Tuning into xanthan: A conserved yet flexible polysaccharide utilization system in Microbacterium

Bacteria encounter structurally complex extracellular polysaccharides in natural environments, yet the regulatory and evolutionary basis of their utilization remains poorly understood. Here, we isolated a soil-derived Microbacterium strain, named Microbacterium xanthanicum UB-LE1, that grows on xanthan as the sole carbon source. We dissected the genetic and regulatory architecture underlying this capability. Genome sequencing combined with transcriptomic and proteomic profiling uncovered a discrete, strongly inducible regulon associated with xanthan utilization, encoding 23 proteins with five secreted proteins and three candidate transcriptional regulators. DNA-affinity purification sequencing confirmed two regulators binding to operons within the xanthan utilization locus. Comparative genomics across the Microbacteriaceae revealed conserved and lineage-specific features of this system and supports recent acquisition and modular integration of the locus, with at least two predominant architectural variants possibly shaped by substrate availability and ecological specialization. Coordinated induction at both the transcript and protein levels, together with two experimentally validated regulators, points to tight regulatory control of complex polysaccharide degradation in Microbacterium xanthanicum UB-LE1. Together, these findings provide mechanistic and evolutionary insight into how bacteria adapt to complex extracellular carbohydrates, expand current knowledge of xanthan turnover in microbial ecosystems, and establish a framework for exploring the emergence and diversification of specialized polysaccharide utilization pathways across bacterial taxa. IMPORTANCEMicroorganisms are central drivers of carbon turnover in soils and other terrestrial ecosystems, determining the availability of nutrients and shaping microbial community structure. A significant portion of soil carbon is contained in extracellular polysaccharides, yet the pathways by which microorganisms degrade these complex polymers remain poorly understood. Xanthan, a structurally complex and widely produced microbial exopolysaccharide, represents a persistent and largely overlooked carbon pool. By dissecting the genetic, regulatory, and evolutionary basis of xanthan utilization in Microbacterium xanthanicum UB-LE1, this study advances our understanding of how soil bacteria adapt to complex extracellular carbohydrates and how substrate availability shapes the emergence and diversification of specialized metabolic pathways. Importantly, the identification of additional xanthan-active enzymes and regulatory components in M. xanthanicum UB-LE1 opens opportunities for targeted modification of xanthan structure and properties, paving the way for new biotechnological applications in food, materials, and industrial biotechnology, while linking microbial ecology to functional innovation.