Functional Role of Bacteria from Invasive Phragmites australis in Promotion of Host Growth

We hypothesize that bacterial endophytes may enhance the competitiveness and invasiveness of Phragmites australis. To evaluate this hypothesis, endophytic bacteria were isolated from P. australis. The majority of the shoot meristem isolates represent species from phyla Firmicutes, Proteobacteria, and Actinobacteria. We chose one species from each phylum to characterize further and to conduct growth promotion experiments in Phragmites. Bacteria tested include Bacillus amyloliquefaciens A9a, Achromobacter spanius B1, and Microbacterium oxydans B2. Isolates were characterized for known growth promotional traits, including indole acetic acid (IAA) production, secretion of hydrolytic enzymes, phosphate solubilization, and antibiosis activity. Potentially defensive antimicrobial lipopeptides were assayed for through application of co-culturing experiments and mass spectrometer analysis. B. amyloliquefaciens A9a and M. oxydans B2 produced IAA. B. amyloliquefaciens A9a secreted antifungal lipopeptides. Capability to promote growth of P. australis under low nitrogen conditions was evaluated in greenhouse experiments. All three isolates were found to increase the growth of P. australis under low soil nitrogen conditions and showed increased absorption of isotopic nitrogen into plants. This suggests that the Phragmites microbes we evaluated most likely promote growth of Phragmites by enhanced scavenging of nitrogenous compounds from the rhizosphere and transfer to host roots. Collectively, our results support the hypothesis that endophytic bacteria play a role in enhancing growth of P. australis in natural populations. Gaining a better understanding of the precise contributions and mechanisms of endophytes in enabling P. australis to develop high densities rapidly could lead to new symbiosis-based strategies for management and control of the host.

Structural Characterization of a Novel Antioxidant Pigment Produced by a Photochromogenic Microbacterium oxydans Strain

The Microbacteriaceae family, such as Microbacterium, is well known for its ability to produce carotenoid-type pigments, but little has been published on the structure of such pigments. Here, we isolated the yellow pigment that is responsible for the yellowish color of a Microbacterium oxydans strain isolated from a decomposing stump of a resinous tree. The pigment, which is synthesized when the bacterium is grown under light, was purified and characterized using several spectroscopic analyses, such as ultraviolet-visible spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR), ¹H and ¹³C nuclear magnetic resonance (¹H NMR, ¹³C NMR), and high-resolution mass spectrometry (HRMS). From these analysis, a molecular formula (C₂₇H₄₂O₂) and a chemical structure (8-hydroxymethyl-2,4,12-trimethyl-14-(2,6,6-trimethyl-cyclohex-2-enyl)-teradeca-3,7,9,11,13-pentan-2-ol) were deduced. The chemical properties of the pigment, such as aqueous stability at different pH, stability in different organic solvents, and antioxidant capacity, are also reported. Together, these data and previous studies have resulted in the identification of a new antioxidant pigment produced by M. oxydans. To the best of our knowledge, this is the first thorough investigation of this carotenoid-like pigment in the Microbacterium genera.

Three new fusidic acid derivatives and their antibacterial activity

Two steroid acids, cephalosporin P1 and isocephalosporin P1, were isolated from Hapsidospora irregularis FERM BP-2511. These compounds are structurally related to fusidic acid. Their NMR data were completely assigned on the basis of the 2D NMR spectra. Incubation of these two compounds with Microbacterium oxydans CGMCC 1788 in Luria-Bertani broth yielded the same set of three new 3-dehydrogenated products, 3-keto-isocephalosporin P1, 3-keto-cephalosporin P1 and 6-deacetyl-3-keto-cephalosporin P1. The final pH of the bacterial culture was 9.0. Incubation of 3-keto-isocephalosporin P1 or 3-keto-cephalosporin P1 in Tris-HCl buffer (pH 9.0) revealed that these two compounds can convert to each other by shifting the acetyl group between C-6 and C-7. The acetyl group at C-6 or C-7 can also be removed by hydrolysis to yield the minor product 6-deacetyl-3-keto-cephalosporin P1. These fusidic acid derivatives were tested for the antibacterial activity against the Gram-positive pathogen Staphylococcus aureus. 3-Keto-cephalosporin P1 showed the highest activity among the five compounds, with a minimal inhibition concentration (MIC) of 4 μg/mL, which is more potent than the substrate cephalosporin P1. Both cephalosporin P1 and 3-keto-cephalosporin P1 were active against methicillin-resistant S. aureus, with the same MIC of 8 μg/mL.

Microbacterium oxydans, a novel alginate- and laminarin-degrading bacterium for the reutilization of brown-seaweed waste

There is a growing demand for the efficient treatment of seaweed waste. We identified six bacterial strains from the marine environment for the reutilization of brown-seaweed waste, and the most potentially useful strain, Microbacterium oxydans, was chosen and further investigated. Plate assays indicated that this bacterial isolate possessed both alginate lyase and laminarinase activities. The optimal inoculum size, pH, temperature and substrate concentration for the degradation of brown-seaweed polysaccharides by the isolate were as follows: 20% (v v(-1)), pH 6.0, 37 °C, and 5 g L(-1) for alginate and 20% (v v(-1)), pH 6.0, 30 °C, and 10 g L(-1) for laminarin, respectively. During 6 d in culture under the optimal conditions, the isolate produced 0.17 g L(-1) of reducing sugars from alginate with 11.0 U mL(-1) of maximal alginate lyase activity, and 5.11 and 2.88 g L(-1) of reducing sugars and glucose from laminarin, respectively. In particular, a fair amount of laminarin was degraded to glucose (28.8%) due to the isolate's exolytic laminarinase activity. As a result, the reutilization of brown-seaweed waste by this isolate appears to be possible for the production of reducing sugars as a valuable resource. This is the first study to directly demonstrate the ability of M. oxydans to degrade both alginate and laminarin.