Understanding different dominance patterns in western Amazonian forests.

Dominance of neotropical tree communities by a few species is widely documented, but dominant trees show a variety of distributional patterns still poorly understood. Here, we used 503 forest inventory plots (93,719 individuals ≥2.5 cm diameter, 2609 species) to explore the relationships between local abundance, regional frequency and spatial aggregation of dominant species in four main habitat types in western Amazonia. Although the abundance-occupancy relationship is positive for the full dataset, we found that among dominant Amazonian tree species, there is a strong negative relationship between local abundance and regional frequency and/or spatial aggregation across habitat types. Our findings suggest an ecological trade-off whereby dominant species can be locally abundant (local dominants) or regionally widespread (widespread dominants), but rarely both (oligarchs). Given the importance of dominant species as drivers of diversity and ecosystem functioning, unravelling different dominance patterns is a research priority to direct conservation efforts in Amazonian forests.

Azospirillum baldaniorum Sp245 exploits Pseudomonas fluorescens A506 biofilm to overgrow in dual-species macrocolonies.

Biofilms are essential for plant-associated bacteria to colonize their host. In this work, we analysed the interaction of Azospirillum baldaniorum Sp245 and Pseudomonas fluorescens A506 in mixed macrocolony biofilms. We identified certain culture conditions where A. baldaniorum Sp245 exploits P. fluorescens A506 to boost its growth. Azospirillum growth increased proportionally to the initial number of pseudomonads building the biofilm, which in turn were negatively affected in their growth. Physical contact with P. fluorescens A506 was essential for A. baldaniorum Sp245 growth increase. Biofilm ultrastructure analysis revealed that Pseudomonas produces a thick structure that hosts Azospirillum cells in its interior. Additional experimentation demonstrated that Azospirillum growth boost is compromised when interacting with biofilm-deficient Pseudomonas mutants, and that a low oxygen concentration strongly induce A. baldaniorum Sp245 growth, overriding Pseudomonas stimulation. In this line, we used a microaerophilia reporter strain of A. baldaniorum Sp245 to confirm that dual-species macrocolonies contain a higher number of cells under microaerophilic conditions. Taking all the results into consideration, we propose that A. baldaniorum Sp245 can benefit from P. fluorescens A506 partnership in mixed biofilms by taking advantage of the low oxygen concentration and scaffold made up of Pseudomonas-derived matrix, to expand its growth.

Antibacterial activity of Bacillus licheniformis B6 against viability and biofilm formation of foodborne pathogens of health importance.

We studied a strain of Bacillus isolated from an artisanal tannery in Salta, Argentina. It was identified as Bacillus licheniformis B6 by 16 S phylogenetic analysis and MALDI TOF (GenBank accession code No. KP776730). The synthesis of lipopeptides by B6 and their antibacterial activity against clinical pathogenic strains was analyzed both in the cell-free supernatant (CFS) and in the crude fraction of lipopeptides (LF). Overall, the CFS did not significantly reduce the viability of the studied strains (Staphylococcus aureus 269 and ATCC 43,300, Escherichia coli 4591 and 25,922, Klebsiella sp. 1087 and 1101). However, LF at 9 mg/mL reduced the viability of those pathogenic strains by 2 and 3 log orders compared to those of the control. When the effects of LF and ampicillin were compared, they showed different sensitivity against pathogenic strains. For example, E. coli 4591 was the strain most resistant to ampicillin, requiring 250 mg/mL of antibiotic to achieve the same inhibitory effect as 9 mg/mL of B6 LF. SEM observations of the effect of LF on biofilm formation by E. coli 4591 and Klebsiella sp. 1087 clearly showed that biofilm structures were destabilized, these strains turning into weak biofilm formers. Signals in the CFS and LF corresponding to kurstakin and iturin were identified by MALDI TOF. Interestingly, surfactin was detected, rather than lichenysin, the expected lipopeptide in B. licheniformis species. Signals of bacitracin and fengycins were also found, the latter with a higher number of homologues and relative intensity in the LF than the other lipopeptides. These results show that the lipopeptides synthesized by B. licheniformis B6 have both potential antibacterial and anti-biofilm activity against pathogenic bacteria of health importance.

Friends or foes in the rhizosphere: traits of fluorescent Pseudomonas that hinder Azospirillum brasilense growth and root colonization.

Bacteria of the Azospirillum and Pseudomonas genera are ubiquitous members of the rhizosphere, where they stimulate plant growth. Given the outstanding capacity of pseudomonads to antagonize other microorganisms, we analyzed the interaction between these two bacterial groups to identify determinants of their compatibility. We could establish that, when in direct contact, certain Pseudomonas strains produce lethality on Azospirillum brasilense cells using an antibacterial type 6 secretion system. When analyzing the effect of Pseudomonas spp. diffusible metabolites on A. brasilense growth on King's B medium, we detected strong inhibitory effects, mostly mediated by siderophores. On Congo Red medium, both inhibitory and stimulatory effects were induced by unidentified compounds. Under this condition, Pseudomonas protegens CHA0 produced a Gac/Rsm-regulated antibiotic which specifically inhibited A. brasilense Sp7 but not Sp245. This effect was not associated with the production of 2,4-diacetylphloroglucinol. The three identified antagonism determinants were also active in vivo, producing a reduction of viable cells of A. brasilense in the roots of wheat seedlings when co-inoculated with pseudomonads. These results are relevant to the understanding of social dynamics in the rhizosphere and might aid in the selection of strains for mixed inoculants.