Under conditions of high aeration and limiting availability of combined nitrogen, A. brasilense cells differentiate into aggregating cells and form dense flocs that are visible to the naked eye (Sadasivan & Neyra, 1985; Burdman et al., 1998). Flocs are formed by cell-to-cell aggregation between nonmotile cells embedded in BMS-354825 clinical trial a dense extracellular matrix (Burdman et al., 2000b). Flocculation correlates with, and likely requires the production of, arabinose-rich exopolysaccharides (Bahat-Samet et al., 2004). Scanning electron and fluorescence microscopy studies of A. brasilense aggregating cells indicate the presence of fibrillar

material connecting cells to each other or to biotic or abiotic substrates (Bashan et al., 1986, 1991). These fibrils seem to be absent in nonaggregating cells or mutant strains that are defective in aggregation, suggesting that they may play a role in promoting this behavior (Burdman et see more al., 1998; Skvortsov &

Ignatov, 1998). The detailed biochemical composition of this fibrillar material remains unknown, although it is possible that it is related to exopolysaccharide production (Bahat-Samet et al., 2004). In support of this idea, the degree of bacterial aggregation appears to correlate with the amount and composition of exopolysaccharide produced by several A. brasilense strains (Burdman et al., 1998). Chemotaxis is perhaps the most-studied signal transduction pathway in bacteria (reviewed in Sourjik, 2004; Wadhams & Armitage, 2004; Parkinson et al., 2005; Hazelbauer enough et al., 2008). Despite the identification of homologous chemotaxis systems in phylogenetically distant bacteria and archaeal species, there is a huge diversity in both the number of chemotaxis operons

encoded within bacterial genomes and their physiological roles (Wadhams & Armitage, 2004). Recent studies have shown that the functions of chemotaxis-like pathways are not limited to the regulation of motility patterns, but also include the regulation of biofilm formation, exopolysaccharide production, and cell-to-cell interactions (Black & Yang, 2003; Hickman et al., 2005; Yang & Li, 2005; Caiazza et al., 2007). In prototypical chemotaxis, the histidine kinase CheA and the response regulator CheY form a two-component signal transduction system, which ultimately modulate the probability of changes in the direction of rotation of flagellar motors in response to specific environmental cues. Changes in the phosphorylation of CheY regulated by the CheA–CheY phosphorylation cascade modulate the affinity of CheY for the flagellar motor switch complex and thus chemotaxis. Surprisingly, in A. brasilense, strains carrying mutations in components of the Che1 chemotaxis-like pathway were found to be affected in their ability to interact by cell-to-cell aggregation and in flocculation.