Strains of uropathogenic Escherichia coli (UPEC) cause the majority of all urinary tract infections. Host defense mechanisms and antibiotic treatments kill most of the bacteria, but some can remain within the bladder tissue. However, the mechanisms involved in bacterial persistence within the urinary tract have been unclear. In the July 4 Science, Gregory G. Anderson and colleagues at the Washington University School of Medicine show that E. coli forms intracellular bacterial biofilm-like pods in bladders, favoring the recurrence of urinary tract infections (Science, 301:105-107, July 4, 2003).

Anderson et al used scanning electron microscopy of infected mouse bladders and observed that E. coli invaded the bladder superficial cells, and these intracellular bacteria matured into biofilms, creating pod-like bulges on the bladder surface. The pods contained bacteria encased in a polysaccharide-rich matrix surrounded by a protective shell of uroplakin. Within the biofilm, bacterial structures interacted extensively with the surrounding matrix, and biofilm associated factors showed regional variation in expression.

"Intracellular biofilms may be vital for UTI progression, and their identification establishes a new paradigm in our understanding of acute and recurrent UTIs. Similar structures may also be involved in the pathogenesis of other chronic or recurrent infections as a common survival strategy for invasive microorganisms," conclude the authors.