Relaxed control of sugar utilization in Lactobacillus brevis

Prioritization of sugar consumption is a common theme in bacterial growth and a problem for complete utilization of five and six carbon sugars derived from lignocellulose. Growth studies show that Lactobacillus brevis simultaneously consumes numerous carbon sources and appears to lack normal hierarchical control of carbohydrate utilization. Analysis of several independent L. brevis isolates indicated that co-utilization of xylose and glucose is a common trait for this species. Moreover, carbohydrates that can be used as a single carbon source are simultaneously utilized with glucose. Analysis of the proteome of L. brevis cells grown on glucose, xylose or a glucose/xylose mixture revealed the constitutive expression of the enzymes of the heterofermentative pathway. In addition, fermentative mass balances between mixed sugar inputs and end-products indicated that both glucose and xylose are simultaneously metabolized through the heterofermentative pathway. Proteomic and mRNA analyses revealed that genes in the xyl operon were expressed in the cells grown on xylose or on glucose/xylose mixtures but not in those grown on glucose alone. However, the expression level of XylA and XylB proteins in cells grown on a glucose/xylose mixture was reduced 2.7-fold from that observed in cells grown solely on xylose. These results suggest that regulation of xylose utilization in L. brevis is not stringently controlled as seen in other lactic acid bacteria, where carbon catabolite repression operates to prioritize carbohydrate utilization more rigorously.