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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Urinary Tract Infections

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Epidemiology, Pathogenesis - Urinary Tract Infections

Bacterial colonization of the urine within this tract(bacteriuria)is common and can,at times, result in microbial invasion of the tissues responsible for the manufacture, trans-port, and storage of urine.

Urinary Tract Infections

Bacterial colonization of the urine within this tract(bacteriuria)is common and can,at times, result in microbial invasion of the tissues responsible for the manufacture, trans-port, and storage of urine. Infection of the upper urinary tract, consisting of the kidney and its pelvis, is known as pyelonephritis. Infection of the lower tract may involve the bladder (cystitis), urethra (urethritis), or prostate (prostatitis), the genital organ that sur-rounds and communicates with the first segment of the male urethra. Because all portions of the urinary tract are joined by a fluid medium, infection at any site may spread to in-volve other areas of the system.

EPIDEMIOLOGY

Urinary tract infection (UTI) is among the most common of diseases particularly among women. Prevalence is age and sex dependent. Approximately 1% of children, many of whom demonstrate functional or anatomic abnormalities of the urinary tract, develop in-fection during the neonatal period. It is estimated that 20% or more of the female popula-tion suffers some form of UTI in their lifetime. Infection in the male population remains uncommon through the fifth decade of life, when enlargement of the prostate begins to in-terfere with emptying of the bladder. In the elderly of both sexes, gynecologic or prostatic surgery, incontinence, instrumentation, and chronic urethral catheterization push UTI rates to 30 to 40%. A single bladder catheterization carries an infectious risk of 1%, and at least 10% of individuals with indwelling catheters become infected.

PATHOGENESIS

The urine produced in the kidney and delivered through the renal pelvis and ureters to the urinary bladder is sterile in health. Infection results when bacteria gain access to this envi-ronment and are able to persist. Access primarily follows an ascending route for bacteria that are resident or transient members of the perineal flora. These organisms are derived from the large intestinal flora, which is uncomfortably nearby. Conditions that create ac-cess are varied, but the most important is sexual intercourse, which has been shown to transiently displace bacteria into the bladder. This puts the female partner is at risk because of the short urethral distance. Other manipulations of the urethra carry risk as well, partic-ularly medical ones such as catheterization. Bacteria may also reach the urinary tract from the bloodstream. This is obviously much less common, because it requires an uncontrolled infection at another site.

For bacteria that reach the urinary tract, the major competing forces are the rich nutrient content of the urine itself and the flushing action of bladder voiding. Persistence is favored by host factors that interrupt or retard the urinary flow such as instrumentation, obstruction, or structural abnormalities. In youth, factors are congenital malformations, and with age these include changes that alter the mechanics of outflow, such as prostatic hypertrophy. Bacterial factors include the ability to adhere to the perineal and uroepithelial mucosa and to produce other classical virulence factors like exotoxins. Escherichia coli is by far the most common and potent UTI pathogen. Urease-producing members of the genus Proteus are associated with urinary stones, which themselves are predisposing factors for infection.

ETIOLOGIC AGENTS

Over 95% of UTIs are caused by a single bacterial species, and 90% of these are E. coli. Other Enterobacteriaceae, Pseudomonas, and Gram-positive bacteria become increas-ingly frequent with chronic, complicated, and hospitalized patients. Of the Gram-positive bacteria enterococci are the most important. Staphylococcus saprophyticus, a coagulase-negative staphylococcus, is now recognized as the etiology in a significant minority of symptomatic infections in young, sexually active women. Yeasts, particularly species of Candida, may be isolated from catheterized patients receiving antibacterial therapy andfrom diabetic individuals, but they seldom produce symptomatic disease.

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