Experimental Parasitology 111 (2005) 87–96 www.elsevier.com/locate/yexpr 0014-4894/$ - see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.exppara.2005.06.002 Trypanosoma cruzi: InXuence of predominant bacteria from indigenous digestive microbiota on experimental infection in mice R. Duarte a , A.M. Silva a , L.Q. Vieira b , L.C.C. Afonso c , J.R. Nicoli a,¤ a Departamento de Microbiologia, ICB, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brazil b Departamento de Bioquímica-Imunologia, ICB, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brazil c Departamento de Ciências Biológicas, ICEB/NUPEB, Universidade Federal de Ouro Preto, Ouro Preto, MG, Brazil Received 4 March 2005; received in revised form 2 June 2005; accepted 7 June 2005 Available online 20 July 2005 Abstract To verify the inXuence of some predominant components from indigenous microbiota on systemic immunological responses dur- ing experimental Chagas disease, germ-free NIH Swiss mice were mono-associated with Escherichia coli, Enterococcus faecalis, Bac- teroides vulgatus or Peptostreptococcus sp. and then infected with the Y strain of Trypanosoma cruzi. All the mono-associations predominantly induced a Th1 type of speciWc immune response to the infection by T. cruzi. A direct correlation was observed between a higher survival rate and increased IFN- and TNF- production (P < 0.05) in E. faecalis-, B. vulgatus-, and Peptostrepto- coccus-associated mice. Moreover, higher levels of anti-T. cruzi IgG1 and anti-T. cruzi IgG2a were also found in mono-associated animals after infection. On the other hand, with the exception of E. faecalis-associated mice, mono-association induced a lower IL-10 production after infection (P < 0.05) when compared with germ-free animals. Interestingly, spleen cell cultures from non-infected germ-free and mono-associated mice spontaneously produced higher levels (P < 0.05) of IL-10 than cultures from infected mono- associated mice, except again for E. faecalis-associated animals. In conclusion, the presence of the components of the indigenous mic- robiota skews the immune response towards production of inXammatory cytokines during experimental infection with T. cruzi in gnotobiotic mice. However, the degree of increase in production of cytokines depends on each bacterial component.  2005 Elsevier Inc. All rights reserved. Keywords: Microbiota; Trypanosoma cruzi; Cytokines; Immunoglobulins; Gnotobiotic mice 1. Introduction The human gastrointestinal tract harbors one of the most complex ecosystems known in microbial ecology with bacterial populations reaching 10 10 –10 11 viable cells/g of contents in its lower portions. These organ- isms may belong to about 400 diVerent bacterial spe- cies, although it is believed that only 20–40 of them reach predominant levels, consisting of 99% of the total community (Berg, 1996). Concerning the human pre- dominant fecal bacteria, two population levels can be distinguished: (i) the dominant microbiota (10 9 – 10 11 cells/g of contents) constituted only by obligate anaerobes (Bacteroides, Bi Wdobacterium, Eubacterium, Peptostreptococcus, and Fusobacterium) and (ii) the sub-dominant microbiota (10 7 –10 8 cells/g of contents) containing predominantly facultative anaerobic and microaerophilic bacteria (Escherichia coli, Enterococ- cus, and Lactobacillus). In healthy hosts, the presence of this microbiota has a very large impact on various aspects of function and metabolism such as metabolic rate, gastrointestinal function, speciWc and quantitative aspects of immune function, and the many aspects of biochemical homeostasis. Only the predominant species have population levels high enough to be considered * Corresponding author. Fax: +55 31 3499 2730. E-mail address: jnicoli@icb.ufmg.br (J.R. Nicoli).