Analysis of Sinorhizobium meliloti native isolates from Argentina revealed a positive correlation between biofilm formation and autoaggregation.

SORROCHE, F., SPESIA, M., ABOD, A.S., ZORREGUIETA, A., GIORDANO, W.

San Miguel de Tucumán

Congreso; VII congreso Argentino de Microbiología General (SAMIGE); 2011

SAMIGE

S. meliloti is an alfaproteobacteria with the potential to establish a symbiotic chronic infection inside plant-derived organs known as root nodules. Beside their symbiotic way of life, these rhizobia can live not only in a planktonic state, but also in a biofilm state. The expR regulator gene, in conjunction with a quorum sensing signal molecule, controls the production of the symbiotically active galactoglucan (EPS II), which is critical for biofilm formation on plastic surfaces and planktonic autoaggregation. Wild type S. meliloti reference strains carrying non-functional expR loci (and thus, unable to synthesize EPS II) fail to autoaggregate and develop a relatively small biomass adhered to plastic surfaces. In an attempt to characterize indigenous, undomesticated S. meliloti strains, we isolated bacteria form alfalfa root nodules growing in fields without previous known inoculation procedures. All isolates developed mucous colonies (indicating EPS II production) and after inoculation, they were able to elicit root nodules formation on alfalfa. 16S rRNA sequencing revealed a high homology with S. meliloti strains. Unlike some reference wild-type strains in which the expR locus is interrupted with an insertion sequence, these native isolates showed a normal-sized expR locus, as revealed by PCR analysis. Planktonic autoaggregation percentages and biofilm formation indexes were obtained from each isolate. Interestingly, we found a positive correlation between both variables. In order to confirm the participation of EPS II in intercellular interactions for native isolates, the expA::Tn5 mutant allele (blocking EPS II synthesis) was mobilized to three highly autoaggregative native strains, and the resulting transductant strains showed a dry colony phenotype, reduced biofilm formation on plastic, and a poorly autoaggregative phenotype. Taken together, these observations indicate that the strength/stability of cell-cell interactions in the context of planktonic and biofilm cells are quantitatively related. In the particular case of highly autoaggregative/biofilm native bacteria, mutant analysis confirmed that EPS II plays a critical role in cell-cell interactions in both, sessile and planktonic cells.