Depending on its pH and chemical constituents, urine is generally a good culture medium. Factors unfavorable to bacterial growth are a low pH (5.5 or less); a high concentration of urea; and the presence of organic acids derived from a diet including fruit juice and methionine, a breakdown product of ingested protein that enhances acidification of the urine. A thin film of urine remains in the bladder after voiding. An intact bladder mucosa removes organisms from the film, probably by the production of organic acids by the mucosal cells and not by antibody formation or phagocytosis. Incomplete bladder emptying renders this mechanism ineffective and is responsible for the increased frequency of infection in patients with a neurogenic bladder, and in postmenopausal women with bladder or uterine prolapse. The latter group also has marked changes in vaginal microflora due to lack of estrogen, with loss of lactobacilli and increased colonization by E. coli.
Frequent and complete voiding has been associated with the reduction in recurrence of UTI. 5 Studies have found that the concentration of bacteria in the bladder may increase tenfold after sexual intercourse due to a "milking action" of the female urethra during intercourse. The use of a diaphragm and spermicide is also associated with recurrence in some patients, probably because the spermicide enhances vaginal colonization with E. coli.6 It is recommended, although unproven, that prompt voiding after intercourse may lessen the frequency of UTI. An increased urinary flow also dilutes the bacterial inoculum that occasionally occurs from sexual intercourse.5,6
Susceptibility to UTIs may have a genetic basis, that is, women who do not secrete blood group antigens (nonsecretors) have a high incidence of recurrent infection. This appears to be due to the presence of specific uroepithelial cell E. coli-binding glycolipids that promote fecal coliform colonization of the vagina.
The majority of uncomplicated UTIs remain in the bladder; the ureteral valves prevent ascent of the bacteria into the kidneys. If these mechanisms fail and ascending infection of the urinary tract occurs, renal defense mechanisms are called into play. Local antibodies are produced in the kidney and kill bacteria in the presence of complement. Local leukocytosis and phagocytosis also help eradicate bacteria.
Urinary infections are categorized into three clinical syndromes. The simplest and most common UTI is acute cystitis, where infection is isolated to the bladder. Subclinical pyelonephritis is characterized by bacterial infection in the upper urinary tract and detected by bladder washout techniques, selective ureteral catheterization, or the presence of antibody-coated bacteria in the urine. However, subclinical pyelonephritis is clinically indistinguishable from acute cystitis and it has been estimated about 25 to 30 percent of patients with the acute cystitis syndrome have subclinical pyelonephritis. Several epidemiologic factors correlate with increased risk for subclinical pyelonephritis; lower socioeconomic status; pregnancy; structural urinary tract abnormality; urinary stone; history of relapse after treatment for a UTI; prior history of acute pyelonephritis; frequent UTIs; symptoms for more than seven days; or diabetes or other immunosuppressing conditions. 2 Acute pyelonephritis is characterized by the presence of bacteria in the kidney with localized pain and tenderness and systemic symptoms of infection (fever, chills, nausea, vomiting, and prostration).
The infective process of acute pyelonephritis can progress into three patterns of renal infection not commonly considered part of the UTI spectrum; acute bacterial nephritis, renal abscess, and emphysematous pyelonephritis.78 These tend to be diagnoses made on imaging studies performed on patients who have an inadequate or atypical response to treatment for presumed acute pyelonephritis. On ultrasound or CT imaging studies, acute pyelonephritis is seen as a diffusely enlarged kidney without focal abnormalities. Acute bacterial nephritis produces ill-defined focal areas, sometimes striated or wedge-shaped, of decreased density. 7 Renal abscesses appear as well-defined areas of decreased density.7 Emphysematous pyelonephritis is a rare gas-forming infection within the kidney, nearly always in diabetics (70 to 90 percent of the time). These patients usually have symptoms and signs of a severe infection, often with dehydration and pyelonephritis. 8
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