Host defense mechanisms in the pathogenesis of urinary tract infection.

Certain microorganisms have a propensity for causing urinary tract infection, and the route (either ascending or hematogenous) by which microorganisms contaminate the urinary tract from external sources is frequently characteristic of the microorganism. There are local defense mechanisms both in the urine and at each anatomic site in the urinary tract (urethra, bladder, ureter, and kidney). The defense mechanisms at one site may have opposing effects on microbial growth at other sites in the urinary tract. The outcome following entrance of microorganisms into the urinary tract is a result of competing forces, which consist of these local urinary defense mechanisms, the initial numbers of microorganisms contaminating the urinary tract, and microbial virulence factors.