytokine release and nitric oxide production in macrophages Stimulate elements of the sepsis cascade

Virulence Factor C5a-peptidase Cleaves and inactivates the complement-derived neutrophil chemoattractant C5a C5a-peptidase-deficient mutants are more rapidly cleared from the lungs of infected animals when compared to the isogenic wild-type strain

Hemolysis on Blood agar

Bacitracin sensitivity Principle: Bacitracin test is used for presumptive identification of group A To distinguish between S. pyogenes (susceptible to B) & non group A such as S. agalactiae (Resistant to B) Bacitracin will inhibit the growth of gp A Strep. pyogenes giving zone of inhibition around the disk Procedure: Inoculate BAP with heavy suspension of tested organism Bacitracin disk (0.04 U) is applied to inoculated BAP After incubation, any zone of inhibition around the disk is considered as susceptible

CAMP test Principle: Procedure: Group B streptococci produce extracellular protein (CAMP factor) CAMP act synergistically with staph. -lysin to cause lysis of RBCs Procedure: Single streak of Streptococcus to be tested and a Staph. aureus are made perpendicular to each other 3-5 mm distance was left between two streaks After incubation, a positive result appear as an arrowhead shaped zone of complete hemolysis S. agalactiae is CAMP test positive while non gp B streptococci are negative

CAMP test

Enhanced Zone of Hemolysis CAMP Factor Test S. aureus (Sphingomyelinase C) Group B Streptococcus (CAMP Factor) Group A Streptococcus Enhanced Zone of Hemolysis

Optochin Susceptibility Test Optochin resistant S. viridans Optochin susceptible S. pneumoniae

Differentiation between -hemolytic streptococci CAMP test Bacitracin sensitivity Hemolysis Negative Susceptible  S. pyogenes Positive Resistant S. agalactiae

Hippurase NEG

Grp B Streptococci and Campylobacter Hippurase POS

Streptococcus agalactiae Diagnosis Culturing Antigen Detection DNA (PCR) test Treatment Penicillin G Pregnant women are give IV 4 hours before delivery

Diagnosis and Screening Culture– Gold Standard Selective broth medium (SMB or Lim Broth) PCR (Berger, et al 2000) Comparable in sensitivity to culture in extremely controlled laboratory environment Limited studied of clinical practice Immunoassay Enzyme and Optical Poor sensitivity Antigen detection may assist with diagnosis of CSF infection Bergeron MG, et al. Rapid detection of GBS in pregnant women at delivery. NEJM 2000;343:175.

Key Identification Characteristics of streptococcus agalactiae   Test Reaction Hemolysis Catalase Bacitracin 0.04U SXT CAMP test Bile esculin Optochin Hydrolysis of hippurate PYR Beta - R +