Urine is Not Sterile, PCR Analysis Shows

Jenni Laidman

April 13, 2012

April 13, 2012 — The bladder holds a world of bacteria that do not show up in standard cultures, regardless of whether a woman has symptoms of a urinary tract infection, according to a study published online January 25 and in the April print issue of the Journal of Clinical Microbiology.

The results of the research, by Alan J. Wolfe, PhD, professor of microbiology and immunology, Stritch School of Medicine, Loyola University, Chicago, Illinois, and colleagues, appears to unseat a long-held belief that the bladder is sterile, suggesting instead that bacterial communities are as normal in the bladder as in other parts of the body. The study further implies that some urinary tract infections go undiscovered and untreated.

"The punch line is, there really are bacteria in the bladders of a lot of women between the ages of 35 and 82. We don't know how it got there. We don't know what it's doing there. We don't know whether it's related to disease, or if it's related to health," Dr. Wolfe told Medscape Medical News. "Now it's time to figure out what they're doing."

"This is going to redefine the way we look at disease," Deborah Lightner, MD, professor of urology, Mayo Clinic, Rochester, Minnesota, said. "It's going to produce a great flurry of research activity.

"It's been the postulate for years that the bladder is a pristine site, [that] it does not have bacteria in it, that urine is always sterile, and if there's anything growing in it, that's disease," Dr. Lightner told Medscape Medical News. "Now this study leaves a whole swath of new questions much bigger than we had before. We have to stop thinking about bacteria as something that only causes disease. Sterility may not be the normal state."

The research team analyzed urine samples from women without symptoms of urinary tract infections (UTIs), with 1 group undergoing surgery for urinary incontinence or pelvic organ prolapse and a second, healthy, control group.

Researchers obtained urine samples via standard collection cups, transurinary catheter (TUC) and suprapubic aspirate (SPA). They also took vaginal swabs, as well as skin swabs from where the needle was inserted into the belly for SPA. They analyzed urine samples, using polymerase chain reaction (PCR) to look for 16S rRNA, part of the protein-making machinery used to classify bacterial taxa.

Regardless of the collection method used, urine samples contained bacteria that are not or cannot be cultured using standard laboratory techniques. In addition, voided urine samples were contaminated with genital tract bacteria. In contrast, comparison of bacteria from skin swabs and urine samples taken by TUC or SPA showed that the TUC and SPA samples were uncontaminated by surface bacteria population.

The bacteria detected in the TUC and SPA samples were similar. The most abundant bacterial sequences corresponded to Aerococcus (50.48% in TUC and 40.43% in SPA) and Actinobaculum (17.34% in TUC and 24.93% in SPA). A small fraction of the population corresponded to Escherichia/Shigella (0.94% in TUC and 2.40% in SPA).

"These data confirm and extend earlier studies clearly showing that urines reported to clinicians as 'culture-negative' or 'insignificant growth' can contain varied bacterial communities that can be simple or extremely diverse and can be composed of typical uropathogens or of genera not identified with standard cultivation techniques," the authors write.

The researchers were unable to amplify bacterial DNA from 2 samples, which Dr. Wolfe said does not necessarily mean the bladders of these women contained no bacteria.

The results indicate that urine cultures may not be "the gold standard" for detecting UTIs, Dr. Wolfe said, citing, in particular, the results of 1 participant for whom standard culture revealed Escherichia coli. When her urine was analyzed via PCR, E coli proved to be a tiny fraction of the patient's bacteria population (between 1% and 3%); the largest population was Aerococcus (~40% for SPA and ~50% for TUC), followed by Actinobaculum (~25% for SPA and ~17% for TUC).

"You have to ask the question: Is she really suffering from E coli?" Dr. Wolfe said.

One coauthor has a research grant from Allergan, and one coauthor is an independent contractor/speakers bureau for Cubist and Merck, Forest Laboratories, bioMerieux, Remel, and Hardy Diagnostics; is a consultant/advisory for Abbott Molecular, Elmhurst Hospital, Edward Hospital, Doctors Data Inc, Forest Laboratories; and participates on review panels, boards, and has research grants from Cepheid, Abbott Molecular, Siemens, Becton-Dickinson, and bioMerieux. The remaining authors and Dr. Lightner have disclosed no relevant financial relationships.

J Clin Microbiol. 2012;50:1376-1383. Abstract


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