Before we discuss the causes of urinary tract infection (UTI), we cannot ignore the significance of definition of UTI. Urinary tract infection (UTI) implies multiplication of organisms in the urinary tract, and is defined by the presence of more than 100,000 organisms per ml in a midstream sample of urine (MSU).
A very important fact must not be ignored at any cost that such infections are much more common in women, about one-third of whom have a UTI at some time. The prevalence of UTI in women is about 30% at the age of 20, increasing by about 1% in each subsequent decade. In males UTI is uncommon except in the first year of life and in men over 60, in whom a degree of urinary tract obstruction due to prostatic hypertrophy is common. UTI causes considerable morbidity, and in small minority of cases, renal damage and chronic renal failure.
Risk factors include pregnancy, urinary tract malformations, urinary tract obstruction, calculus or renal stones, prostatic obstruction, bladder diverticulum, spinal injury, trauma, urinary tract tumor, diabetes mellitus and immunosuppression as in case of AIDS.
Now we move on to the causes of urinary tract infection. UTI may be uncomplicated or complicated; the latter may result in permanent renal damage, the former rarely (if ever) do so. Uncomplicated infections are almost invariably due to single strain of organisms.
Outside hospitals, E. coli derived from fecal reservoir accounts for about 75% of infections, the remainder being due to Proteus, Pseudomonas species, streptococci or Staphylococcus epidermidis. In hospitals a greater proportion of infections are due to organisms such as Klebsiella or streptococci, but fecal E. coli still predominates. Certain strains of E. coli have a particular propensity to invade the urinary tract. They possess surface fimbriae, at the tips of which are lectin molecules, which bind to glycolipid or glycoprotein surface receptors on the urothelium.
The first stage in the development of UTI is colonization of the periurethral zone with pathogenic fecal organisms. The urothelium of susceptible persons may have more receptors to which virulent strains of E. coli become adherent. Colonization by the pathogenic bacteria may be facilitated by lack of personal hygiene, wearing of sanitary towels and local infections like vaginitis.
In women, the ascent of organisms into the bladder is facilitated by the short urethra and absence of bactericidal prostatic secretions, while the longer male urethra protects against transfer of bacteria to the bladder. It must be kept in mind that multiplication of bacteria occurs in bladder, from where they reach the ureters and kidneys easily, facilitated by vasicoureteric reflux and dilatation of hypo tonic ureters.
Sexual intercourse naturally causes minor urethral trauma and may transfer bacteria from the perineum into the bladder.
Instrumentation of the bladder may also introduce organisms.
Residual urine left after voiding interferes with mucosal defense mechanisms; thus patients with bladder outflow obstruction, gynecological abnormalities, pelvic floor weakness or neurological problems are susceptible to infection.
Injury to the mucosa and the presence of a foreign body in the bladder also depress vesical defense mechanisms.