Urinary Tract Infections


Urinary Tract Infections

Contributed by Brian S. Schwartz, MD



Urinary tract infections are a group of common diseases that occur predominantly by ascension of normal enteric flora through the urethra into the bladder. These infections more frequently affect women due to anatomic differences including a shorter urethra. Diagnosis is made by identifying related clinical symptoms in combination with an abnormal urinalysis and growth on urine culture. Antibiotics are often effective therapy, although antibiotic resistance is increasing.


Urine microscopy is the use of a microscope to look at urine. In patients with urinary tract infections, one can often find pyuria (elevated white blood cells [WBCs] in urine) and hematuria (red blood cells in urine), and sometimes bacteria can be seen. The presence of WBC casts indicates pyelonephritis rather than cystitis. A urine sample that has abundant squamous epithelial cells suggests that it is contaminated and the results of the culture are not reliable.

Urine dipsticks use different chemicals reagents on a strip that is dipped in urine to diagnose urinary tract diseases. Certain dipstick test results are suggestive of infection, namely positive leukocyte esterase, positive nitrite, and positive hemoglobin. The positive nitrite occurs from the conversion of nitrate to nitrite by Enterobacteriaceae.

Urine culture allows identification of the organism causing infection. Urine in the bladder is normally sterile. Because contamination of samples can occur as urine passes through the outer third of the urethra, a numeric threshold of colony-forming units (CFUs) per milliliter has been established to confirm infection. In samples obtained from a midstream void, ≥1 × 105 CFU/μL is consistent with infection. In samples collected via catheterization, ≥1 × 102 CFU/μL is consistent with infection.



Cystitis is an infection of the bladder. The term “cysto” refers to bladder, and “itis” refers to inflammation. Uncomplicated cystitis is defined as cystitis in otherwise healthy women, whereas complicated cystitis is defined as cystitis in all other groups such as men, pregnant women, diabetics, those with anatomic and neurologic problems, and those with recurrent urinary tract infections.


Bacteria (rarely fungi) reach the bladder via ascension through the urethra. This is much more common in women due to the short urethra and close approximation of the urethra to the vagina and anus. Preceding infection, the vagina, which is normally colonized by Lactobacillus species, will become colonized by enteric organisms such as Escherichia coli instead. E. coli are able to adhere to the urethral mucosa via pili. Once bacteria enter the bladder, they are able to reproduce and cause an inflammatory response, resulting in the symptoms of infection.

Medical conditions that cause abnormal emptying of bladder increase risk for urinary tract infections. These include anatomic abnormalities such as cystoceles, neurologic disorders such as spinal cord injuries and multiple sclerosis, and the presence of foreign bodies such as indwelling Foley catheters. In infants less than 3 months of age, uncircumcised boys are at higher risk for urinary tract infections than girls. However, after infancy, girls are at higher risk for infection than all boys.

Clinical Manifestations

The most common clinical manifestations of cystitis include dysuria (pain with urination); frequent, low-volume urination; suprapubic tenderness; and gross hematuria. Men may experience some penile discharge. Most patients with cystitis do not have fever or other systemic symptoms of infection, and when they are present, an upper urinary tract infection (pyelonephritis) should be considered.


E. coli is by far the most common cause of urinary tract infections. Other enteric gram-negative rods such as Klebsiella species and Proteus species are regular culprits. Pseudomonas aeruginosa

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Jan 26, 2017 | Posted by in MICROBIOLOGY | Comments Off on Urinary Tract Infections
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