I. UTIs due to bacterial pathogens

Urinary infections can be divided into:

• upper tract infections, which involve the kidneys (pyelonephritis), and
• lower tract infections, which involve bladder (cystitis), urethra (urethritis), and prostate (prostatitis).

As bacterial pathogenic infections often spread from one area to the other, differentiating between the sites may be difficult or impossible, especially in children.

Although urethritis (inflammation of the urethra) and prostatitis (prostate infection) are infections that involve the urinary tract, the term UTI commonly refers to:

• pyelonephritis (kidney infection) and
• cystitis (bladder infection).

In most cases, urinary infections are caused by a variety of pathogens normally present in the intestinal tract, especially in the colon and rectum.

Nearly all infections of the lower urinary tract are caused by a few strains of E. coli bacteria, called uropathogenic Escherichia coli (UPEC).

The second most frequent pathogenic organism causing UTIs is Klebsiella pneumoniae.

Other pathogens include:

• Staphylococcus aureus (Staph)
• Pseudomonas aeruginosa,
• Chlamydia trachomatis (a common sexually transmitted disease),
• Staphylococcus saprophyticus,
• Group B Streptococcus (GBS),
• Mycoplasma hominis,
• Enterococcus faecalis,
• Proteus mirabilis, and
• Citrobacter freundii.

Urethritis is usually caused by an STD (sexually transmitted disease). Prostatitis is typically due to pathogenic bacteria (see the above) and sometimes an STD.

II. UTIs due to nonbacterial pathogens

The most common nonbacterial pathogens that may cause urinary tract infections are:

• (more commonly) fungi, usually candida species and
• (less commonly) mycobacteria, the two most widely known species of mycobacteria are Mycobacterium tuberculosis, which causes tuberculosis, and Mycobacterium leprae, which causes leprosy,
• (less commonly) viruses, other than adenoviruses which are implicated in hemorrhagic cystitis, viruses have no major contribution to UTI in people with a normal immune response/immunocompetent, and
• (less commonly) parasites.

Nonbacterial pathogens causing urinary tract infections usually affect people who:

• are immunocompromised (with an impaired immune system),
• have diabetes (high blood sugar levels),
• have obstruction (blockage that inhibits the flow of urine through the urinary tract); or
• have structural urinary tract abnormalities, especially in pediatric age group; or
• have had recent urinary tract instrumentation (catheterization, cystoscopy, transurethral surgery, ureteroscopy or percutaneous nephrostomy).