Can a Silent Kidney Infection or Genetic Predisposition Underlie Recurrent UTIs?

, University of Calgary and Alberta Children's Hospital


Medscape General Medicine. 1996;1(1) 

In This Article

Microbiology of UTIs in Women

E coli, a gram-negative bacillus that is one of the most common commensals of the fecal flora, accounts for 80%-85% of all episodes of uncomplicated cystitis and pyelonephritis in women and 95% of all UTIs in women.[26] Six O serogroups, designated as uropathogenic E coli clones (ie, O2, O4, O8, O18ab, O75, and O150) are responsible for most of the UTIs caused by E coli.[5]

Staphylococcus saprophyticus, a coagulase-negative Staphylococcus, has also been clearly shown to play a pathogenic role in UTIs in young women.[12] Approximately 5%-15% of acute uncomplicated infections are caused by S saprophyticus.[12] The resistance of S saprophyticus to novobiocin facilitates differentiating it from other coagulase-negative staphylococci that may contaminate urine cultures.

While recurrent infections may be caused by either E coli or S saprophyticus, there is also an increased likelihood of finding other organisms such as members of the gram-negative enteric species (Proteus, Klebsiella, and Enterobacter) or Group D enterococci. In addition, whenever organisms persist or recur in the urinary system, they tend to become more resistant to antibiotics.

Common flora in the periurethral, perineal, and urethral skin areas include Staphylococcus epidermidis and other coagulase-negative staphylococci (except for S saprophyticus), lactobacilli, corynebacteria, and streptococci (other than enterococci). It is therefore not surprising that these organisms are also the most common contaminants of urine cultures despite adherence to proper urine culture collection procedures by laboratory personnel.

Although clinically difficult to sort out, common skin organisms (eg, coagulase-negative Staphylococci, not S saprophyticus) may also be the cause of an uncomplicated UTI, but this occurs only rarely compared with the incidence found in cases of complicated or catheter-associated UTIs.[8,13,27,28] In my experience, women have usually been persistently treated with multiple courses of antimicrobial agents for confirmed or suspected UTI due to gram-negative organisms, which may then select for increased colonization with gram-positive organisms.