How Urinary Tract Infection (UTI) Occurs?

Urinary tract infection (UTI) is frequent in women, in whom it typically occurs in an anatomically normal urinary tract. On the other hand, it is infrequent in men and children, and the urinary tract may be abnormal and requires investigation. The incidence of UTI is 50,000 per million persons per year and accounts for 1-2% of patients in primary care. Recurrent infection causes substantial morbidity; if complicated, it can result in severe renal disease including the gravest renal failure. It is also a frequent source of life threatening Gram-negative septicemia.

Percentage of causative organisms responsible for UTI

Escherichia coli and other 'coliforms' – 68+ %
Proteus mirabilis – 12%
Klebsiella aerogenes – 4%
Enterococcus faecalis – 6 %
Staphylococcus (saprophyticus or epidermidis) - 10%


How Urinary tract gets infected?

Infection is mostly due to bacteria from the patient's own gut flora. Transfer to the urinary tract may be through the bloodstream, the lymphatics or by direct portal (e.g. from a vesicocolic fistula), but is most commonly via the ascending transurethral route. Symptomatic infection is correlated to the virulence of the organisms, which competes with the natural body defense system, However, inflammation and injury are determined by the body’s response and not by the bacterium.

Criteria for virulence of a bacteria:

Ability to stick on to epithelial cells determines the level of virulence of the organism. For E. coli, these factors include flagellae (for motility), aerobactin (for iron acquisition in the iron poor environment of the urinary tract), haemolysin (for pore forming) and most importantly, the presence of adhesins on the bacterial fimbriae and on the cell surface.

There are two types of E. coli: those with type 1 fimbriae (with adhesin known as FimH) responsible for cystitis; and those with type P fimbriae (with adhesion known as PapG) commonly associated with pyelonephritis.

Bacterial adhesins are required for stickiness of bacteria to the mucous membranes of the perineum and urothelium. There are quite a few molecular forms of adhesins. The most studied is the Pap-G- adhesin, which is positioned on the tip of P fimbriae.

Natural defense mechanisms of body, those prevent UTI:

The following body defense mechanisms are needed to prevent UTI.

#Neutrophils - adhesins activate receptors, e.g. Toll 4, on the mucosal surface, resulting in IL-8 production and expression of its receptor CXCR1 on neutrophil
surfaces. Activa tion of neutrophils is important for bacterial destruction. Defective IL-8 production or reduced expression of CXCR1 results in impaired function of neutrophils making a person susceptible to severe UTI.

#Urine osmolality and pH - urinary osmolality more than 800 m osm/kg and low or high pH is detrimental for bacterial survival.

#Complement - complement activation with IgA production by uroepithelium (acquired immunity) is also a major factor in defense against UTI.

#Commensal organisms - such as lactobacilli, coryne bacteria, streptococci and bacteroides are part of the normal host defense. Destructions of these commensal
organisms by spermicidal jelly or disruption by certain antibiotics helps for overgrowth of E. coli.

#Urine flow - urine flow and normal passage of urine washout bacteria. Urine stasis due to any reason promotes UTI.

#Uroepithelium - mannosylated proteins such as Tamm- Horsfall proteins (THP), which are present in the mucus and glycocalyx covering uro epithelium, have antibacterial properties. These proteins hinder with bacterial attachment to uroepithelium. Trauma or disruption of this uro epithelium as in sexual intercourse or catheterization predisposes to UTI.

#Cranberry juice (blueberry juice) has a large-molecular-weight factor (p roanthrocyanidins) that prevents attachment of E. coli to the uroepithelium.

#Blood group antigens - women who are non-secretors of ABH blood group antigens are three to four times susceptible to have recurrent UTIs.

How UTI Occurs?
Complicated, Uncomplicated, recurrent and relapsing UTI
Symptoms and diagnosis of UTI
Treatment of UTI
Special types of UTI