VIRULENCE FACTORS AND HOST DEFENSES (POSTER PRESENTATION) Stimulatory E¡ect of Bi¢dobacteria on the Host Mononuclear Phagocyte System Using Gnotobiotic Animal Models G.I. Podoprigora 1 * , L.B. Comunian, E.F. Pimentel, L.N. Moura, D.C. Cara 2 , J.R. Nicoli 3 , and E.C.Vieira 1 1 Department of Biochemistry and Immunology; 2 Department of General Pathology; 3 Department of Microbiology, Institute of Biological Sciences, Federal University of Minas Gerais, Belo Horizonte, Minas Gerais, Brazil Introduction Bifidobacteria are being widely used as the basis for probiotic preparations and correction of microbioce- nosis. However, the mechanisms of direct action of Bifidobacteria on the host's nonspecific resistance mechanisms are still poorly understood. Gnotobiotic animals (both germ-free and monoassociated ones) have proved to be a unique tool for the study of the direct immunomodulating action of various probio- tics on the host without coinfluence of concomitant microorganisms. The aim of the present work was to investigate an upregulating potential of the probiotics based on anaerobic Gram-positive bacteria like Bifidobacteria for the host's Mononuclear Phagocyte System (MPS) activity against pathogenic bacteria (Escherichia coli B41 or Salmonella typhimurium) using gnotobiotic models. Materials and Methods Animal models Germfree and conventional 1-month-old NIH mice (Taconic, Germantown, U.S.A.) were used in the experiments. Probiotic monoassociation experiments The probiotic used in the study was Bifidobacterium bifidum strain 791 (`Bifidumbacterin', Moscow, Russia). Germfree mice were inoculated intragastri- cally with a single dose containing 10 8 colony forming units (CFU) of the probiotic bacteria in 0.2 mL saline. The same doses of probiotic were inoculated into control animals every 2 days for 7±9 days. Control of the probiotic population dynamics in monoassociation The kinetics of probiotic colonization of the intestinal tract of gnotobiotic animals were studied during the *Corresponding author. Tel.: 31 449 26 51; Fax: 31 441 59 63. E-mail: podoprig@mono.icb.ufmg.br 1075±9964/99/030509 + 04 $30.00/0 # 1999 Academic Press Anaerobe (1999) 5, 509±512 Article No. anae.1999.0241