Application of phytohemagglutinin (PHA) in weaning feed has been suggested to stimulate intestinal epithelium maturation. In this study, PHA strongly affected the fecal bacterial population structure of rats. Escherichia coli overgrowth was not prevented by probiotic mannose-adhering Lactobacillus plantarum 299v. Therefore, use of PHA in weaning feed deserves careful evaluation.Plant lectins such as kidney bean-derived phytohemagglutinin (PHA) have gained renewed interest for potential use in animal feed and infant nutrition since a positive impact on weaning adaptation and gut health has been suggested based on stimulation of the functional maturation of the gastrointestinal tract of suckling rats and piglets (24,25,34,39). In contrast, several detrimental effects of PHA on gut microbiota and physiology such as reduced growth rates, loss of body weight, diarrhea, and epithelial hyperplasia have been demonstrated in several species (4,5,12,33).Previously, PHA has been reported to dose dependently induce bacterial overgrowth in the intestine, which is associated with weight loss, malabsorption, and villus damage (2, 3). It has been suggested that PHA increases the turnover of epithelial cells, promoting the expression of mannosylated receptor glycans on the gut surface and thus leading to increased adhesion and subsequent proliferation of mannose-sensitive type 1-fimbriated Escherichia coli (31, 32). One would assume that based on this mechanism multiple bacterial strains besides E. coli that adhere to these additional receptor sites are stimulated by PHA. Therefore, the first aim of the present study was to investigate the influence of PHA on overall composition and dynamics of the intestinal and fecal microbiota of young adult rats. To this end, fragment length polymorphism analysis of the amplified 16S ribosomal terminal restriction fragment (T-RFLP) was performed and the 16S-23S ribosomal intergenic spacer (IGS) was evaluated, in addition to bacterial plate counts and real-time PCR quantification of exemplary bacteria.In addition to E. coli, mannose-specific adhesion has been demonstrated for Lactobacillus plantarum, e.g., the probiotic strain 299v (1, 29). Addition of mannose-adhering probiotics may prevent disturbances of the intestinal microbiota like overgrowth of E. coli and could therefore contribute to an improved introduction of PHA in weaning feeds. Therefore, the second objective of the present study was to assess whether an orally administered mannose-adhering L. plantarum strain could prevent E. coli stimulation in the PHA-affected rat intestine by mannose-specific competitive exclusion and thereby counteract the adverse effects of PHA.Crude red kidney bean extract containing PHA was purchased from the Laboratory of Biochemistry and Glycobiology at the University of Ghent, Belgium. It was isolated from Phaseolus vulgaris basically according to the method of Pusztai et al. (32).Specific-pathogen-free male Wistar rats (WU, Harlan, Horst, The Netherlands), eight per group, 8 weeks old, were housed i...