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In spite of advantages in bio-based chemical and energy production, lignocellulosic biomass is still not a top choice of carbon source for microorganisms. Sugar heterogeneity derived from lignocelluloses hamper the flexibility of process design and decrease the productivity and yield. Prioritization of sugar consumption is a common theme in bacterial growth and a problem for complete utilization of five and six carbon sugars. Growth studies show that Lactobacillus brevis, Lactobacillus buchneri, and Lactobacillus pentosus JH5XP5 simultaneously consumes numerous carbon sources and appears to lack normal hierarchical control of carbohydrate utilization. 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. Genomics and proteomics analysis of L. buchneri and L. pentosus JH5XP5 exhibit the same trait showing the constitutive expression of xyl operons. It suggests regulation of xylose utilization in is not stringently controlled as seen in other lactic acid bacteria, where carbon catabolite repression operates to prioritize carbohydrate utilization more rigorously. Also, the important of transport system to overcome the hierarchical regulation of sugar utilization.