Pathogenesis of Infection
The main route of infection of the urinary tract is ascendance of commensal bacteria colonizing the periurethral and perineal surface into the urethra and bladder (Fig. 1). Once established in large enough numbers in the bladder, infecting bacteria may then further move upwards through the ureters to the kidneys to establish pyelonephritis. Bacteria that are able to adhere tightly onto the mucosal surface of the uroepithelium are therefore the main agents of infection.
Figure 1. A woman's short urethra facilitates the ascendance of commensal bacteria colonizing the periurethral and perineal surface into the urethra and up to the bladder. While UTI may be limited to localized bladder infection, or cystitis, it also may extend to an upper tract infection of the kidneys, or pyelonephritis. Adapted from Salit IE: Pathogenesis of UTI. In Urinary Tract Infection (slides and lecture notes), Copyright (c) 1990, Irving E. Salit, MD.
Patients with normal urinary tract structure and function have mechanisms that offset bacterial colonization in several ways. Frequent voiding, which flushes out the urinary system, is important in preventing infection. Although small numbers of bacteria regularly move upwards into the bladder, micturition with complete emptying of the bladder prevents organisms from multiplying enough to cause infection.
The mucosal surface of the bladder is also coated with a uromucoid or uromucin layer that may help to prevent bacteria from adhering to the epithelial surface as a critical step in establishing infection. There is also a surface mucopolysaccharide glycosaminoglycan (GAG), which may protect by a similar mechanism. In addition, urine itself is normally a poor culture medium and usually inhibits the growth of many organisms due to the lack of a carbon source, such as glucose, for bacterial growth, as well as its acidity, high urea concentration, and high osmolality. However, some genera within the Enterobacteriaceae (such as Proteus sp., Morganella sp. and Providencia sp.) produce an enzyme called urease that permits these types of bacteria to more readily live in urine. Collectively these factors usually prevent the establishment of a urinary tract infection via inoculation of small numbers of bacteria in the normal urinary tract, but the inoculation or growth of large numbers of bacteria or particularly virulent bacteria into the bladder may overwhelm the host's defenses.
In addition, inadequate drainage due to urinary system abnormalities (ie, structural abnormality, obstruction, or malfunction) may predispose patients with complicated urinary tract problems to recurrent infections. Most commonly, these patients have a structural abnormality of the urinary tract, either congenital or acquired, have a foreign body such as a stone-causing obstruction, or have neurological dysfunction (ie, spinal cord injury, multiple sclerosis, etc.). Foreign bodies such as stones may not only cause urinary tract obstruction but in themselves may act as niduses for bacterial adhesion. In addition, the presence of a stone or other foreign body within the urinary tract causes uroepithelial abrasions to which bacteria may more avidly adhere.
Medscape General Medicine. 1996;1(1) © 1996
Cite this: Can a Silent Kidney Infection or Genetic Predisposition Underlie Recurrent UTIs? - Medscape - Oct 07, 1996.