Fig. 3: Levan induces SSM in B. subtilis and other soil bacteria.

A A schematic diagram of sucrose metabolism and the signal relay triggering SSM in B. subtilis. On the left of the diagram, it indicates that the extracellular sucrose is imported into cells by the SacP transporter, and then hydrolyzed into glucose-6-P and fructose by the SacA hydrolase. On the top, it illustrates that sucrose is metabolized extracellularly. The levansucrase SacB uses sucrose to synthesize polymeric fructoses (levan), and glucose. When needed, levan can be degraded into levanoligosaccharides by LevB, and into monomeric fructoses by SacC, which is then imported into cells by the transporter composed of LevDEFG. Levan likely indirectly actives the srfAA-AD operon, whose product (surfactin) triggers SSM in B. subtilis. B SSM by B. subtilis 3610 and various mutants on solid LB plates supplemented with sucrose (5 g/L), levan (0.2 g/L), or levanbiose (0.2 g/L). Pictures are representatives of at least three independent assays. C Pure levan, when supplied at 0.2 g/L, induces SSM and hyper-flagellation in B. subtilis 3610. D The structural analogues of levan (inulin, FOS, and dextran) cannot induce SSM by B. subtilis 3610 when provided at 0.2 g/L. E Levan (0.2 g/L) induces SSM by some soil bacteria (Serratia marcescens T4-3, Pectobacterium carotovorum subsp. carotovorum Z3-3, Xanthomonas oryzae pv. oryzae PXO99F, Pseudomonas protegens pf-5, and P. fluorescens 2p24). All pictures are representatives of at least three independent assays. All Petra dishes shown here have a diameter of 10 cm.