This tab covers the framework that underlies every urinary tract infection — how UTIs are defined and classified, who gets them, how organisms reach and colonise the tract, and how the diagnosis is established. The condition-specific tabs (cystitis, pyelonephritis, special hosts) build on these principles.
Definitions
- Cystitis — a clinical syndrome of dysuria, frequency, urgency, and occasionally suprapubic pain. These symptoms usually indicate bacterial cystitis but may also reflect urethral or vaginal infection, or non-infectious causes (interstitial cystitis, bladder carcinoma, calculi). Conversely, a patient may be asymptomatic yet have bladder — and possibly upper-tract — infection.
- Acute pyelonephritis — a clinical syndrome of chills, fever, and flank pain accompanied by bacteriuria and pyuria. The term should not be used if flank pain is absent. It may have no morphologic or functional correlate on routine imaging, and is difficult to diagnose in spinal-cord-injured and elderly patients who cannot localise discomfort.
- Chronic pyelonephritis — a shrunken, scarred kidney diagnosed by morphologic, radiologic, or functional evidence. It may be post-infectious but is frequently not associated with UTI.
Uncomplicated vs complicated: an uncomplicated infection occurs in a healthy patient with a structurally and functionally normal urinary tract (the majority are women). A complicated UTI requires at least one of the following:
- Anatomic or functional abnormality (outlet obstruction, stone disease, diverticulum, neurogenic bladder, vesicoureteral reflux).
- Urinary instrumentation or foreign bodies (catheters, stents, nephrostomy tubes).
- Systemic disease (renal insufficiency, diabetes, immunodeficiency, organ transplantation).
- Pregnancy.
- Multidrug-resistant bacteria.
"Chronic" is a poor term in the context of UTIs (except chronic pyelonephritis or chronic bacterial prostatitis) because the duration is undefined. UTIs are also defined by their relationship to one another:
- First / isolated — occurs in someone who has never had a UTI, or whose prior infection was remote.
- Unresolved — has not responded to therapy and is documented to be the same organism with a similar resistance profile.
- Recurrent — occurs after documented successful resolution of a prior infection. Two subtypes: bacterial reinfection (re-introduction of bacteria from outside) and bacterial persistence (the same organism re-emerging from a focus within the tract — an infection stone, the prostate, or an infected atrophic kidney). Relapse is often used interchangeably.
The distinction matters because reinfection and persistence drive different evaluations and treatments. Two related terms: antimicrobial prophylaxis (preventing reinfection) and antimicrobial suppression (preventing growth of a persistent focus that cannot be eradicated).
Epidemiology
- UTIs are the most common bacterial infection. About 30% of women have had a symptomatic UTI requiring antimicrobials by age 24, and ~50% experience one in their lifetime.
- Catheter-associated UTIs (CAUTIs) are the most common nosocomial infection; 80% of nosocomial UTIs are due to an indwelling urethral catheter.
- Once a patient has one infection, further infections are likely: the probability of recurrence rises with the number of prior infections and falls in inverse proportion to the interval between the first and second. About 70% of recurrences are reinfection with a different organism, and most occur after 2 weeks but within 5 months.
- Incidence is increased in spinal cord injury, women with diabetes, multiple sclerosis, and HIV/AIDS. Diabetic women have more asymptomatic and symptomatic UTIs (no substantial increase in diabetic men); most diabetic UTIs are asymptomatic but diabetes predisposes to more severe infection. UTIs are 5× more prevalent in HIV-positive individuals. Of all patients with bacteriuria, those with spinal cord injury have the most severe UTIs and morbidity.
Pathogenesis
A UTI results from interaction between uropathogen and host, determined by bacterial virulence factors, inoculum size, and inadequacy of host defences.
Routes of Infection
- Ascending (most common) — bacteria from the bowel ascend the urethra into the bladder; adherence to introital and urothelial mucosa is key, and the route is enhanced by faecal soiling of the perineum, spermicide use, and catheters. Most pyelonephritis arises from retrograde ascent from bladder → ureter → renal pelvis and parenchyma. Reflux is probably not required, but cystitis-related oedema can alter the ureterovesical junction enough to permit it. Ascent is increased by anything that impairs ureteral peristalsis (gram-negative endotoxins, pregnancy, obstruction); bacteria then enter the parenchyma through the collecting ducts at the papillary tips.
- Haematogenous — uncommon in normal individuals; the kidney may be seeded in S. aureus bacteraemia (often oral origin) or Candida fungaemia.
- Lymphatic — direct extension from adjacent organs via lymphatics in unusual circumstances (severe bowel infection, retroperitoneal abscess).
Uropathogens
Most UTIs are caused by facultative anaerobes originating from bowel flora; S. epidermidis and C. albicans come from vaginal/perineal skin. (Recall: staphylococci are gram-positive cocci in clusters, streptococci in chains.) A useful list of common pathogens is KEEPPS — Klebsiella, E. coli, Enterococcus, Proteus, Pseudomonas, S. saprophyticus.
- Gram-negative: Enterobacteriaceae — E. coli is the most common cause (≈85% of community-acquired and 50% of hospital-acquired UTIs); the ST131 (O25b:H4) clone is a rapidly emerging multidrug-resistant strain with common β-lactamase and fluoroquinolone resistance. Also Proteus, Klebsiella, and (Pseudomonadaceae) Pseudomonas.
- Gram-positive (most of the remaining community-acquired infections): Enterococcus faecalis and Staphylococcus saprophyticus.
- Nosocomial: E. coli, Klebsiella, Enterobacter, Citrobacter, Serratia, Pseudomonas, Providencia, E. faecalis, and S. epidermidis.
- Age effects: S. saprophyticus causes ≈10% of symptomatic lower UTIs in young, sexually active women but rarely infects males or the elderly. In neonates and young infants, empirically cover Enterococcus (higher incidence in early infancy).
- Fastidious organisms: symptomatic UTIs growing only obligate anaerobes are rare but these are frequent in suppurative GU infections — usually Bacteroides (incl. B. fragilis), Fusobacterium, anaerobic cocci, and Clostridium perfringens. Suspect them when irritative symptoms plus cocci or gram-negative rods on microscopy coexist with negative routine aerobic cultures. M. tuberculosis and non-TB mycobacteria do not grow aerobically and may surface during a sterile pyuria work-up.
- Uropathogenic E. coli (UPEC): expresses fimbrial and afimbrial adhesins. Type 1 (mannose-sensitive) pili are found on both pathogenic and non-pathogenic E. coli; "P" pili (P for pyelonephritis) are found in most pyelonephritogenic strains.
Host Defences and Susceptibility
- Epithelial receptivity — women susceptible to UTI may have vaginal (and buccal) epithelial cells with increased bacterial adherence, suggesting a genetic trait; blood-group determinants on uroepithelial cells also influence susceptibility. Uropathogens adhere more in menopausal than premenopausal women, and estrogen decreases recurrent-UTI susceptibility. At the bladder, FimH-mediated binding is the initial step and is essential for UPEC invasion.
- Natural defences — the normal flora of the introitus, periurethral area, and urethra (lactobacilli, corynebacteria, coagulase-negative staphylococci, streptococci) form a barrier. The most inhibitory urinary factors are osmolality, urea concentration, organic-acid concentration, and pH; growth is inhibited by very dilute urine or by high osmolality with low pH. Uromodulin (Tamm-Horsfall protein) saturates the mannose-binding sites of type 1 pili, blocking attachment to uroplakin receptors.
- Immune response — pathogen-associated molecular pattern receptors (e.g. Toll-like receptors) link recognition to the innate response (primarily local inflammation; faster than adaptive; PMNs, neutrophils, macrophages, eosinophils, NK cells, mast cells, dendritic cells). Adaptive immunity (T/B lymphocytes, high-affinity antibodies) develops 7–10 days after infection. UPEC modulates TLR signalling to extend a "window of opportunity" for infection.
Altered Host Defences and Renal Papillary Necrosis
- Obstruction is the key factor increasing susceptibility; stasis promotes bacterial growth and adherence.
- Vesicoureteral reflux — children with gross reflux and UTIs typically develop progressive renal damage (scarring, proteinuria, renal failure); lesser reflux usually improves or resolves. In adults, reflux does not reduce renal function unless there is stasis with concurrent UTIs.
- Renal papillary necrosis (RPN) — necrosis of the papillae and inner medulla (which receives 10% of renal blood flow). Associated conditions follow POSTCARDS: Pyelonephritis, Obstruction, Sickle cell disease, Tuberculosis, Cirrhosis, Analgesic abuse, Renal vein thrombosis, Diabetes mellitus (most common), Systemic vasculitis — plus miscellaneous causes (cryoglobulinaemia, renal candidiasis, contrast media, amyloidosis, calyceal arteritis, necrotizing angiitis, rapidly progressive glomerulonephritis, hypotensive shock, acute pancreatitis, transplant rejection, dehydration, hypoxia, neonatal jaundice). The role of infection is controversial. Acute ureteral obstruction from a sloughed papilla with concurrent UTI is a urologic emergency.
Natural History
- Complicated UTIs can cause progressive renal damage; the long-term effects of uncomplicated recurrent UTIs are not fully known, and no association has been established with renal scarring, hypertension, or progressive azotaemia.
- 57–80% of untreated (or placebo-treated) bacteriuric women clear their infection spontaneously.
- The risk of recurrent bacteriuria is the same whether a patient receives no treatment or short-term, long-term, or prophylactic therapy — prophylaxis reduces reinfections but does not change the underlying predisposition. Extended prophylaxis (≥6 months) lowers infection rates during treatment, but the rate returns to baseline once prophylaxis stops.
Diagnosis and Evaluation
History and Physical Exam
- Cystitis — dysuria, frequency, and/or urgency; suprapubic pain and haematuria are less common. Lower-tract symptoms usually precede upper-tract symptoms by several days.
- Pyelonephritis — fever, chills, flank pain; nausea and vomiting may occur.
- Renal or perirenal abscess — indolent fever with a flank mass and tenderness.
- Elderly — symptoms may be subtle (epigastric/abdominal discomfort) or absent.
- Indwelling catheters — often asymptomatic bacteriuria, but fever from bacteraemia can develop rapidly and become life-threatening.
Urine Collection
- Males (voided): circumcised men need no preparation; uncircumcised men should retract the foreskin, wash the glans with soap, and rinse. Collect the first 10 mL (urethral) and a midstream specimen (bladder). Prostatic fluid is obtained by digital massage onto a slide; the first 10 mL voided after massage reflects added prostatic fluid. Catheterisation for culture is not indicated unless the patient cannot urinate.
- Females (voided): midstream contamination with introital bacteria and WBCs is common — instruct the patient to spread the labia, cleanse the periurethral area with moist gauze, then collect midstream. Antiseptic cleansing is not recommended (risk of false-negative culture). A specimen showing vaginal epithelial cells and lactobacilli is contaminated, and a catheterised sample should be obtained.
- Suprapubic aspiration: highly accurate but carries some morbidity; reserved for patients who cannot urinate on command (spinal cord injury, newborns).
- Bag specimens: unreliable and unacceptable in high-risk populations and infants — in a child who is not toilet-trained, only a catheterised or needle-aspirated specimen is diagnostic (bagged specimens have a high false-positive rate). A negative bag or diaper specimen collected after good perineal cleansing and processed promptly can help exclude bacteriuria.
Urinalysis
Urinalysis gives rapid identification of bacteria and WBCs and a presumptive diagnosis (assess bacteria, epithelial cells, pyuria, haematuria, nitrites), but does not replace urine culture, which confirms the diagnosis. Sediment from ≈5–10 mL centrifuged for 5 minutes at 2000 rpm is examined.
- Bacteriuria — bacteria in normally sterile urine; "significant bacteriuria" is a count exceeding contamination. Found in >90% of infections at ≥10⁵ cfu/mL (highly specific); bacteria are usually not seen microscopically at lower counts (10²–10⁴/mL). False-negatives: early infection, dilute urine — a negative result never excludes bacteria. False-positives: contamination (numerous squamous epithelial cells), with risk rising as technique reliability falls from suprapubic aspiration → catheter → voided.
- Pyuria — WBCs in urine, indicating infection or inflammation from bacteria, stones, or a foreign body. Its absence should make you question the UTI diagnosis until culture returns. Bacteriuria without pyuria suggests colonisation rather than infection. Sterile pyuria warrants evaluation for tuberculosis, stones, or cancer (among many causes); almost any urinary-tract injury can elicit PMNs. Leukocyte-esterase tests detect pyuria.
- Nitrites — bacteria convert urinary nitrates to nitrites. Enterobacteriaceae (E. coli, Klebsiella, Proteus, Enterobacter, Serratia, Citrobacter) convert; gram-positives (enterococcus, staphylococcus) generally do not. The important exception is Pseudomonas, a gram-negative that lacks the enzyme.
- Haematuria — an inflammatory marker; microscopic haematuria appears in 40–60% of cystitis and is uncommon in other dysuric syndromes.
Urine Culture
Two techniques: direct surface plating and dip slides. Refrigerate immediately and culture within 24 hours. A cut-off of ≥10⁵ cfu/mL is the traditional definition of significant bacteriuria from a midstream specimen — but 20–40% of women with symptomatic UTIs present with only 10²–10⁴ cfu/mL, so in dysuric patients an appropriate threshold is 10² cfu/mL of a known pathogen. The ≥10⁵ cut-off can also over-diagnose contaminated specimens.
Localisation and Imaging
Fever and flank pain are common in pyelonephritis but can occur with bladder infection; ureteral catheterisation can separate bacterial persistence into upper versus lower tract and identify which kidney is involved.
Imaging is not required in most UTIs, since clinical and laboratory findings suffice. Indications include: risk factors for complicated UTI; febrile infection; failure to respond to appropriate therapy; potential obstruction (stricture, tumour, prior GU surgery, calculi — especially struvite stones); potential papillary necrosis; polycystic kidneys in dialysis/severe renal insufficiency; neurogenic bladder; and unusual organisms (tuberculosis, fungus, or urea-splitting organisms such as Proteus, Pseudomonas, Klebsiella, Staphylococcus, Mycoplasma). Options are ultrasound, CT/MRI, voiding cystourethrogram (VCUG, for reflux), and radionuclide studies.