Intracellular multiplication and toxic destruction of cultured macrophages by Capnocytophaga canimorsus.

Capnocytophaga canimorsus is a gram-negative rod that causes opportunistic infections resulting in bacteremia, septicemia, meningitis, and death in immunocompromised, splenectomized, and alcoholic individuals. Infections caused by a related species, Capnocytophaga cynodegmi, remain localized at the site of the wound where the organism is introduced. Both organisms are part of the normal canine oral flora and are introduced through puncture wounds via dog bites. We found that both C. canimorsus and C. cynodegmi attach, are phagocytized, and multiply intracellularly in J774 mouse macrophage cells. After 48 h of infection by C. canimorsus, large sections of the macrophage cell layer were observed to detach and lyse, while the monolayer infected with C. cynodegmi demonstrated no cytotoxic effects. Tissue culture supernatants from the C. canimorsus-infected J774 cells filtered through a 0.22-micron-pore membrane produced a similar effect on fresh monolayers, while filtrates from C. cynodegmi and uninfected controls produced no effect. No endotoxin release was observed in these supernatants. We conclude that the cytotoxic phenotype of C. canimorsus is the likely result of a toxin produced by this organism.