Production of ethanol from xylan by indigenous xylanolytic and ethanologenic bacteria isolated from fruit wastes

Xylan-containing fruit wastes can be valorized efficiently by xylanolytic microbes for the production of biofuels. The current study aimed to isolate and screen ethanol-producing bacteria from rotten fruit wastes that ferment xylose. Sixteen bacteria were isolated by spread-plate method from apple, banana, guava, and grape wastes on a solid medium supplemented with 5% xylan. The potential of ten bacterial isolates to degrade xylose was checked by biochemical tests. Bacterial strains XA2, XG1, and XG3 showed maximum CO 2 production in Durham tubes. XA2, XG2, and XG3 showed maximum zones (1–2.6 cm) by Congo-red staining. XG2, XG3, and XA2 showed maroon and pink color with triphenyltetrazolium demonstrating degradation of xylan. The selected bacterial isolates were subjected to ferment 5% xylan-supplemented minimal medium for ten days to investigate ethanologenesis. Bacterial isolate XG2 produced maximum ethanol of 3.069 ± 0.738 g/L on day 8. The sugar contents in the fermentation medium continued to be decreased as the experiments proceeded up to 10 days. Maximum enzyme activity was observed by both XA2 and XG2 as 0.817 ± 0.036 and 0.917 ± 0.059 μmol/min/L. Strains XA2 and XG2 were characterized as Enterococcus faeceum XA2 and Bacillus cereus XG2 (Accession Nos. OM971654 and OM970803) by comparing their 16 S rRNA gene sequences with the available sequences in GenBank each having resemblance of 98 %. The present study highlights the dual capacity of bacteria to degrade xylan as well as promote ethanol production by employing xylan as a carbon source.