UTIs lecture

Urinary Tract Infections (UTIs)

  • Objectives:

    • Describe different types of UTIs.
    • Understand general risk factors.
    • Understand how UTIs are diagnosed.
    • Consider the impact of microbial communities (biofilms) on UTIs.

  • Common Uropathogens:

    • Focus on uropathogenic E. coli (UPEC).
    • Reference catheter-associated infections from previous lectures.

Defining UTIs

  • Symptomatic Infection:

    • UTI is diagnosed only when symptoms are present.
    • Historically, urine was considered sterile, but now it's known to have a microbiome.

  • Asymptomatic Bacteriuria:

    • Significant bacterial numbers in urine without symptoms.
    • Clinical data based on culture defines it as a single species at approximately 10^5 per milliliter from a midstream urine sample.

Types of UTIs

  • Lower UTIs:

    • Involve the bladder (cystitis) or urethra (urethritis).

  • Upper UTIs:

    • Affect the kidneys or ureters.

Anatomy of the Urinary Tract

  • Two kidneys, two ureters, bladder, and urethra.
  • Ureters connect kidneys to the bladder, and the urethra allows urine to exit.

Urine Composition

  • Complex fluid with micronutrients and micro compounds.
  • Artificial urine contains about 17 components, including water, urea, and various salts.
  • Human urine contains lysozyme, amylase, albumin, and other components.

Clinical Significance of UTIs

  • Common bacterial infection, accounting for 1-3% of general practitioner visits.
  • More than 1 in 3 women experience a UTI by age 24.
  • Female anatomy increases the risk.
  • Treatment incidence increases with age, particularly in women.
  • Pregnancy increases risk.

Risk Factors

  • Anatomical: Shorter urethra in females allows easier bacterial access to the bladder.
  • Birth Control: Some types increase risk.
  • Hormonal Changes: Menopause-related estrogen changes.
  • Sexual Activity: Increases risk.

Classification of UTIs

  • Recurrent vs. Single Episode:
    • Recurrent UTIs can be caused by the same or different organisms.
  • Uncomplicated vs. Complicated:
    • Uncomplicated: Infection in healthy individuals without urinary tract problems.
      • Example: Simple cystitis cleared with antibiotics.
    • Complicated: Infection associated with other factors.
      • Presence of urinary catheter (biofilm formation).
      • Urinary obstruction (stones).
      • Kidney failure.
      • Urinary retention (inability to release urine).

Catheter-Associated Infections

  • Indwelling catheters for more than 30 days have nearly a 100% chance of infection or blockage.

Development of Infection

  • Colonization at the urethra entrance, movement to the bladder, and colonization within the bladder tissue (lower UTI).
  • Pathogens ascending into the ureters lead to an upper UTI, potentially causing acute kidney injury.

Special Concerns

  • Elderly People: UTIs can be serious, potentially causing symptoms confused with dementia.

    • Medical protocol: Rule out UTI in elderly patients showing memory loss or confusion.

  • Review Paper:

    • Refer to review paper on Blackboard summarizing bacterial invasion of the urinary tract.

Common Bacteria

  • Uncomplicated UTIs:
    • E. coli (uropathogenic E. coli or UPEC) is the most prevalent.
  • Other Pathogens:
    • Klebsiella, Staphylococcus, Pseudomonas aeruginosa, Candida, Enterococcus, Acinetobacter baumannii.

Is Urine Sterile?

  • Urine is generally not sterile.
  • Historically, sterility was assumed based on observations from the 1800s.
  • Sequencing shows low levels of complex bacterial communities.
  • The threshold of 10^5 colony-forming units per milliliter dates back to a 1950s report.
  • The Human Microbiome Project initially excluded urine.

Diagnostic Tests

  • Rely on dipstick chemical tests and culture when indicated.
  • Antibiotics given rapidly after diagnosis.
  • Symptoms:
    • Dysuria (painful urination).
    • Polyuria (excessive urination).
    • Pyuria (white blood cells in urine).
    • Hematuria (blood in urine).
    • Suprapubic (related to catheters inserted through the abdominal wall).

Dipstick Tests

  • Quick chemical tests for pH, white blood cells, leukocytes, and nitrites.
  • Nitrites indicate the presence of nitrate-reducing bacteria like E. coli and Klebsiella.
  • Limitations: Not very sensitive; requires a high concentration of organisms.

Leukocyte Test

  • Microscopic examination for white blood cells.
  • More than three white blood cells per field of view indicate infection.
  • Enzymatic color change test.
  • Limitations: Issues around time and detection level.

Diagnostic Pathway

  • Clear pathway for diagnosing UTIs based on symptoms and bacteria results.
  • Criteria for UTI Diagnosis:
    • More than 10^4 per milliliter of a single organism.
    • More than 10^5 mixed growth, but one predominant organism.
    • More than 10^3 of specific pathogens.
  • Mixed growth was previously considered contamination.

Uropathogenic E. coli (UPEC)

  • E. coli colonizes the GI tract first, followed by transfer to the urinary system.
  • The urinary tract is a hostile environment with salts, urea, and varying pH levels.
  • UPEC strains have virulence mechanisms for survival.

Virulence Factors

  • Adhesion:
    • Critical due to rapid urine flow.
  • Iron Acquisition:
    • Essential in a low-nutrient environment.
  • Immune Evasion:
    • Avoiding host defenses.
  • Inflammation:
    • Causing inflammation.

Opportunistic Intracellular Pathogens

  • UPEC invades cells.

  • Adherence is crucial.

  • Primary Adherence Factors:

    • Pili and fimbriae (Type 1, P, and F1C pili).

  • Type 1 Pili:

    • Required for colonization, invasion, and persistence in the bladder.

Bladder Epithelium

  • Covers the urinary system.
  • Outer layer has umbrella cells that break away.
  • Bacteria need to colonize and persist despite this defense.

Adhesion System

  • Type 1 pili bind with specific receptors on host cells.
  • Initiates invasion cascade.
  • Colonization and invasion into umbrella cells.
  • Signal transduction cascade causes actin rearrangement, leading to engulfment.

Intracellular Communities

  • Intracellular Bacterial Communities (IBCs):
    • Transient and active.
  • Quiescent Intracellular Reservoirs (QIRs) or Shells:
    • Long-term, persisting communities.
    • Small, membrane-bound compartments with 4-10 bacteria.
    • Covered in actin to avoid host detection.
    • Allow long-term viability and reinfection of the bladder.
  • UPEC can be cleared in the majority by many antibiotics, but these communities will persist.

P Pili

  • Bind to glycan lipids, modulate immune response in upper UTIs.
  • Interact with toll-like receptor 4, affecting IgA transport.

Other Virulence Factors

  • Siderophores:
    • Help scavenge iron.
  • Alpha-Hemolysin:
    • Causes holes in umbrella cells, promoting lysis and releasing nutrients and iron.
  • Cytotoxic Necrotizing Factor 1:
    • Causes membrane ruffling, increasing bacterial involvement.

Catheter-Associated UTIs

  • Pili adhere to abiotic surfaces.
  • Proteus mirabilis:
    • Produces urease, breaking down urea and increasing pH.
    • Leads to crystal formation and catheter blockage.
  • Biofilm EPS traps crystals, creating complex structures.
  • Toxins like hemolysin and agglutinin lead to cell damage and nutrient release.
  • Enterococcus:
    • Forms biofilms on catheters.
    • Produces and uses fibrinogen as a food source, creating a structural biofilm matrix.

Multidrug Resistance

  • UTIs are a hotspot for resistance development.
  • Early reports of multidrug resistance came from urine isolates.
  • Factors promoting resistance: close contact, stress on cells, biofilm formation.
  • WHO priority pathogens are commonly found in UTIs.