Urinary Tract Infection

Urinary tract infection (UTI) is among the commonest bacterial infections encountered in clinical practice and a perennial favourite in NEET PG across Medicine, Microbiology, Pharmacology and Community Medicine. These notes consolidate the classification, microbiology, clinical syndromes, investigations, antibiotic selection and prevention — with the cut-offs and one-liners that examiners love.

Definitions and classification

UTI denotes the presence of microbial pathogens within the urinary tract, ordinarily sterile above the distal urethra. The anatomical distinction drives both severity and management.

• Lower UTI — cystitis (bladder), urethritis (urethra), prostatitis (prostate). Presents with dysuria, frequency, urgency, suprapubic pain; usually no fever.
• Upper UTI — pyelonephritis (renal pelvis and parenchyma). Presents with fever, rigors, flank/loin pain and costovertebral angle (CVA) tenderness; systemic upset.

A second axis classifies UTI as uncomplicated or complicated, which is operationally the most important distinction in choosing therapy.

Feature	Uncomplicated UTI	Complicated UTI
Host	Healthy, non-pregnant women, premenopausal	Men, pregnancy, children, elderly
Tract anatomy	Structurally normal	Obstruction, stones, reflux, catheter, neurogenic bladder
Comorbidity	None significant	Diabetes, immunosuppression, renal failure, transplant
Typical organisms	E. coli predominantly	Broader spectrum, more resistance, Pseudomonas, Enterococcus, Candida
Treatment duration	Short course (3 days)	Longer (7–14 days), imaging often needed

High-yield: A UTI in any man, in pregnancy, or with a catheter/structural abnormality is automatically considered complicated until proven otherwise. NEET PG repeatedly tests that UTI in a male is never "simple".

Other terms worth defining:

• Asymptomatic bacteriuria — significant bacteriuria without symptoms. Treated only in pregnancy and before urological instrumentation/TURP.
• Recurrent UTI — ≥2 infections in 6 months or ≥3 in 12 months. Subdivided into relapse (same organism within 2 weeks, suggests a focus/stone) and reinfection (new organism, more common).
• Significant bacteriuria — the classic Kass criterion of ≥10⁵ colony-forming units (CFU)/mL on a clean-catch midstream specimen.

Etiology — the pathogens

Causative organisms are predominantly Gram-negative enteric flora ascending from the periurethral/faecal reservoir.

Organism	Notes / exam cue
Escherichia coli	Most common cause of both community and uncomplicated UTI (~80%); uropathogenic strains have P fimbriae
Staphylococcus saprophyticus	Second commonest in young sexually active women; novobiocin-resistant
Proteus mirabilis	Urease producer → alkaline urine → struvite (triple phosphate) staghorn calculi; "swarming" on agar
Klebsiella pneumoniae	Catheter-associated, hospital UTI, urease positive
Enterococcus faecalis	Catheter-associated, elderly, post-instrumentation
Pseudomonas aeruginosa	Nosocomial, catheter, post-instrumentation, resistant
Candida albicans	Catheter, diabetes, broad-spectrum antibiotics
Staphylococcus aureus	If present in urine, suspect haematogenous spread (e.g. endocarditis) rather than ascending

High-yield: Proteus with its urease raises urinary pH and produces struvite staghorn stones — a triple-tested association (UTI + alkaline urine + recurrent stones = Proteus).

Pathophysiology

The dominant route is ascending infection: periurethral colonisation → urethra → bladder → (via vesicoureteric reflux) → ureter → renal pelvis/parenchyma. Haematogenous seeding is rare and points to S. aureus, Candida or M. tuberculosis.

Host defences that normally keep urine sterile: unidirectional urine flow and complete bladder emptying, low urinary pH and high osmolarity, Tamm–Horsfall (uromodulin) glycoprotein trapping fimbriated bacteria, and urothelial mucosal IgA.

Risk factors (predisposing to UTI):

• Female sex — short urethra, proximity to anus (women >> men).
• Sexual intercourse ("honeymoon cystitis"); diaphragm and spermicide use.
• Pregnancy — progesterone-mediated ureteric dilatation and stasis.
• Obstruction/stasis — stones, BPH, stricture, neurogenic bladder.
• Vesicoureteric reflux — chief mechanism of pyelonephritis in children.
• Diabetes mellitus — predisposes to emphysematous pyelonephritis and Candida.
• Catheterisation and instrumentation.

Bacterial virulence: uropathogenic E. coli (UPEC) use type 1 fimbriae (bind bladder mannose receptors → cystitis) and P fimbriae/pap pili (bind globoseries glycolipids on renal epithelium → pyelonephritis), plus haemolysin and aerobactin (iron acquisition).

Clinical features

Acute cystitis (lower UTI): dysuria, frequency, urgency, suprapubic discomfort, cloudy or malodorous urine, occasionally microscopic/gross haematuria. Notably, fever is absent or low-grade; high fever suggests upper-tract involvement.

Acute pyelonephritis (upper UTI): high fever with rigors, loin/flank pain, costovertebral angle (CVA) tenderness, nausea and vomiting, often with preceding lower-tract symptoms. May progress to urosepsis with hypotension.

Acute prostatitis (men): fever, perineal/low back pain, obstructive voiding symptoms, exquisitely tender, boggy prostate on examination (avoid vigorous massage — bacteraemia risk).

Special groups:

• Elderly — may present atypically with confusion/delirium, falls, incontinence rather than classic dysuria.
• Neonates/infants — non-specific: fever, poor feeding, failure to thrive, jaundice.
• Catheterised patients — symptoms often blunted; fever may be the only clue.

High-yield: Costovertebral angle tenderness + fever + flank pain = acute pyelonephritis (upper UTI). The presence of systemic features distinguishes upper from lower UTI clinically.

Diagnosis and investigation of choice

Urine collection: clean-catch midstream specimen; in catheterised patients sample from the port (never the bag). Suprapubic aspiration is the gold standard in infants where any growth is significant.

Urinalysis (dipstick + microscopy):

• Leucocyte esterase — surrogate for pyuria.
• Nitrite — positive with Enterobacteriaceae (e.g. E. coli) that reduce nitrate to nitrite; negative with Enterococcus, S. saprophyticus, Pseudomonas (cannot reduce nitrate). A common MCQ trap.
• Pyuria — ≥10 WBC/mm³ (or ≥10 WBC/hpf in spun sediment).
• WBC casts — pathognomonic of pyelonephritis (localise infection to the kidney).

Urine culture — the investigation of choice for confirmation:

Specimen / setting	Significant count
Clean-catch midstream (women, symptomatic)	≥10⁵ CFU/mL (Kass); ≥10² CFU/mL acceptable if symptomatic with pyuria
Symptomatic men	≥10³ CFU/mL
Catheterised specimen	≥10² CFU/mL
Suprapubic aspirate	Any growth

High-yield: The Kass count ≥10⁵ CFU/mL defines significant bacteriuria in asymptomatic clean-catch samples, but a symptomatic woman with pyuria needs only ≥10² CFU/mL of a uropathogen — examiners test that lower symptomatic threshold.

Sterile pyuria (pyuria with negative routine culture) — think of genitourinary tuberculosis, Chlamydia/gonococcal urethritis, partially treated UTI, analgesic nephropathy, or interstitial cystitis. A classic NEET PG one-liner: sterile pyuria → rule out renal TB.

Imaging — when and what:

• Uncomplicated cystitis needs no imaging.
• Ultrasound first-line to detect obstruction, hydronephrosis or abscess in complicated/recurrent UTI or poor response.
• CT (contrast) is the investigation of choice for complications: perinephric/renal abscess, emphysematous pyelonephritis (gas in renal parenchyma in a diabetic), and obstruction.
• DMSA scan — best for detecting renal scarring in children after pyelonephritis.
• MCUG (micturating cystourethrogram) — investigation of choice to demonstrate vesicoureteric reflux in children.

Management and drug of choice

The approach is stratified by syndrome and complexity.

Approach to a symptomatic woman: Assess symptoms → dipstick (leucocyte esterase/nitrite) → if uncomplicated cystitis, empirical short-course oral antibiotic → culture only if recurrent, complicated, pregnant or treatment fails → review with sensitivities.

Stepwise: Recognise syndrome → classify uncomplicated vs complicated → send culture if indicated → start empirical therapy → de-escalate per sensitivity → arrange imaging/follow-up if complicated or recurrent.

Uncomplicated cystitis (non-pregnant women)

First-line oral agents:

Drug	Regimen	Caution
Nitrofurantoin	100 mg BD × 5 days	Avoid if eGFR <30–45; avoid at term/G6PD deficiency (haemolysis); no tissue/renal penetration so not for pyelonephritis
Trimethoprim ± sulfamethoxazole (co-trimoxazole)	3 days	Avoid in first trimester (folate antagonist, neural tube defects) and near term
Fosfomycin	Single 3 g dose	Convenient single-dose option
Fluoroquinolones (ciprofloxacin)	Reserve	Not first-line for simple cystitis — keep for pyelonephritis/complicated

High-yield: Nitrofurantoin achieves high urinary but poor tissue concentrations — excellent for cystitis but never for pyelonephritis or prostatitis. Conversely fluoroquinolones penetrate renal and prostatic tissue, making them suited to upper-tract and prostatic infection.

Acute pyelonephritis

• Mild/moderate, outpatient: oral fluoroquinolone (ciprofloxacin) is the traditional drug of choice (good tissue penetration).
• Severe/admitted: IV third-generation cephalosporin (ceftriaxone) or a fluoroquinolone, ± an aminoglycoside; broaden (piperacillin–tazobactam, carbapenem) if resistant organisms or sepsis. Duration 7–14 days; de-escalate to oral once afebrile 48–72 h.

Complicated UTI

Culture-guided therapy for 7–14 days; relieve obstruction (stent/nephrostomy) — antibiotics alone fail an obstructed, infected system, which is a urological emergency.

Pregnancy

Treat asymptomatic bacteriuria (unlike non-pregnant adults) to prevent pyelonephritis and preterm labour. Safe agents: nitrofurantoin (avoid at term), cephalexin, amoxicillin/amoxicillin–clavulanate, fosfomycin. Avoid: fluoroquinolones (cartilage), trimethoprim (1st trimester), sulfonamides (near term — kernicterus).

Acute bacterial prostatitis

Prolonged course (4–6 weeks) of a fluoroquinolone or co-trimoxazole because of the prostatic penetration barrier; longer (6–12 weeks) for chronic prostatitis.

Recurrent UTI in women

• Behavioural: post-coital voiding, adequate hydration, avoid spermicides.
• Post-coital single-dose prophylaxis (co-trimoxazole or nitrofurantoin) if related to intercourse.
• Continuous low-dose prophylaxis (nitrofurantoin/trimethoprim) for 6 months.
• Vaginal oestrogen in postmenopausal women (restores lactobacilli, lowers pH).
• Cranberry products and methenamine have weaker evidence.

Catheter-associated UTI (CAUTI) and prevention

CAUTI is the commonest healthcare-associated infection and a high-yield infection-control topic. Biofilm on the catheter shelters organisms (E. coli, Klebsiella, Pseudomonas, Enterococcus, Candida).

High-yield: Do not treat asymptomatic catheter bacteriuria/candiduria — treat only if symptomatic. If treatment is needed and the catheter has been in >2 weeks, replace the catheter before/at the start of antibiotics.

Prevention bundle (examiner favourites):

1. Avoid unnecessary catheterisation and remove as early as possible (single most effective measure).
2. Strict aseptic insertion technique.
3. Maintain a closed drainage system; keep the bag below bladder level and off the floor.
4. Ensure unobstructed downhill flow; secure the catheter.
5. Hand hygiene; no routine antibiotic prophylaxis and no routine bladder irrigation.
6. Consider intermittent catheterisation over indwelling where feasible.

Complications

• Acute pyelonephritis → renal scarring, especially in children with reflux.
• Renal and perinephric abscess — persistent fever despite antibiotics; CT diagnostic; drainage needed.
• Emphysematous pyelonephritis — necrotising infection with gas, almost exclusively in diabetics, usually E. coli/Klebsiella; high mortality; CT shows gas; may require nephrectomy.
• Pyonephrosis — pus in an obstructed system; emergency drainage.
• Urosepsis / septic shock.
• Struvite (staghorn) calculi with urease-producing organisms (Proteus).
• Chronic pyelonephritis / reflux nephropathy → hypertension and chronic kidney disease.
• Xanthogranulomatous pyelonephritis — chronic destructive infection (Proteus, obstructing stone), "bear-paw" sign on imaging, foamy lipid-laden macrophages; nephrectomy is curative.

Key differentials

Mimic	Distinguishing clue
Urethritis/STI (Chlamydia, gonococcus)	Sexually active, discharge, sterile pyuria, partner symptoms
Vaginitis	Discharge, external dysuria, no frequency/urgency
Interstitial cystitis / painful bladder	Chronic pelvic pain, negative cultures, relief on voiding
Genitourinary tuberculosis	Sterile pyuria, haematuria, weight loss; needs AFB/culture/PCR
Renal/ureteric colic	Colicky loin-to-groin pain, haematuria, afebrile (unless infected)
Acute appendicitis / PID	Lower abdominal/pelvic signs; check pregnancy, examine adnexa

Mnemonics and eponyms

• Pathogens of UTI — "KEEPS": Klebsiella, E. coli, Enterococcus/Enterobacter, Proteus, Staph saprophyticus/Pseudomonas.
• Nitrite-negative uropathogens — "SEP": Staph saprophyticus, Enterococcus, Pseudomonas (cannot reduce nitrate).
• Kass criterion — ≥10⁵ CFU/mL = significant bacteriuria.
• Bear-paw sign — xanthogranulomatous pyelonephritis.
• Honeymoon cystitis — post-coital UTI in young women.

Recently asked / exam angle

• Most common cause of UTI → E. coli (single most repeated one-liner).
• UTI in a young sexually active woman, coagulase-negative → Staph saprophyticus (novobiocin-resistant).
• Recurrent UTI + alkaline urine + staghorn stone → Proteus (urease).
• Drug for uncomplicated cystitis but NOT pyelonephritis → nitrofurantoin (no tissue penetration).
• Asymptomatic bacteriuria — when to treat? → pregnancy and before urological instrumentation only.
• Sterile pyuria → renal TB (also Chlamydia, partially treated UTI).
• Investigation of choice for VUR in a child → MCUG; renal scarring → DMSA.
• Gas in renal parenchyma in a diabetic → emphysematous pyelonephritis (CT diagnostic).
• Best single measure to prevent CAUTI → avoid/remove the catheter early.
• Drug to avoid in pregnancy → fluoroquinolones (cartilage), trimethoprim (1st trimester), sulfa (term).
• WBC casts in urine → localises infection to the kidney (pyelonephritis).

Rapid revision

1. E. coli is the commonest uropathogen overall; S. saprophyticus in young sexually active women.
2. UTI in a man, pregnancy, or with a catheter/abnormal tract = complicated by definition.
3. Kass criterion = ≥10⁵ CFU/mL; symptomatic woman with pyuria needs only ≥10² CFU/mL.
4. Nitrite-negative organisms: Enterococcus, S. saprophyticus, Pseudomonas.
5. Nitrofurantoin → cystitis only; avoid in pyelonephritis, term pregnancy, G6PD deficiency, eGFR <30–45.
6. Fluoroquinolones/co-trimoxazole penetrate renal and prostatic tissue → pyelonephritis and prostatitis.
7. CVA tenderness + fever + flank pain = acute pyelonephritis; WBC casts confirm renal involvement.
8. Proteus (urease) → alkaline urine → struvite staghorn calculi; swarming growth.
9. Treat asymptomatic bacteriuria only in pregnancy and pre-instrumentation.
10. Sterile pyuria → think renal TB.
11. Emphysematous pyelonephritis is a diabetic emergency; CT is diagnostic.
12. Best CAUTI prevention = avoid catheter and remove early; maintain a closed drainage system below bladder level.