Abstract
We examined the possibility that segmented, filamentous bacteria (FBs) in the ileum of mice play a role in host resistance to pathogens by comparing four groups of mice, namely germ-free mice, mice mono-associated with FBs, FB-free mice with a specified pathogen-free (SPF) flora, and FB-positive SPF mice, after oral administration of viable cells of either Salmonella enteritidis or Enterobacter cloacae. The SPF gut flora with or without FBs reduced colonisation of host tissues by pathogens to a similar extent. FBs alone did not reduce mean colonisation, except for salmonella counts in spleen. Mice mono associated with FBs had a lower incidence of animals with salmonella-positive livers and spleens than did germ-free mice. When compared with the germ-free status, both the SPF flora and FBs raised the serum IgA concentration irrespective of the type of pathogen challenge. The SPF flora consistently raised the concanavalin A-induced proliferative response of isolated mesenteric lymph node cells (MLNC). In MLNC from FB-positive SPF mice, the concanavalin A-induced proliferative response was lower than in FB-free SPF mice, but the salmonella antigen-induced response of spleen cells (SC) was higher; these effects were independent of the type of pathogen challenge. The in vitro proliferative response of Peyer's patch cells to various antigens was not affected by the type of gut flora of mice challenged with either Salmonella enteritidis or Enterobacter cloacae.