Detection, Characterization, and Biological Effect of Quorum-Sensing Signaling Molecules in Peanut-Nodulating Bradyrhizobia

Abstract

Bacteria of the genus Bradyrhizobium are able to establish a symbiotic relationship with peanut (Arachis hypogaea) root cells and to fix atmospheric nitrogen by converting it to nitrogenous compounds. Quorum sensing (QS) is a cell-cell communication mechanism employed by a variety of bacterial species to coordinate behavior at a community level through regulation of gene expression. The QS process depends on bacterial production of various signaling molecules, among which the N-acylhomoserine lactones (AHLs) are most commonly used by Gram-negative bacteria. Some previous reports have shown the production of QS signaling molecules by various rhizobia, but little is known regarding mechanisms of communication among peanut-nodulating strains. The aims of this study were to identify and characterize QS signals produced by peanut-nodulating bradyrhizobial strains and to evaluate their effects on processes related to cell interaction. Detection of AHLs in 53 rhizobial strains was performed using the biosensor strains Agrobacterium tumefaciens NTL4 (pZLR4) and Chromobacterium violaceum CV026 for AHLs with long and short acyl chains, respectively. None of the strains screened were found to produce AHLs with short acyl chains, but 14 strains produced AHLs with long acyl chains. These 14 AHL-producing strains were further studied by quantification of β-galactosidase activity levels (AHL-like inducer activity) in NTL4 (pZLR4). Strains displaying moderate to high levels of AHL-like inducer activity were subjected to chemical identification of signaling molecules by high-performance liquid chromatography coupled to mass spectrometry (LC-MS/MS). For each AHL-producing strain, we found at least four different AHLs, corresponding to N-hexanoyl-dl-homoserine lactone (C6), N-(3-oxodecanoyl)-l-homoserine lactone (3OC10), N-(3-oxododecanoyl)-l-homoserine lactone (3OC12), and N-(3-oxotetradecanoyl)-l-homoserine lactone (3OC14). Biological roles of 3OC10, 3OC12, and 3OC14 AHLs were evaluated in both AHL-producing and -non-producing peanut-nodulating strains. Bacterial processes related to survival and nodulation, including motility, biofilm formation, and cell aggregation, were affected or modified by the exogenous addition of increasing concentrations of synthetic AHLs. Our results clearly demonstrate the existence of cell communication mechanisms among bradyrhizobial strains symbiotic of peanut. AHLs with long acyl chains appear to be signaling molecules regulating important QS physiological processes in these bacteria.

Exopolyphosphatase of Pseudomonas aeruginosa is essential for the production of virulence factors, and its expression is controlled by NtrC and PhoB acting at two interspaced promoters

“…Biofilm formation capacity was determined macroscopically (Nievas et al, 2012). Briefly, glass tubes were inoculated with 800 ml LB medium (OD 600 0.5) and incubated with shaking for 24 h at 37 uC.…”

Effect of the plant flavonoid luteolin on Ensifer meliloti 3001 phenotypic responses

“…In certain species, the biofilm formation is an important feature for colonization and/or host invasion (Nievas et al 2012;Perez-Montano et al 2014). The biofilm formation assay performed on E. meliloti 3001 revealed that luteolin affected the biofilm formation on rich medium and minimal medium (Fig.…”

“…The ICDs for which a significant reduction, t test, P < 0.05 or t test, P < 0.01, of lag time induced by luteolin was detected are marked with * or **, respectively. ICDs for which statistical analysis was not performed, since the lag time of untreated cultures (0 μM luteolin) was higher than 90 h, are marked as Bnd^(not determined) Effect of luteolin on motility proficiency In rhizobia, motility has been shown not to be essential for nodulation, but it allows the bacteria to find their specific host legume and establish symbiosis (Nievas et al 2012). Bacterial motility is an energetically costly process, and therefore must be finely regulated (Nievas et al 2012).…”

Bacterial biofilms and quorum sensing: fidelity in bioremediation technology

“…QS positively regulates the auto-aggregation (or co-aggregation) process, enhancing the survival of microorganisms and attachment to the substratum (Nievas, Bogino, Sorroche, & Giordano, 2012). The aggregation process is beneficial for aerobic granule (AG) formation.…”