Hypermucoviscous Phenotype Expressed by an Isolate of Uropathogenic Escherichia coli: an Overlooked and Underappreciated Virulence Factor

Uropathogenic Escherichia coli (UPEC) strains are classified as extraintestinal pathogenic E. coli with a special predilection for the urinary tract. Among the virulence factors enabling urinary tract invasion are fimbriae, afimbrial adhesions, siderophores, and several secreted toxins. Biofilm formation and capsular K antigens also aid colonization of the urinary tract. This report describes perhaps the first documentation of a hypermucoviscous phenotype of a UPEC strain isolated from the urine of a 54-year-old patient with chronic emphysematous pyelonephritis leading to nephrectomy of his right kidney. Culture of the patient's urine grew highly viscous colonies which rendered a “stringing” phenomenon when an inoculating needle was passed through the colonies analogous to that produced by liver-invasive strains of Klebsiella pneumoniae. Stained preparations of colony teasings emulsified in India ink and examined microscopically (×1,000) revealed two distinct halos enveloping the bacillary form. The inner halo was indicative of a capsule, while the second viscous halo enveloped the entire complex. While mucoid E. coli strains have been isolated from various clinical specimens, none have been reported to have been tested for the stringing phenomenon indicative of the hypermucoviscous phenotype, which may be a common occurrence but overlooked as a distinctive virulence characteristic of UPEC strains.