Virulence Mechanism of Bacteria in Mixed Infection: Attenuation of Cytokine Levels and Evasion of Polymorphonuclear Leukocyte Phagocytosis

Background: The objective of the present study is to evaluate the effect of bacterial viability on the virulence of mixed infection. Methods: Expression of pro- and anti-inflammatory cytokines (interleukin [IL]-1 beta and IL-10, respectively) was tested in vivo, following live versus heat-killed infection (mono or mixed), using the mouse chamber model of infection. Ex vivo, phagocytosis of fluorescently labeled bacteria was tested in primary mouse polymorphonuclear leukocytes by flow cytometry. Results: In monoinfection, heat-killed Porphyromonas gingivalis led to augmented levels of IL-1 beta 2 hours postinfection, whereas IL-10 levels remained unaffected. Phagocytosis of heat-killed P. gingivalis was reduced compared with that of the live P. gingivalis, whereas phagocytosis of heat-killed Fusobacterium nucleatum was augmented compared with that of live F. nucleatum. In mixed infection, both IL-1 beta and IL-10 levels were augmented 24 hours postinfection when the bacteria were heat-killed. Although the phagocytosis pattern of F. nucleatum in the mixed infection remained similar to that upon monoinfection, phagocytosis of P. gingivalis was reduced following mixed infection. Conclusions: The inflammatory response to live mixed infection is attenuated with reduced phagocytosis, compared with that of heat-killed mixed infection. The lower response to live mixed infection could stem from a mechanism enabling the bacteria to evade the host response, thereby increasing bacterial survival.