Lactobacillus casei Ferments the N -Acetylglucosamine Moiety of Fucosyl-α-1,3- N -Acetylglucosamine and Excretes l -Fucose

We have previously characterized fromLactobacillus casei BL23 three α-l -fucosidases, AlfA, AlfB, and AlfC, which hydrolyzein vitro natural fucosyl-oligosaccharides. In this work, we have shown thatL. casei is able to grow in the presence of fucosyl-α-1,3-N -acetylglucosamine (Fuc-α-1,3-GlcNAc) as a carbon source. Interestingly,L. casei excretes thel -fucose moiety during growth on Fuc-α-1,3-GlcNAc, indicating that only theN -acetylglucosamine moiety is being metabolized. Analysis of the genomic sequence ofL. casei BL23 shows that downstream fromalfB , which encodes the α-l -fucosidase AlfB, a gene,alfR , that encodes a transcriptional regulator is present. Divergently fromalfB , three genes,alfEFG , that encode proteins with homology to the enzyme IIAB (EIIAB), EIIC, and EIID components of a mannose-class phosphoenolpyruvate:sugar phosphotransferase system (PTS) are present. Inactivation of eitheralfB oralfF abolishes the growth ofL. casei on Fuc-α-1,3-GlcNAc. This proves that AlfB is involved in Fuc-α-1,3-GlcNAc metabolism and that the transporter encoded byalfEFG participates in the uptake of this disaccharide. A mutation in the PTS general component enzyme I does not eliminate the utilization of Fuc-α-1,3-GlcNAc, suggesting that the transport via the PTS encoded byalfEFG is not coupled to phosphorylation of the disaccharide. Transcriptional analysis withalfR andccpA mutants shows that the two gene clustersalfBR andalfEFG are regulated by substrate-specific induction mediated by the inactivation of the transcriptional repressor AlfR and by carbon catabolite repression mediated by the catabolite control protein A (CcpA). This work reports for the first time the characterization of the physiological role of an α-l -fucosidase in lactic acid bacteria and the utilization of Fuc-α-1,3-GlcNAc as a carbon source for bacteria.