Uropathogenic Escherichia coli Strains Generally Lack Functional Trg and Tap Chemoreceptors Found in the Majority of E. coli Strains Strictly Residing in the Gut

Escherichia coli, the predominant facultative anaerobe of the diverse microflora found in the gastrointestinal tract in humans and animals (19), causes both diarrheal and intestinal diseases as well as extraintestinal disease, including urinary tract infection (UTI) (12). Uropathogenic E. coli (UPEC) strains, particularly strains belonging to phylogenetic group B2 (20), are the cause of more than 80% of uncomplicated UTIs among otherwise healthy individuals (28). The intestinal tract is considered the primary reservoir for urinary tract infection, where bacteria, including UPEC, from the anus gain access to the periurethral area and establish infection in an ascending manner. The gastrointestinal tract and the urinary tract represent two different nutritional niches for UPEC. The function of the gastrointestinal tract of humans and other animals is to break down ingested food into simpler nutrients and expel the remaining, unused food as waste. In the large intestine, approximately 1012 bacteria per gram of contents have been recovered; diverse genera, including Escherichia coli, are represented in these bacteria (19, 23). UPEC must compete with this array of bacteria in the large intestine for the remaining nutrients that have not been absorbed by the small intestine. Conversely, the urinary tract is primarily a sterile environment. Metabolic by-products are excreted from the kidneys and combine with water to form urine. Trace amounts of protein (<10 mg/dl) and amino acids (0.4 to 1.1 g/day) can be detected in the urine by urinalysis or chromatography; however, glucose and other simple carbohydrates are generally not detectable in the urine of healthy individuals (4, 7, 8, 24). Bacteria utilize multiple mechanisms to detect changes in the chemical composition of the surrounding environment (3). The most extensively studied mechanism is flagellum-mediated chemotaxis, where transmembrane chemoreceptors relay signals to the flagellar motor to direct movement toward a more favorable environment (27). In the urinary tract, motility and chemotaxis offer a selective advantage (14). Four transmembrane chemoreceptors, or methyl-accepting chemotaxis proteins (MCPs), that generally mediate chemotaxis to amino acids (Tsr to serine and Tar to aspartate), saccharides (Trg to ribose and galactose), and dipeptides (Tap) have been identified for E. coli (26). Additionally, the Tar receptor has been shown to mediate chemotaxis to maltose (22). Tsr, Tar, Trg, and Tap are composed of four domains: a periplasmic ligand-binding domain (proximal to the N terminus), two transmembrane domains, and a cytoplasmic signaling domain (proximal to the C terminus) (6, 13). To explore potential uropathogen-specific traits, our laboratory has also examined the chemotaxis of UPEC and other intestinal E. coli strains toward additional simple chemical attractants that have been functionally and genetically linked to the four well-characterized MCP receptors. Our initial observations revealed that representative UPEC pyelonephritis and cystitis strains, CFT073 and F11, respectively, did not possess functional Trg and Tap chemotactic responses; this was supported by inspection of both genomes. To assess whether this was a uropathogen-specific trend, we determined the prevalence of each of the four MCP receptor genes for 21 uropathogenic and 13 fecal-commensal isolates by Southern blotting or PCR analysis. From these analyses, four different genotypes were observed: tsr+ tar+ trg+ tap+ (11 strains), tsr+ tar+ trg+ (5 strains), tsr+ tar+ (17 strains), and negative for all four chemoreceptors (1 nonmotile strain). Six uropathogenic and four fecal-commensal strains, representing different receptor profiles, were tested for their ability to respond chemotactically to known chemoattractants for the four MCP receptors with a modified capillary chemotaxis assay (1, 21). Additionally, MCP sequence analysis was conducted with a genomic sequence available for nine diarrheagenic E. coli strains; of these strains, four were chosen to determine MCP functionality. In 49 of 56 cases, the presence of the intact gene correlated with the function of that particular receptor. After the genomic and functional results were combined, Trg and Tap were found to be significantly less prevalent or functional among UPEC strains than among fecal-commensal or diarrheagenic strains (P < 0.02). Overall, data from these studies suggest that UPEC strains have evolved under selective pressures that are different from other intestinal E. coli strains, reflective of the urinary tract and gut environments, respectively.