Microbial physiology

The impact of mixtures of xylose and glucose on the microbial diversity and fermentative metabolism of sequencing-batch or continuous enrichment cultures

Julius L. Rombouts, Galvin Mos, David G. Weissbrodt, Robbert Kleerebezem,
Mark. C.M. van Loosdrecht

Delft University of technology, Department of Biotechnology, j.l.rombouts@tudelft.nl

Glucose and xylose are the two most abundant monomers found in lignocellulosic waste streams Microbial diversity plays an important role in the functioning of mixed-culture biotechnologies. Complexity in the carbohydrate source could be one of the governing factors. The effect of feeding equivalent substrates to a microbial community, such a xylose and glucose is not well understood in terms of number of dominant species and how these species compete for substrate. We compared the metabolism and microbial community structure in a continuous-flow stirred tank reactor (CSTR) and a sequencing batch reactor (SBR) fed with a mixture of xylose and glucose.  We hypothesise that a CSTR will select for generalist species, taking up both substrates. We used 16S rRNA gene sequencing and fluorescent in situ hybridisation to accurately determine the microbial community structures. Both enrichments were stoichiometrically and kinetically characterised. The CSTR enrichment culture was dominated by Clostridium intestinale (91%±2%), which confirms the generalist hypothesis. The closest related strain (99% identity of the 16 rRNA gene) showed presence of both a high affinity xylose and glucose transporter. The SBR showed a large fraction of Enterobacteriaceae (75%±8%), which was dominated by Citrobacter freundii and a minor fraction of Raoultella ornithinolytica. Citrobacter freundii ferments xylose and glucose in a non-diauxic fashion. Clearly, a non-diauxic generalist outcompetes specialists and diauxic generalists in a SBR environment. A non-diauxic generalist fermenting xylose and glucose simultaneously is thus a good candidate organism when designing an industrial fermentation process using a lignocellulosic feed.