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Rhizobium Species Form Biofilms: A Survival Strategy?

Saturday, April 13, 2002 - 2:20pm - 3:10pm
Keller 3-180
Ann Hirsch (University of California, Los Angeles)
Joint work with N.A. Fujishige, K.S. Jankaew, and C.J. Butcher (Department of Molecular, Cell and Developmental Biology, University of California, Los Angeles).

We found that transfer of Sinorhizobium meliloti nod genes to Bradyrhizobium japonicum and to Rhizobium leguminosarum bv. viciae allowed these strains, which normally nodulate soybean and pea, respectively, to form nodules on the roots of transgenic alfalfa plants that carried either the soybean (SBL) or the pea (PSL) lectin genes. Moreover, significantly more bacteria attached to the transgenic roots than to the non-transgenic roots (van Rhijn et al., 2001). From preliminary studies using the electron microscope, we observed that the transconjugant bacterial strains were decorated with numerous pili/fimbriae, which as a rule are very difficult to see with TEM in rhizobial species. Because fimbriae are important for biofilm formation in numerous other bacteria, these observations led us to examine whether or not rhizobial species were capable of forming biofilms on abiotic surfaces, in addition to biotic surfaces, with the goal of identifying genes that are important for biofilm formation. Rhizobial species do not form spores and hence, it is unclear how they survive in the absence of their hosts or in soils that are N-sufficient where their hosts do not develop nitrogen-fixing nodules. Because biofilms are a strategy that many bacteria use to survive in aquatic and terrestrial environments, we analyzed biofilm formation by rhizobial species. We focused our studies on S. meliloti because the genome has been completely sequenced, although we have data demonstrating that R. leguminosarum bv. viciae also forms biofilms in vitro. Preliminary experiments indicate that there are major differences between the one-dimensional protein profiles of biofilmed versus planktonic rhizobia.

van Rhijn, P., Fujishige, N.A., Lim, P.-O., and Hirsch, A.M. 2001. Sugar-binding activity of pea (Pisum sativum) lectin is essential for heterologous infection of transgenic alfalfa (Medicago sativa L.) plants by Rhizobium leguminosarum biovar viciae. Plant Physiol. 125:133-144.