Intestinal bacteria comprise one-third of the contents of the large intestine in humans. Their interactions with the gastrointestinal immune system induce characteristic immunological responses which stimulate or suppress the host's defense system. RAW 264.7 murine cell line was used as a macrophage model to assess the effects of the exposure to the isolated human intestinal bacteria, Bacteroides, Bifidobacterium, Eubacterium, Streptococcus, and E. coli, on NO (nitric oxide), (hydrogen peroxide), and cytokines IL (interleukin)-6 and TNF (tumor necrosis factor)-a production. RAW 264.7 cells were cultured in the presence of heat-killed bacteria for 24 h at concentrations of 0-g/ml. Our results showed that Bacteroides and E. coli stimulated IL-6, TNF-, NO, and production at high levels even at g/ml, whereas Bifidobacterium, Eubacterium, and Streptococcus showed a low level of stimulation at g/ml, and a gradual increase as the cell concentration increased up to g/ml. This result suggests that gram-negative Bacteroides and E. coli are better able to stimulate macrophage than gram-positive Bifidobacterium, Streptococcus, and Eubacterium. The in vitro approaches employed here should be useful in further characterization of the effects of intestinal bacteria on gastrointestinal and systemic immunity.

Keywords: Bifidobacterium, intestinal bacteria, macrophage, H2O2, nitric oxide, cytokine