Characterization of a gene cluster for sialoglycoconjugate utilization in Bacteroides fragilis

Recent analysis of the whole genome sequence of Bacteroides fragilis revealed extensive duplication of polysaccharide utilization genes in this anaerobe. Here we analyzed a unique 27-kb gene cluster (sgu) comprised of the 13 sialoglycoconjugates-utilization genes, which include the sialidase gene (nanH1) in B. fragilis strain YCH46. The genes were tightly organized and transcribed polycistronically. Comparative PCR scanning demonstrated that the sgu locus was conserved among the Bacteroides strains tested. Based on the transcriptional profiles generated by reverse transcriptase PCR, the sgu locus can be classified into at least three regulatory units : 1) sialic acid- or sialooligosaccharide-inducible genes, 2) constitutively expressed genes that can be down-regulated by catabolite repression, and 3) constitutively expressed genes. In vitro comparison of the growth of a sgu locus deletion mutant (SGUM172941) with a wild type strain indicates that this locus is necessary for B. fragilis to efficiently utilize mucin as a carbon source. Furthermore, SGUM172941 was defective in colonization of the intestines of germfree mice under competitive conditions. These data indicate that the sgu locus in B. fragilis plays a crucial role in the utilization of host-derived sialoglycoconjugates and the stable colonization of this anaerobe in the human gut.