Production of Experimental Endocarditis by Coagulase-Negative Staphylococci: Variability in Species Virulence

The abilities of 11 coagulase-negative staphylococcal isolates of human origin to establish infective endocarditis in a rat model were compared. Seven of the strains were Staphylococcus epidermidis, the species most commonly causing endocarditis in humans. The other four strains were Staphylococcus hominis, an infrequent cause of endocarditis. Four of the seven strains of S. epidermidis and two of the four strains of S. hominis elaborated surface slime. With an inoculum of 107 colony-forming units, endocarditis was established in all of 77 rats injected with S. epidermidis but in only five (12.5%) of 40 animals injected with S. hominis (P < .001). Strains of S. epidermidis were significantly more resistant to phagocytic killing in vitro than were strains of S. hominis. These data indicate that there are major differences in the virulence of various species of coagulase-negative staphylococci, both in vivo and in vitro, and that these differences are reflected in the observed incidence of disease caused in humans by the respective species.