The genome of the Azorhizobium caulinodans ORS571 contains a unique chemotaxis gene cluster (che) including five chemotaxis genes: cheA, cheW, cheY, cheB, and cheR. Analysis of the role of the chemotaxis cluster of A. caulinodans using deletion mutant strains revealed that CheA or the Che signaling pathway controls chemotaxis behavior and flagella-driven motility and plays important roles in formation of biofilms and production of extracellular polysaccharides (EPS). Furthermore, the deletion mutants (ΔcheA and ΔcheA-R) were defective in competitive adsorption and colonization on the root surface of host plants. In addition, a functional CheA or Che pathway promoted competitive nodulation on roots and stems. Interestingly, a nonflagellated mutant, ΔfliM, displayed a phenotype highly similar to that of the ΔcheA or ΔcheA-R mutant strains. These findings suggest that through controlling flagella-driven motility behavior, the chemotaxis signaling pathway in A. caulinodans coordinates biofilm formation, EPS, and competitive colonization and nodulation.
Bioaugmentation can facilitate hydrogen production from complex organic substrates, but it still is unknown how indigenous microbial communities respond to the added bacteria. Here, using a Hydrogenispora ethanolica LX-B (named as LX-B) bioaugmentation experiments, the distribution of metabolites and the responses of indigenous bacterial communities were investigated via batch cultivation (BC) and repeated batch cultivation (RBC). In BC the LX-B/sludge ratio of 0.12 achieved substantial high hydrogen yield, which was over twice that of control. In RBC one-time bioaugmentation and repeated batch bioaugmentation of LX-B resulted in the hydrogen yield that was average 1.2-fold and 0.8-fold higher than that in control, respectively. This improved hydrogen production performance mainly benefited from a shift in composition of the indigenous bacterial community caused by LX-B bioaugmentation. The findings represented an important step in understanding the relationship between bioaugmentation, a shift in bacterial communities, and altered bioreactor performance.
Azorhizobium caulinodans ORS571 can induce nodule formation on the roots and the stems of its host legume, Sesbania rostrata. Plant exudates are essential in the dialogue between microbes and their host plant and, in particular, amino acids can play an important role in the chemotactic response of bacteria. Histidine, arginine, and aspartate, which are the three most abundant amino acids present in S. rostrata seed exudates, behave as chemoattractants toward A. caulinodans. A position-specific-iterated BLAST analysis of the methyl-accepting chemotaxis proteins (MCPs) (chemoreceptors) in the genome of A. caulinodans was performed. Among the 43 MCP homologs, two MCPs harboring a dCache domain were selected as possible cognate amino acid MCPs. After analysis of relative gene expression levels and construction of a gene-deleted mutant strain, one of them, AZC_0821 designed as TlpH, was confirmed to be responsible for the chemotactic response to the three amino acids. In addition, it was found that these three amino acids can also influence chemotaxis of A. caulinodans independently of the chemosensory receptors, by being involved in the increase of the expression level of several che and fla genes involved in the chemotaxis pathway and flagella synthesis. Thus, the contribution of amino acids present in seed exudates is directly related to the role as chemoattractants and indirectly related to the role in the regulation of expression of key genes involved in chemotaxis and motility. This “dual role” is likely to influence the formation of biofilms by A. caulinodans and the host root colonization properties of this bacterium.
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