초록
<P><B>Background</B></P><P>One major problem of ABE (acetone, butanol and ethanol) fermentation is high oxygen sensitivity of <I>Clostridium acetobutylicum</I>. Currently, no single strain has been isolated or genetically engineered to produce butanol effectively under aerobic conditions. In our previous work, a symbiotic system TSH06 has been developed successfully by our group, and two strains, <I>C. acetobutylicum</I> TSH1 and <I>Bacillus cereus</I> TSH2, were isolated from TSH06.</P><P><B>Results</B></P><P>Compared with single culture, TSH06 showed promotion on cell growth and solvent accumulation under microaerobic conditions. To simulate TSH06, a new symbiotic system was successfully re-constructed by adding living cells of <I>B. cereus</I> TSH2 into <I>C. acetobutylicum</I> TSH1 cultures. During the fermentation process, the function of <I>B. cereus</I> TSH2 was found to deplete oxygen and provide anaerobic environment for <I>C. acetobutylicum</I> TSH1. Furthermore, inoculation ratio of <I>C. acetobutylicum</I> TSH1 and <I>B. cereus</I> TSH2 affected butanol production. In a batch fermentation with optimized inoculation ratio of 5 % <I>C. acetobutylicum</I> TSH1 and 0.5 % <I>B. cereus</I> TSH2, 11.0 g/L butanol and 18.1 g/L ABE were produced under microaerobic static condition. In contrast to the single culture of <I>C. acetobutylicum</I> TSH1, the symbiotic system became more aerotolerant and was able to produce 11.2 g/L butanol in a 5 L bioreactor even with continuous 0.15 L/min air sparging. In addition, qPCR assay demonstrated that the abundance of <I>B. cereus</I> TSH2 increased quickly at first and then decreased sharply to lower than 1 %, whereas <I>C. acetobutylicum</I> TSH1 accounted for more than 99 % of the whole population in solventogenic phase.</P><P><B>Conclusions</B></P><P>The characterization of a novel symbiotic system on butanol fermentation was studied. The new symbiotic system re-constructed by co-culture of <I>C. acetobutylicum</I> TSH1 and <I>B. cereus</I> TSH2 showed excellent performance on butanol production under microaerobic conditions. <I>B. cereus</I> TSH2 was a good partner for <I>C. acetobutylicum</I> TSH1 by providing an anaerobic environment. During fermentation process, the high ratio of <I>Clostridium</I> and low ratio of <I>Bacillus</I> composition indicated that this symbiotic system was an effective and easily controlled cultivation model for ABE fermentation under microaerobic conditions.</P>