Use of suppressor analysis to find genes involved in the colonization deficiency of a Bacteroides thetaiotaomicron mutant unable to grow on the host-derived mucopolysaccharides chondroitin sulfate and heparin.

Bacteroides thetaiotaomicron, one of the numerically predominant species of human colonic bacteria, can ferment two types of host-derived mucopolysaccharides, chondroitin sulfate (CS) and heparin (HP). Originally, the pathways for utilization of CS and HP appeared to be completely independent of each other, but we have recently identified a gene, chuR, that links the two utilization systems. chuR is probably a regulatory gene, but it controls only a small subset of genes involved in CS and HP utilization. Some of the genes controlled by chuR are important for survival of B. thetaiotaomicron in the colon because a mutant that no longer produced ChuR was unable to compete with the wild type for colonization of the intestinal tract of germfree mice. In an attempt to identify genes that either were controlled by ChuR or encoded proteins that interacted with ChuR, we used transposon mutagenesis to generate suppressor mutations that restored the ability of a chuR disruption mutant to grow on CS and HP. Two classes of suppressors were isolated. One class grew as well as the wild type on CS and HP and had recovered the ability to compete with the wild type for colonization of the germfree mouse intestinal tract. A second class grew more slowly on CS and HP and reached only a half-maximum level on CS. This mutant still had a colonization defect. Representatives of both classes of suppressor mutants have been characterized, and the results of this analysis suggest that the transposon insertions in the suppressor mutants probably affected regulatory genes whose products interact with ChuR.