مجتبی محسنی

Ethanol is a renewable and safe fuel and it is mainly produced based on microbial fermentation. The present study aims to isolate and identify ethanol producing Zymomonas spp. from natural environments with characterization, optimization and evaluation of their ethanol productivity. Samples from various fruits and sap of plants were screened for ethanol producing bacteria on RM medium. Ethanol producing isolates were selected for characterization. In addition, bacterial growth and ethanol production conditions were optimized based on pH, temperature, agitation, time and initial glucose concentration. The morphological, physiological and molecular characterization was investigated for identification of the isolates. Of all the 10 ethanol producing isolates, the two highest producing isolates were selected for further studies. Both of them were motile and catalase positive but failed to hydrolyze gelatin and produce H2S. Among them, isolate ZYM6 exhibited highest ethanol yield 6.28 gL-1 with optimum pH 6 and growth temperature of 30 ˚C. In addition, isolates ZYM6 and ZYM10 exhibited highest ethanol yield: 15.00 gL-1 and 12.00 gL-1 with xylose and tryptophan, respectively. Thus, the optimum condition for ethanol production was a medium composed of pH 6, growth temperature of 30-35 ˚C for 24-48 hours and xylose and tryptophan as carbon and nitrogen sources. The results of morphological and physiological characteristics showed that isolates ZYM6 and ZYM10 belong to Zymomonas. Moreover, 16S rRNA sequencing and phylogenetic analyses exhibited that isolates ZYM6 and ZYM10 were similar to Zymomonas mobilis with 99% homology. These native Zymomonas spp. can produce ethanol with high yield. In addition, xylose is a feasible feedstock for ethanol fermentation with high efficiency while using these isolates.