Over the years several β-glucan transferases from yeast and fungi have been reported, but enzymes with such an activity from bacteria have not been characterized so far. In this work, we describe the cloning and expression of genes encoding β-glucosyltransferase domains of glycosyl hydrolase family GH17 from three species of proteobacteria: Pseudomonas aeruginosa PAO1, P. putida KT2440 and Azotobacter vinelandii ATCC BAA-1303. The encoded enzymes of these GH17 domains turned out to have a non-Leloir trans-β-glucosylation activity, as they do not use activated nucleotide sugar as donor, but transfer a glycosyl group from a β-glucan donor to a β-glucan acceptor. More particularly, the activity of the three recombinant enzymes on linear (β1 → 3)-linked gluco-oligosaccharides (Lam-Glc4–9) and their corresponding alditols (Lam-Glc4–9-ol) was studied. Detailed structural analysis, based on thin-layer chromatography, matrix-assisted laser desorption ionization time-of-flight mass spectrometry, electrospray ionization mass spectrometry, and 1D/2D 1H and 13C nuclear magnetic resonance data, revealed diverse product spectra. Depending on the enzyme used, besides (β1 → 3)-elongation activity, (β1 → 4)- or (β1 → 6)-elongation, or (β1 → 6)-branching activities were also detected.