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. 2019 Dec 11;9(70):40903-40909.
doi: 10.1039/c9ra08045g. eCollection 2019 Dec 9.

Flavin-mediated extracellular electron transfer in Gram-positive bacteria Bacillus cereus DIF1 and Rhodococcus ruber DIF2

Tian Tian  1 , Xiaoyang Fan  1 , Man Feng  2 , Lin Su  1 , Wen Zhang  1   3 , Huimei Chi  1   2 , Degang Fu  1
Affiliations

Flavin-mediated extracellular electron transfer in Gram-positive bacteria Bacillus cereus DIF1 and Rhodococcus ruber DIF2

Tian Tian et al. RSC Adv. .

Abstract

Flavin-mediated extracellular electron transfer was studied in two Gram-positive bacteria: Bacillus cereus strain DIF1 and Rhodococcus ruber strain DIF2. The electrochemical activities of these strains were confirmed using amperometric I-t curves and cyclic voltammetry (CV). Spent anodes with biofilms in fresh anolytes showed no redox peaks, while new anodes in the spent broth showed relative redox peaks using CV measurements, indicating the presence of a redox electron mediator secreted by bacteria. Adding riboflavins (RF) and flavin mononucleotide (FMN) improved the electron transfer of the microbial fuel cells inoculated with the two strains. The redox peaks indicated that flavins existed in the anolyte, and HPLC analysis showed that RF and FMN were secreted by the two bacterial strains. The concentration of RF increased until the bacteria grew to the log phase in microbial fuel cells. The concentration of RF decreased and that of FMN increased after the log phase. The two strains secreted FMN only in the microbial fuel cell. These results confirmed that the electrochemical activity mediated by flavins and FMN is essential in the extracellular electron transfer process in the strains DIF1 and DIF2.

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Conflict of interest statement

There are no conflicts of interest to declare.

Figures

Fig. 1
Fig. 1. (A) Scanning electron micrograph of the Bacillus cereus strain DIF1 on a carbon paper anode. (B) Scanning electron micrograph of the Rhodococcus ruber strain DIF2 on a carbon paper anode.
Fig. 2
Fig. 2. Cyclic voltammograms of new electrodes in spent anolytes and spent electrodes in new anolytes compared with the original operation in MFCs. (A) Bacillus cereus strain DIF1, (B) Rhodococcus ruber strain DIF2.
Fig. 3
Fig. 3. CVs for the different culture media. (A) Bacillus cereus strain DIF1. (B) Rhodococcus ruber strain DIF2.
Fig. 4
Fig. 4. Amperometric I–t curves of the two bacteria when adding RF and FMN. (A) Bacillus cereus strain DIF1, (B) Rhodococcus ruber strain DIF2.
Fig. 5
Fig. 5. HPLC chromatograms of anolytes compared with those of riboflavins (A) HPLC chromatograms for reference standard solutions and samples. (B) Partial enlargement of HPLC chromatograms. Sample 1 and sample 2 represent the anolytes of strain DIF1 and strain DIF2, respectively.
Fig. 6
Fig. 6. The growth curve of bacteria and the concentration curve of flavins. (A) Bacillus cereus strain DIF1, (B) Rhodococcus ruber strain DIF2.
Fig. 7
Fig. 7. HPLC chromatograms of RF and FMN in anolytes compared with those in general medium. Peaks of FMN indicating the anolytes from MFC.
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