Flavins mediate extracellular electron transfer in Gram-positive Bacillus megaterium strain LLD-1.

The extracellular electron transfer (EET) mechanism of an isolated Gram-positive Bacillus megaterium strain (LLD-1), identified by 16S rRNA gene sequencing and physiological analysis, was investigated in the present study. The electrochemical activity of strain LLD-1 was confirmed by electrochemical E-t and amperometric I-t tests. Flavins in culture suspension from strain LLD-1 were further proved to be able to act as electron shuttles, strengthening the electron transfer from LLD-1 to the electrode. The output voltage and current output were increased 2.8 times and 3.7 times, respectively, by adding 100nM exogenetic flavins into microbial fuel cells inoculated with LLD-1. Electricity generation by LLD-1 from different carbon sources can be enhanced by adding 100nM exogenetic flavins. This study indicated that flavins were essential to the EET process of the Gram-positive strain LLD-1. Furthermore, a putative EET model for B. megaterium strain LLD-1 and even for Gram-positive bacteria was proposed.

Effective methods for extracting extracellular polymeric substances from Shewanella oneidensis MR-1.

Extracellular polymeric substances (EPS) play crucial roles in bio-aggregate formation and survival of bacterial cells. To develop an effective but harmless method for EPS extraction from Shewanella oneidensis MR-1, five extraction methods, i.e. centrifugation (control), heating (40, 45, 50, and 60 °C), and treatments with H2SO4, ethylenediaminetetraacetic acid (EDTA) and NaOH, were examined, respectively. Results from scanning electron microscope and flow cytometric analyses indicate that MR-1 cells were severely broken by H2SO4, NaOH and heating temperature ≥45 °C. Proteins and polysaccharides in EPS extracted by heating at 40 °C were 7.12 and 1.60 mg g-1 dry cell, respectively. Although EDTA treatment had a relatively lower yield of EPS (proteins and polysaccharides yields of 5.15 and 1.30 mg g-1 dry cell, respectively), cell lysis was barely found after EPS extraction. Three peaks were identified from the three-dimensional excitation-emission matrix spectrum of each EPS sample, suggesting the presence of protein-like substances. Furthermore, the peak intensity was in good accordance with protein concentration measured by the chemical analysis. In short, heating (40 °C) and EDTA treatments were found the most suitable methods for EPS extraction considering the cell lysis and EPS content, composition and functional groups together.