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From a microbiological view point, urinary tract infection (UTI) is indicated when pathogenic microorganisms are found in the urine, bladder, urethra, kidney and prostate, and a microorganism growth of greater than 105 per mL is isolated from a mid-stream or clean catch urine specimen.
Asymptomatic urinary tract infection is the absence of symptoms in a patient having urinary tract infection. The presence of symptoms in a patient having infection of the urinary tract is referred to as symptomatic urinary tract infection.
In both cases, urine culture may be positive or negative. In symptomatic patients, lower microorganism count (102-104 per mL) may signify infection. Urine specimen from catheterization with colony counts of 102 - 104 per mL indicates infection. However, occasionally due to mid-stream urine contamination, a colony count of greater than 105 per mL accompanied by multiple bacteria specie growth may occur.
Acute urinary tract infection can either be lower urinary tract infection (urethritis and cystitis) or upper urinary tract infection (acute pyelonephritis).
Clinical Features
Acute cystitis and acute pyelonephritis are two major clinical syndromes mostly encountered. Patients with acute cystitis have infection localized in the bladder. In a typical acute pyelonephritis patients, in which the infection has spread to the kidney, localized kidney pain, fever, nausea, vomiting, chills and malaise are usually observed.
Acute urethritis patients often present with symptoms of dysuria, urgency, frequency and non-significant bacteria growth. Catheter-associated UTIs cause minimal symptoms often resolved after catheter is removed.
 Pathogenosis
Most UTIs result when bacteria gain access to the bladder via the urethra. Ascent of bacteria from the bladder may follow and is probably the pathway for most renal parenchyma infections.
Some strains of E. coli and Proteus are uropathogenic. These strains have violent genes (e.g. genes encoding fimbriae) that mediate attachment to uroepithelial cells. UTIs are caused by a subset of fecal microbial, flora of which E. Coli is the most common. Some hosts, primarily women, are especially susceptible to infection.
In females prone to the development of cystitis, however, enteric Gram-negative organisms residing in the bowel colonize the introitus, the periurethral skin and distal urethra before and during bacteriuria episodes.
Alteration of the normal vaginal flora (e.g loss of normal dominant H2 02- producing lactobacilli) appears to facilitate colonization of E. Coli. Antibiotic treatment and other genital infections or contraceptive are contributive factors.
Etiology
Uncomplicated community-acquired UTIs are caused by E. Coli in 80-85 percent of cases. Other organisms such as Klebsiella, Proteus and Enterobacter are Gram-negative rods which account for smaller proportions of UTI cases.
Gram-positive etiologic agents of UTI include Staphylococcus Saprophyticus which causes 5-10 percent acute UTIs among schoolgirls. E. Coli, Proteus, Klebsiella, Enterobacter, Serratia, and Pseudomonas are commonly associated with recurrent UTIs and UTI of the calculi.
Sexually transmitted organisms such as Chlamydia trachomatis and Neisseria gonorrhoeae are urethritis producing agents. Ureplasma urelyticum is associated with acute dysuria, while Candida and other fungi species have frequently been isolated from the urine of catheterized patients.
Epidemiology
The magnitude of the epidemiology of UTI can be considered from two perspectives: catheter- associated (nosocomial) and non-catheter associated (community acquired) infections. In both groups, UTIs can either be symptomatic or asymptomatic. Community-acquired UTIs results in more than 7 million doctor's office visits and 1 million hospital emergency department visits, resulting in 100,000 hospitalizations annually in the United States.
These visits involve about 1.2-3.2 percent of sexually active young girls. Acute symptomatic infections is common among young women accounting for about 0.9 percent infection per patient every year among this category.
Asymptomatic bacteriuria is frequently reported among elderly men and women with rates as high as 50 percent in some case studies. UTIs are uncommon in males younger than 50 years. However, the incidence of UTI in men tends to rise after the age of 50 years.
Among older patients residing in nursing homes, UTIs are the most common bacterial infection. UTIs are also the most common reason for antimicrobial drug prescriptions. About 20 percent to 60 percent of antimicrobial drug treatments are initiated among older patients in nursing homes.
The occurrence of not so complicated UTIs among females who reside in the communities and who are between the ages of 18-49 years is about 28.2 percent among every 10,000 women. Approximately 10 percent of adult women in the United States have at least one UTI each year.
Nearly 1 in 3 women will experience UTI which requires antimicrobial therapy by the age of 24.The incidence of UTI among hospitalized patients with indwelling catheters is about 10.2-15.2 percent. The risk of infection is 3-5 percent per day of catheterization. Catheterized urinary tract is found to facilitate Gram negative bacteremia in about 1.2-2.3 percent cases of hospitalized patients with indwelling catheters.
Risk Factors
Under certain special settings, an individual may become predisposed to symptomatic or asymptomatic urinary tract infection. These special settings include:
- Socio-economic status--UTI is prevalent among individuals who belong to very low socio economic class.
- Pregnancy--UTIs are detected among 3-9 percent of pregnant women. Twenty to 30 percent of pregnant women with asymptomatic bacteriuria subsequently develop pyelonephritis.
- Structural urinary tract abnormality--any obstruction to the free flow of urine results in hydronephrosis which greatly increases the frequency of UTI.
- Sexual behavioral practices--UTIs are prevalent among young women who are sexually active.
- Menopause--post menopausal women have reduced natural estrogen level which favors the colonization of uropathogenic Gram-negative bacilli.
- Vesicoureteral reflux--an anatomically impaired vesicoureteral junction facilitates the reflux of bacteria, hence upper urinary tract infection.
- Genetic factors--host genetic factors influence susceptibility to UTIs. Women with maternal history of UTI often experience recurrent UTIs.
- Bacterial virulence factor--certain uropathogenic organisms like E.coli and Proteus specie have fimbriae that mediate bacterial attachment to specific receptors on epithelial cells. This stimulates UTIs.
- Diabetes--the presence of glucose in urine favors the growth of glucose utilizing bacteria. Women who have hyperglycemia and poorly managed diabetes are at risk of UTIs.
Other special conditions under which UTIs may be engendered include history of UTI relapse after treatment, prior history of acute pyelonephritis, frequent UTIs with symptoms longer than 7 days and neurogenic bladder dysfunction.
Diagnostic Testing
Laboratory evaluation of mid-stream urine specimen or a sample from urethral catheterization is essential. The mid-stream urine should be analyzed for nitrite and leukocyte reactions, pyuria, bacteriuria and hematuria.
A positive urine nitrite strongly suggests the diagnosis of UTI. A positive urine leukocyte esterase reaction from pyuria is also a strong indicator. Abnormal pyuria in women is defined as 2 to 5 leukocytes per high power field from a centrifuged urine specimen. The presence of 1 to 2 leukocytes per high power field from the centrifuged urine specimen of a man, accompanied by bacteriuria, is a strong indicator of UTI.
Systemic leukopenia may produce a false-negative urine leukocyte esterase reaction. In women with chlamydia, bacteriuria may be absent. However, more than 15 bacteria per oil immersion field in a centrifuged urine suggests the diagnosis of UTI.
In symptomatic patients, Gram-stained uncentrifuged urine specimens should be microscopically evaluated. The detection of bacteria by urine microscopy accompanied by colony count of 105 per mL is an evidence of UTI. Asymptomatic bacteriuria is defined as a urine culture with more than 105 per mL of a single bacterial specie in an asymptomatic patient.
In the urinalysis for hematuria, confirmatory microscopic urine analysis should be performed as a false-positive blood urine test strip reaction can occur due to the presence of free hemoglobin, myoglobin, porphyrins or providone-iodine in urine. More than five red blood cells (RBC) per high power field in centrifuged urine is one of the indicators of UTI.
UTI Treatment
Fluoroquinolone therapy is a first-line treatment choice as bacteria resistance is sometimes observed when patients with UTI are treated with routinely used antibiotics. Fluoroquinolones such as ciprofloxacin, levofloxacin or ofloxacine, can be used. Other routinely used antibiotics include, co-trimoxazole or trimethoprim, Amoxicillin/Clavulanate, amoxicillin, cephalexin and nitrofurantoin.
Kehinde Lawal is a medical laboratory technologist with specialization in medical microbiology.
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