Search

Two new ethanologenic strains (FBR4 and FBR5) of Escherichia coli were constructed and used to ferment corn fiber hydrolysate. The strains carry the plasmid pLOI297, which contains the genes from Zymomonas mobilis necessary for efficiently converting pyruvate into ethanol. Both strains selectively maintained the plasmid when grown anaerobically. Each culture was serially transferred 10 times in anaerobic culture with sugar-limited medium containing xylose, but no selective antibiotic. An average of 93 and 95% of the FBR4 and FBR5 cells, respectively, maintained pLOI297 in anaerobic culture. The fermentation performances of the repeatedly transferred cultures were compared with those of cultures freshly revived from stock in pH-controlled batch fermentations with 10% (w/v) xylose. Fermentation results were similar for all the cultures. Fermentations were completed within 60 h and ethanol yields were 86-92% of theoretical. Maximal ethanol concentrations were 3.9-4.2% (w/v). The strains were also tested for their ability to ferment corn fiber hydrolysate, which contained 8.5% (w/v) total sugars (2.0% arabinose, 2.8% glucose, and 3.7% xylose). E. coli FBR5 produced more ethanol than FBR4 from the corn fiber hydrolysate. E. coli FBR5 fermented all but 0.4% (w/v) of the available sugar, whereas strain FBR4 left 1.6% unconsumed. The fermentation with FBR5 was completed within 55 h and yielded 0.46 g of ethanol/g of available sugar, 90% of the maximum obtainable.

Thirty strains of Butyrivibrio fibrisolvens isolated in diverse geographical locations were examined for esterase activity by using naphthyl esters of acetate, butyrate, caprylate, laurate, and palmitate. All strains possessed some esterase activity, and high levels of activity were observed with strains 49, H17c, S2, AcTF2, and LM8/1B. Esterase activity also was detected in other ruminal bacteria (Bacteroides ruminicola, Selenomonas ruminantium, Ruminobacter amylophilus, and Streptococcus bovis). For all B. fibrisolvens strains tested, naphthyl fatty acid esterase activity paralleled culture growth and was predominantly cell associated. With strains 49, CF4c, and S2, the activity was retained by protoplasts made from whole cells. Esterase activity was detected with all strains when grown on glucose, and some strains showed higher activity levels when grown on other substrates (larchwood xylan or citrus pectin). When nitrophenyl esters of fatty acids were used to measure esterase activity, generally four- to sevenfold-higher activity levels were detected, and with a number of strains substantial levels were found in the culture fluid. Cultures of these strains (H17c, NOR37, D1, and D30g) contained xylanase and acetyl xylan esterase activities, neither of which was associated to any great extent with the cells. Acetyl xylan esterase has not been previously detected in ruminal bacteria and may be important to overall digestion of forage by these organisms.