SUSCEPTIBILITY OF CHICKEN EMBRYOS TO GROUP-A STREPTOCOCCI - CORRELATION WITH FIBRINOGEN BINDING

One problem in investigating group A streptococcal infections and viru lence is the lack of appropriate in vivo models. In this study we intr oduce the chicken embryo model for determining virulence of Streptococ cus pyogenes. We found that M protein positive strains, if administere d intravenously, were highly virulent for 12-day-old chicken embryos. The LD(50) of the strains tested could be correlated directly with the amount of cell wall exposed M protein, which has been determined by t he capacity of streptococci to bind fibrinogen and by the ability of s treptococci to survive in fresh normal human blood. The number of colo ny forming units (cfu) of M(+) strains necessary to kill 50% of embryo nated eggs was significantly lower ( < 10(2) cfu) than for M(.) varian ts (> 10(4) cfu). Albumin and/or IgG binding to streptococal cells, wh ich can also take place in proteins of the M protein family which do n ot bind to fibrinogen, did not show that clear correlation of the viru lence in chicken embryos that did fibrinogen binding. Application of a nti-streptococcal M protein antisera from chicken and rabbit reduced t he lethality of the chicken embryos. In contrast, no correlation was f ound between lethality of chicken embryos and the in vitro production of erythrogenic toxins by the administered strains. Thus the results i ndicate that the presence of M-protein with its fibrinogen binding act ivity on the streptococcal cell surface is necessary for virulence of group A streptococci in the chicken embryo model.