Studies of Staphylococcal Infections. I. Virulence of Staphylococci and Characteristics of Infections in Embryonated Eggs *

Abstract

Staphylococci patho-genic for man are extremely lethal to embryonated eggs. Intraallantoic infection with less than 100 colony forming units of strains of Staphylo-coccus aureus isolated from clinical infections almost uniformly pro-duced fatality rates of 50% or greater. The virulence of individual strains was reproducible over prolonged periods and tended to parallel their clinical behavior. Staphylococci isolated from clinical infections were more lethal than strains obtained from healthy carriers, strains isolated from severe staphylococcal infections were more lethal than strains isolated from milder infections, and strains of S. epidermidis were essentially nonlethal for chick embryos. The age of the embryo also modified lethality, embryos of less than 10 days of age being more susceptible to fatal infection with both coagulase-positive and coagulase -negative staphylococci than older embryos. Differences in growth between coagulase-positive and coagulase-negative staphylococci during infection were observed but did not explain the marked differences in virulence of the two species. Both grew equally well in allantoic fluid in vitro and in vivo and invaded the embryo. Coagulase-positive staphylococci persisted at essentially the same numbers that were attained after maximal growth in allantoic fluid and in the embryo, whereas the numbers of coagulase-negative staphylococci progressively decreased after initial logarithmic growth until less than 1% remained viable 10 days after infection. The mechanism of death for the embryo, the role of exotoxins, and explanation of the differences in virulence of coagulase positive and coagulase negative staphylococci remain to be determined.