Microbiology of Urinary Tract Infections

Infections of the Urinary Tract

• Urinary tract infections (UTIs) are common, affecting men and women of all ages.
• UTIs are a frequent reason for prescribing antibiotics.
• Most UTIs are not life-threatening, but can be significant in certain situations.
• Bacterial virulence and host defense mechanisms influence the development of UTIs.
• Proper diagnosis and treatment are key to resolving UTIs effectively.
• Urine is not sterile; bacteriuria may be protective against UTIs in some instances.
• Overuse of antibiotics should be avoided in UTI cases, especially in light of MDR bacteria.
• Appropriate antibiotic stewardship is crucial to manage UTIs.

Definitions

• UTI: Inflammatory response of the urothelium to bacterial invasion, often associated with bacteriuria and pyuria.
• Bacteriuria: Presence of bacteria in the urine, often considered an indicator of infection or colonization.
• Pyuria: Presence of white blood cells (WBCs) in the urine, indicative of infection.

Types of UTIs

• Uncomplicated UTIs: Infections in healthy individuals with a normal urinary tract, often resolved by a short course of oral antibiotics.
• Complicated UTIs: Infections in individuals with underlying conditions or abnormalities (e.g., obstruction, diabetes, pregnancy) that increase risk, often require more invasive treatment and can be more difficult to treat due to the presence of antibiotic-resistant organisms.

Routes of Infection

• Ascending route: Bacteria enter from the bowel or skin, ascending through the urethra to the bladder then possibly to the kidney.
• Hematogenous route: Rare; bacteria spread through the bloodstream to the kidney
• Lymphatic route: Bacteria spread from adjacent organs through lymphatics; a less common route.

Urinary Pathogens

• Escherichia coli (E. coli): The most common cause of UTIs, contributing to 85% of community-acquired and 55% of hospital-acquired infections.
• Other Gram-negative Enterobacteriaceae (e.g., Proteus, Klebsiella) and Gram-positive bacteria (e.g., Enterococcus, Staphylococcus saprophyticus) also cause UTIs.

Incidence and Epidemiology

• UTIs are a common reason for emergency room visits and physician office visits in the United States.
• Incidence increases with age, especially in the elderly.
• Other risk factors include diabetes, pregnancy, and spinal cord injury.
• UTIs are highly recurrent in susceptible individuals.

Host Defenses of the Urinary Tract

• Periurethral and Urethral Region: Normal flora (e.g., lactobacilli, coagulase-negative staphylococci, and streptococci).
• Urine: Has antimicrobial properties (pH, urea concentration, organic acids), inhibiting pathogen growth.
• Uromodulin: Plays a role in limiting bacterial attachment to the urothelium.

Bladder Infections

• Uncomplicated Cystitis: Common infection presenting with dysuria, frequency, and urgency.
• Acute Pyelonephritis: Presents with flank pain, fever, chills and is often accompanied by bacteriuria and pyuria.

Kidney Infections

• Renal Infection: Presents with acute onset of fever, chills and flank pain, often accompanied by pyuria and bacteriuria.
• Emphysematous Pyelonephritis: Rare; characterized by gas in the renal parenchyma, commonly associated with diabetes.
• Renal Abscess: Accumulation of pus within the renal parenchyma, often caused by spreading infection from other sites.
• Xanthogranulomatous Pyelonephritis (XGP): Chronic infection characterized by granulomatous inflammation, often linked to urolithiasis or obstruction.
• Renal Echinococcosis: Parasitic infection, the cysts tend to appear in the cortex.
• Suppurative processes: include perinephric abscess.

Other Infections

• Fournier gangrene: Severe necrotizing fasciitis of the perineum that rapidly progresses.
• Periurethral Abscess: Infections in the periurethral compartment. Typically, patients have a history of prior trauma, instrumentation, or UTIs.

Bacterial Relapse

• The concept that bacteria persist in the kidney between episodes of bacteriuria to cause “relapsing” UTI is not accurate in most cases.

Bacterial persistence

• If a UTI fails to respond to treatment, a possibility of bacterial persistence should be considered, often due to failure to eradicate the infection due to presence of the infection nidus, or the presence of antibiotic resistance.

Treatment of UTIs

• Empiric antibiotic therapy based on the clinical presentation and likelihood of common pathogens; treatment is usually shortened in most uncomplicated UTI cases.
• Treatment should include urine cultures, determining antibiotic susceptibility of the isolated organism.
• Consideration of underlying anatomical and functional anomalies.
• Specific patient factors influencing the choice and duration of therapy will alter treatment regimens.

Catheter-Associated Infections (CAUTIs)

• Infections associated with indwelling urinary catheters.
• Can be caused by contamination introduced at insertion or by ascending infection through the catheter lumen.

Surgical Antimicrobial Prophylaxis

• Peri-procedural antibiotic treatment of the urinary tract to prevent local and systemic infections.
• AP is commonly used; indications, duration, and choice of antibiotics are determined by the specific procedure and the patient's risk profile.

Pregnancy and UTI

• Pregnant women have an increased risk of UTI due to anatomical and physiological changes during pregnancy.
• Routine screening for bacteriuria is recommended for all pregnant women.
• Treatment of asymptomatic bacteriuria (ASB) is crucial during pregnancy to prevent pyelonephritis and its associated outcomes.

UTIs in the Elderly

• UTIs present differently in the elderly; they may be asymptomatic.
• The presence of bacteriuria does not indicate a clinically significant UTI in the elderly.
• Screening for ASB is not recommended for community-dwelling elderly adults.
• Treatment should be patient-specific based on symptoms.

Complications

• Premature births
• Low-birth weight
• Maternal Sepsis/Death
• Kidney damage
• Renal scarring
• Renal failure
• Other systemic infections

Principles of Antimicrobial Therapy

• Understanding the spectrum of antibiotic activity: crucial in treating uncomplicated UTIs.
• Consideration of the risk of developing antimicrobial resistance and collateral damage: A common risk of frequent and inappropriate antibiotic use.

Management of infections

• Early recognition of UTI is essential for minimizing complications
• Specific treatment protocols should be implemented based on factors such as age, health status of the patient, and the presence of anatomical anomalies.