The principal causative organisms in acute pyelonephritis are enteric gram-negative bacilli. Escherichia coli is by far the most common. Other important organisms are Proteus, Klebsiella, Enterobacter, and Pseudomonas; these usually are associated with recurrent infections, especially in individuals who undergo urinary tract manipulations or have congenital or acquired anomalies of the lower urinary tract (see later). Staphylococci and Streptococcus faecalis also may cause pyelonephritis, but they are uncommon pathogens in this setting.
Bacteria can reach the kidneys from the lower urinary tract (ascending infection) or through the bloodstream (hematogenous infection) (
Fig. 14.12 ). Ascending infection from the lower urinary tract is the most important and frequent route by which bacteria reach the kidney. Adhesion of bacteria to mucosal surfaces is followed by colonization of the distal urethra (and the introitus in females). Genetically determined properties of both the urothelium and the bacterial pathogens may facilitate adhesion to the urothelial lining by bacterial fimbriae (proteins that attach to receptors on the surface of urothelial cells), conferring susceptibility to infection. The organisms then reach the bladder, by expansive growth of the colonies and by moving against the flow of urine. This may occur during urethral instrumentation, including catheterization and cystoscopy.
In the absence of instrumentation, UTI most commonly affects females. Because of the close proximity of the female urethra to the rectum, colonization by enteric bacteria can occur more readily in females. Furthermore, the short urethra and trauma to the urethra during sexual intercourse facilitate the entry of bacteria into the urinary bladder. Normally, bladder urine is sterile, because of the anti-microbial properties of the bladder mucosa and the flushing mechanism associated with periodic voiding of urine. With outflow obstruction or bladder dysfunction, however, the natural defense mechanisms of the bladder are overwhelmed, setting the stage for UTI. In the presence of stasis, bacteria introduced into the bladder can multiply undisturbed, without being flushed out or http://ebooksmedicine.net
Common agents:
E. coli
Pyelonephritis Foci of pyelonephritis
Common agents:
E. coli Proteus Enterobacter
Fig.14.12 Pathwaysofrenalinfection.Hematogenousinfectionresultsfrom bacteremicspread.Morecommonisascendinginfection,whichresultsfrom acombinationofurinarybladderinfection,vesicoureteralreflux,andintra-renalreflux.
destroyed by the bladder wall. Accordingly, UTI is particularly frequent among patients with urinary tract obstruction, as may occur with benign prostatic hyperplasia and uterine prolapse. From the contaminated bladder urine, the bacteria ascend along the ureters to infect the renal pelvis and parenchyma. The frequency of UTI also is increased in diabetes because of the increased susceptibility to infection and neurogenic bladder dysfunction, which predispose to urine stasis. Increased incidence of UTI during pregnancy is attributed to urine stasis due to pressure on the bladder and ureters from the growing uterus.