Urinary Tract Infection (UTI) — Causes, E. coli, Nitrofurantoin & Treatment Guide — Symptoms, Causes & Treatment | MyMedicPlus

A urinary tract infection (UTI) is a microbial infection of any part of the urinary tract — from the urethra and bladder (lower UTI — cystitis or urethritis) to the ureters and kidneys (upper UTI — pyelonephritis or urosepsis). UTIs are the most common bacterial infections globally, affecting 150 million people per year and accounting for approximately 20-30% of community antibiotic prescriptions in the UK. Approximately 50-60% of women develop at least one UTI during their lifetime; 25-30% experience recurrent UTIs (3 or more per year). Men are substantially less affected due to anatomical differences (longer urethra, prostatic secretions), but UTI incidence in men increases markedly with age, particularly due to benign prostatic hyperplasia causing urinary stasis. UTIs are classified as: uncomplicated (in non-pregnant women with no structural or functional abnormality — the vast majority of community cystitis cases); complicated (in men, pregnant women, children, catheterised patients, immunocompromised patients, or those with urological abnormalities — requiring broader investigation and treatment); and catheter-associated UTI (CAUTI — the most common healthcare-associated infection).

Uropathogens: Escherichia coli accounts for 80-85% of uncomplicated community UTIs — it expresses Type 1 and P fimbriae (pili) that bind to uroepithelial receptors, facilitating ascent. Staphylococcus saprophyticus (5-15% — characteristically in young sexually active women); Klebsiella pneumoniae (5%); Enterococcus faecalis; Proteus mirabilis (associated with urease production — urease splits urea to ammonia, alkalising urine and causing struvite stones; Proteus UTI in a man suggests structural problem); Pseudomonas aeruginosa (healthcare-associated, catheter-related). Risk factors in women: female anatomy (short urethra — 4 cm — close proximity to perineum facilitates uroepithelial colonisation); sexual intercourse (post-coital UTI — bacteria mechanically inoculated into bladder); diaphragm and spermicide use (kills protective vaginal Lactobacillus, raises vaginal pH); postmenopausal oestrogen deficiency (atrophic urethritis and vaginitis — loss of Lactobacillus colonisation, mucin, and protective vaginal flora); pregnancy (physiological ureteric dilatation and bladder displacement impair urinary drainage); and prior UTI. Risk factors in men: urethral stricture, benign prostatic hyperplasia (urinary stasis), uncircumcised (prepuce colonises uropathogens), prostatitis. General risk factors: urinary catheterisation (CAUTI risk increases 3-7% per catheter-day); diabetes mellitus (glycosuria promotes bacterial growth, autonomic bladder dysfunction); immunosuppression; renal calculi; vesicoureteric reflux; structural uropathy.

Lower UTI (cystitis): dysuria (burning or stinging pain during urination — the cardinal symptom); urinary frequency (frequent small-volume voids); urgency (sudden compelling need to urinate, difficult to defer); nocturia; suprapubic pain or discomfort; haematuria (30-40% of cystitis episodes — visible or microscopic); cloudy or foul-smelling urine (from pyuria and bacteriuria). Systemic features are absent in uncomplicated lower UTI — their presence suggests upper tract involvement. Upper UTI (pyelonephritis): loin pain (unilateral or bilateral flank pain — from renal capsule distension); renal angle tenderness (costovertebral angle tenderness on percussion); fever (temperature above 38°C) and rigors; nausea and vomiting; preceding or concurrent lower UTI symptoms (though not always present). Severe pyelonephritis causing urosepsis: high fever, rigors, tachycardia, hypotension, and confusion — requires hospital admission and IV antibiotics. Important clinical patterns: complicated UTI (any UTI in a man, child, pregnant woman, or catheterised patient — requires MSU culture and targeted treatment); asymptomatic bacteriuria (significant bacteriuria in the absence of UTI symptoms — in most adults, treatment is not indicated and antibiotics increase resistance; exceptions: pregnancy where untreated ASB causes pyelonephritis in 40% and is treated).

Clinical diagnosis: in uncomplicated lower UTI in women, a clinical diagnosis based on characteristic symptoms (dysuria, frequency, urgency) in the absence of vaginal discharge is sufficient — NICE recommends antibiotic treatment without urine culture in uncomplicated cystitis with typical symptoms. Urine dipstick: nitrites (from bacterial reduction of urinary nitrates — positive with most gram-negative organisms; E. coli, Klebsiella, Proteus); leucocyte esterase (from pyuria — white blood cells); haematuria. A positive nitrites result with leucocyte esterase supports UTI; negative dipstick in an uncomplicated lower UTI makes UTI less likely. Midstream urine (MSU) culture: indicated for complicated UTI (men, pregnant women, children, recurrent UTI, treatment failure, pyelonephritis, catheter-associated) — clean-catch midstream specimen sent promptly; significant bacteriuria is conventionally over 10^5 colony-forming units (CFU)/mL of a single uropathogen with pyuria. Blood tests for pyelonephritis: full blood count (neutrophilia); CRP; blood cultures (positive in 25-30% of pyelonephritis — repeat positive culture not needed if initial blood culture positive); U&E and creatinine (assess renal function). Imaging: renal ultrasound for recurrent UTI, complicated UTI, pyelonephritis not responding to treatment within 48 hours (exclude obstruction — infected hydronephrosis is a urological emergency), and all first UTIs in men and children.

Uncomplicated lower UTI in non-pregnant women (NICE NG112, 2023): first-line nitrofurantoin 100 mg modified-release twice daily for 5 days (concentrated in urine; active against most E. coli; avoid in eGFR below 45 mL/min/1.73m2); second-line trimethoprim 200 mg twice daily for 7 days (high local resistance rates of 25-30% in the UK — use only when previous susceptibility confirmed or local resistance is below 20%). Pivmecillinam 400 mg TDS for 3 days and fosfomycin 3 g sachet single dose are alternatives. Avoid fluoroquinolones (ciprofloxacin, ofloxacin) for uncomplicated UTI — reserve for complicated infection. Phenazopyridine or sodium citrate sachets (Cymalon) — urinary alkalinisers — may provide symptomatic relief but do not treat infection. Uncomplicated pyelonephritis (outpatient): co-amoxiclav 500/125 mg TDS for 7-10 days; cefalexin 500 mg QDS for 7-10 days; review culture results and adjust. Severe pyelonephritis (hospital admission): IV gentamicin once daily (guided by weight, renal function, and drug levels) or IV ceftriaxone 2 g once daily; step-down to oral once blood cultures are negative and patient improving. Complicated UTI: send MSU culture first; empirical treatment based on local antibiogram while awaiting culture; treat underlying cause (relieve obstruction, remove infected catheter). Catheter-associated UTI (CAUTI): remove or change catheter; send catheter urine culture; targeted antibiotic 5-7 days. Recurrent UTI in women (3+ per year): continuous prophylaxis: nitrofurantoin 50-100 mg nocte (75-95% reduction in recurrence); or trimethoprim 100 mg nocte; post-coital prophylaxis for coitally triggered UTI; self-start therapy (patient-initiated 3-day course with antibiotic kept at home, starting at symptom onset); D-mannose 2 g daily (binds E. coli Type 1 fimbriae, preventing uroepithelial adhesion — evidence comparable to low-dose antibiotic prophylaxis in a 2014 RCT); topical oestrogen cream for postmenopausal women (restores Lactobacillus colonisation).

Pyelonephritis: ascending infection from untreated cystitis can cause pyelonephritis — particularly in pregnant women (asymptomatic bacteriuria progresses to pyelonephritis in 40% without treatment) and in patients with vesicoureteric reflux. Acute pyelonephritis causes renal scarring, particularly in children with vesicoureteric reflux — repeated episodes cause chronic pyelonephritis with progressive loss of renal function (reflux nephropathy). Renal abscess (perinephric or intrarenal): from untreated or inadequately treated pyelonephritis — requires CT-guided aspiration or surgical drainage. Infected hydronephrosis: urinary obstruction combined with infection (pus under pressure — pyonephrosis) — a urological emergency requiring urgent nephrostomy and antibiotics; rapidly fatal without drainage. Urosepsis: systemic sepsis from urinary tract infection (most common cause of gram-negative septicaemia) — carries 20-30% mortality. In pregnancy, UTI complications include preterm labour, low birth weight, and maternal sepsis. Chronic recurrent UTI causes significant morbidity — anxiety, pain, social disruption, reduced sexual function, and antibiotic resistance — which is increasingly important as antibiotic resistance to trimethoprim and fluoroquinolones rises in the UK.

Hydration: drink 1.5-2 litres of fluid daily to maintain dilute urine and regular bladder emptying — dehydration concentrates urine and slows washout of bacteria. Urinate promptly and do not defer urination — 'holding on' allows bacteria to multiply. Post-coital voiding: urinating after sexual intercourse significantly reduces post-coital UTI risk by washing bacteria introduced during intercourse out of the urethra. Avoid douching and aggressive genital washing — disrupts protective vaginal flora; use soap and water externally only. Wipe front to back after bowel movements (women) — prevents faecal uropathogens entering the urethra. Avoid spermicides and diaphragm contraception in women with recurrent UTIs — switch to condoms or alternative methods. Cranberry products: widely used but evidence is mixed — a 2023 Cochrane review found cranberry products modestly reduce UTI recurrence in women with frequent UTIs and children (RR 0.73); the active compound is A-type proanthocyanidins (PAC) that block E. coli fimbriae adhesion; use high-PAC standardised products. D-mannose 2 g daily: an evidence-based, non-antibiotic option for UTI prevention with a 2014 RCT showing effectiveness comparable to low-dose antibiotic prophylaxis. Topical oestrogen cream for postmenopausal women with recurrent UTI — the most effective preventive intervention in this population.

Self-treat with pharmacy advice (Cymalon, increase fluids) for mild initial UTI symptoms if you are a non-pregnant adult woman with typical symptoms (dysuria, frequency, no fever). Contact a GP for: symptoms of cystitis not improving within 48 hours of self-care or antibiotic treatment; any UTI in a man (all UTIs in men are considered complicated and require culture and targeted treatment); a pregnant woman with any urinary symptoms (asymptomatic bacteriuria in pregnancy requires treatment to prevent pyelonephritis and preterm birth); and any child with a febrile UTI. Seek urgent medical care (same-day GP or A&E) for: UTI symptoms with fever, rigors, loin pain, or vomiting (possible pyelonephritis); and any known kidney or bladder abnormality with UTI symptoms. Attend A&E immediately for: fever, confusion, rapid heart rate, and low blood pressure with UTI symptoms (urosepsis — a medical emergency); inability to pass urine; and severe loin pain with fever (possible infected obstruction — urological emergency). Any man or child with a first UTI and any adult with recurrent or treatment-resistant UTI requires renal ultrasound to exclude structural abnormality.