Urinary Tract Infection

Classification

By level: Lower UTI (cystitis/urethritis, bladder/urethra) vs Upper UTI (pyelonephritis, kidney — ↑systemic illness/scarring risk). By host: Uncomplicated (normal urinary tract, classic: young sexually-active nonpregnant woman) vs Complicated (structural/functional abnormality, catheter, stone, immunosuppression, pregnancy, male sex) — affects Rx duration + workup threshold.

Etiology

E. coli = COMMONEST, 70-95% uncomplicated. UPEC strains: P fimbriae (adherence), hemolysin, aerobactin (iron) — distinguish from commensal E. coli.

Context-specific:

• S. saprophyticus: 2nd commonest, YOUNG SEXUALLY ACTIVE WOMEN specifically. CoNS but NOVOBIOCIN-RESISTANT (distinguishing test).
• Klebsiella, Proteus, Enterobacter, Pseudomonas: complicated/catheter/hospital-acquired UTI.
• Proteus mirabilis: linked to URINARY CALCULI — urease splits urea→ammonia→alkaline urine→struvite (Mg-NH4-PO4) stone precipitation. Mechanistic link, not coincidence.

Pathogenesis

DOMINANT route: ASCENDING infection. Periurethral/fecal flora → urethra → bladder. Women: shorter urethra + anal proximity = why UTI >> common in women (gap narrows in old age, prostatic disease raises male risk).

Bacterial factors (adhesins, esp. P fimbriae for pyelonephritis strains) + host factors (sexual activity, spermicide, postmenopausal estrogen decline, incomplete emptying, VUR, catheterization, pregnancy ureteral dilation) → colonization progresses to infection.

Hematogenous spread to kidney: LESS common, mainly disseminated bacteremia/sepsis (e.g. S. aureus).

Clinical Manifestations

Lower UTI (cystitis): dysuria, frequency, urgency, suprapubic discomfort, ±hematuria. NO fever/systemic symptoms (confined to bladder mucosa).

Upper UTI (pyelonephritis): fever, chills/rigors, flank pain, CVA tenderness, ±N/V. Systemic symptoms + flank findings = distinguishes from cystitis (lower symptoms often ALSO present, not absent).

Asymptomatic bacteriuria: common (elderly, catheterized). NOT treated except specific situations (pregnancy, pre-urological procedure) — see CAUTI topic under HAI.

Laboratory Diagnosis

Urine culture = DEFINITIVE, also yields susceptibility data. Collection technique CRITICAL:

• Clean-catch midstream: ≥10⁵ CFU/mL (classic threshold, symptomatic)
• Lower threshold ≥10²-10³ CFU/mL: symptomatic + compatible picture, OR contamination-bypass technique (catheter specimen, suprapubic aspiration) — sterile-collection specimen with ANY significant growth = more diagnostic weight than same count from midstream.

Urinalysis (dipstick/microscopy): rapid initial screen while culture pending. Leukocyte esterase (pyuria proxy) + Nitrite (nitrate-reducing organisms, mainly Enterobacteriaceae — NOT produced by S. saprophyticus/enterococci, negative nitrite ≠ ruled out). Microscopy: pyuria + bacteriuria direct. Normal urinalysis ≠ substitute for culture if suspicion persists (imperfect sensitivity).

Treatment

Uncomplicated cystitis: short-course oral (nitrofurantoin, TMP-SMX, fosfomycin — per local resistance), 3-5 days. Pyelonephritis: longer course (7-14 days), severe → initial IV (fluoroquinolone or 3rd-gen cephalosporin, per local resistance — rising FQ resistance). Complicated UTI: longer course + ADDRESS UNDERLYING FACTOR (catheter removal/change, stone treatment) — antibiotics alone often fail if anatomical/functional abnormality persists.

Prevention

Hydration, post-coital voiding, avoid spermicide, recurrent UTI: low-dose prophylactic antibiotics OR (postmenopausal women) topical vaginal estrogen (addresses estrogen-decline flora shift specifically).

Classification

Urinary tract infection (UTI) is classified by anatomical level — lower UTI (cystitis, urethritis, confined to the bladder/urethra) and upper UTI (pyelonephritis, involving the kidney itself, with correspondingly greater risk of systemic illness and renal scarring) — and by host context, since the same organism behaves very differently depending on the patient: uncomplicated UTI occurs in an anatomically and functionally normal urinary tract (classically a young, sexually active, non-pregnant woman), while complicated UTI occurs in the presence of a structural/functional abnormality, catheter, stone, immunosuppression, pregnancy, or male sex — categories that matter directly for treatment duration and threshold for further workup, since a complicated UTI both responds less predictably and carries higher risk of progression/relapse.

Etiology

Escherichia coli is overwhelmingly the commonest cause of both uncomplicated and complicated UTI, accounting for roughly 70–95% of uncomplicated cases — specific uropathogenic E. coli (UPEC) strains carry dedicated virulence factors (P fimbriae/pili mediating adherence to uroepithelium, haemolysin, aerobactin for iron acquisition) that distinguish them from ordinary gut commensal E. coli. Other organisms follow a recognizable pattern by context: Staphylococcus saprophyticus is a genuinely distinctive second-commonest cause specifically in young, sexually active women (coagulase-negative but notably novobiocin-resistant, the test used to distinguish it from other CoNS); Klebsiella, Proteus, Enterobacter, and Pseudomonas feature more in complicated/catheter-associated/hospital-acquired UTI; Proteus mirabilis is specifically associated with urinary calculi, since its powerful urease activity splits urea into ammonia, alkalinizing urine and precipitating struvite (magnesium ammonium phosphate) stones — a genuinely important mechanistic link between organism and complication, not a coincidental association.

Pathogenesis

The dominant route is ascending infection — periurethral/faecal flora colonizes the urethra and ascends into the bladder, a route anatomically favoured in women by their shorter urethra and its proximity to the anus, which is the principal reason UTI is dramatically more common in women than men across most of adult life (this gap narrows in old age, as prostatic disease raises male UTI risk). Bacterial virulence factors (adhesins, particularly P fimbriae for pyelonephritis-causing strains, which bind specific uroepithelial receptors) combine with host factors (sexual activity, spermicide use, post-menopausal oestrogen decline, incomplete bladder emptying, vesicoureteric reflux, catheterization, pregnancy-related ureteral dilation) to determine whether colonization progresses to true infection. Haematogenous spread to the kidney is a distinctly less common route, seen mainly in disseminated bacteraemia/sepsis (e.g. S. aureus bacteraemia seeding the kidney) rather than as the typical UTI mechanism.

Clinical Manifestations

Lower UTI (cystitis): dysuria, urinary frequency and urgency, suprapubic discomfort, sometimes visible haematuria — notably without fever or systemic symptoms, since the infection stays confined to the bladder mucosa. Upper UTI (pyelonephritis): fever, chills/rigors, flank pain, costovertebral angle tenderness, and often nausea/vomiting — the systemic symptoms and flank findings are what actually distinguish it clinically from simple cystitis, since lower urinary symptoms (dysuria, frequency) frequently accompany pyelonephritis too rather than being absent. Asymptomatic bacteriuria — significant bacterial counts without any symptoms — is common, particularly in the elderly and catheterized patients, and (as covered under Catheter-Associated UTI in Hospital Acquired Infections) is deliberately not treated except in specific situations (pregnancy, pre-urological-procedure) given the risk/benefit balance of unnecessary antibiotic exposure.

Laboratory Diagnosis

Urine culture remains the definitive diagnostic test, with collection technique critical to interpretation — a properly collected clean-catch midstream sample minimizes (but never fully eliminates) contamination by periurethral flora, which is exactly why quantitative thresholds exist to separate true infection from contamination: ≥10⁵ CFU/mL for a symptomatic midstream specimen is the classic threshold; a lower threshold (≥10² –10³ CFU/mL) is accepted for a symptomatic patient with a compatible clinical picture, or for a specimen obtained by a technique that bypasses contamination risk (catheter specimen, suprapubic aspiration) — since a genuinely sterile-collection specimen showing any significant growth carries more diagnostic weight than the same count from a contamination-prone midstream sample. Culture additionally provides the antimicrobial susceptibility data needed to guide or refine therapy, particularly important given rising resistance in community uropathogens.

Urinalysis (dipstick or microscopy) serves as the rapid initial screen while culture is pending: leukocyte esterase (a proxy for pyuria) and nitrite (produced by nitrate-reducing organisms, chiefly Enterobacteriaceae — notably not produced by some UTI-causing organisms like S. saprophyticus or enterococci, so a negative nitrite does not rule out UTI) together have reasonable but imperfect predictive value; microscopy directly shows pyuria (white cells) and bacteriuria. A normal urinalysis makes UTI less likely but, given its imperfect sensitivity, does not substitute for culture where clinical suspicion remains.

Treatment

Uncomplicated cystitis: short-course oral therapy (nitrofurantoin, trimethoprim-sulfamethoxazole, or fosfomycin, depending on local resistance patterns), typically 3–5 days. Pyelonephritis needs a longer course (typically 7–14 days) and, for more severe presentations, initial IV therapy with a fluoroquinolone or third-generation cephalosporin, guided by local resistance data given rising fluoroquinolone resistance in many settings. Complicated UTI is managed with a longer course and, critically, addressing the underlying complicating factor (removing/changing a catheter, treating an obstructing stone) wherever feasible — antibiotics alone frequently fail to clear infection if the anatomical/functional abnormality driving it persists.

Prevention

Adequate hydration, post-coital voiding, avoiding spermicide use, and, for recurrent UTI in specific patient groups, low-dose prophylactic antibiotics or, in postmenopausal women, topical vaginal oestrogen (addressing the oestrogen-decline-related vaginal flora shift that predisposes to recurrent infection in that group specifically).

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