Urinary Tract Infection: Causes

Urinary tract infections (UTIs) usually occur due to bacteria, which enter the urinary tract via the urethra. Certain bacteria from the own bowel flora, live on the skin around the perineum. They can ascend and multiply in the urethra and the bladder. Sometimes, the bacteria can spread from the blood stream or the lymphatics to the urinary tract.

The urinary tract is designed to avoid the invading bacteria. Sometimes, these defence mechanisms can fail. Therefore, bacteria grow in the urinary tract, causing a UTI. There are several defence mechanisms to avoid urinary tract infections. They are:

• White blood cells, which help destroy invading bacteria.

• High urinary concentration (osmolality) and low or high pH, which reduce bacterial survival.

• Antibodies in the blood

• Commensal organisms like Lactobacilli, Streptococci, Corynebacteria, and Bacteroides, which minimise the growth of other pathogens. (Spermicidal jelly or certain antibiotics can eradicate commensals and predispose to overgrowth of Escherichia coli.)

• Urine flow, which washes out bacteria.

• The lining of the urinary tract (uroepithelium), which prevents the bacterial growth. (Sexual intercourse and catheterization can damage the uroepithelium and predispose to UTI.)

Urinary tract infections are common among women compared to men and children. The most UTIs affect the lower urinary tract, which consists of the urethra and the bladder.

Symptoms of a UTI are associated with the virulence of the bacteria, which competes with the immune system. However, inflammation and injury are based on the host response and not by the bacteria.

Escherichia coli and other coliform bacteria, which are frequently found in the gastrointestinal tract, are the most common pathogens of urinary tract infections. They account for about 70 % of UTIs. Other causative organisms include Staphylococcus saprophyticus, Staphylococcus epidermidis, Proteus mirabilis, Klebsiella aerogenes, and Enterococcus faecalis. UTI caused by Staphylococci is more common among young women (20-30 %).

Sexual intercourse may cause lower urinary tract infections, especially cystitis in females. However, you don’t need to be sexually active to get cystitis. All women have a higher risk of developing cystitis compared to men, because the short distance from the urethral opening to both the anus and the bladder. Sometimes, certain sexually transmitted infections, like gonorrhoea, herpes, and chlamydia, can lead to urethritis in women, because their urethra is near to the vagina.



Related Links:

How to Recognise a Urinary Tract Infection?
Who Is at Risk of UTI?
UTI: Complications and Diagnosis
UTI: Treatments
How to Prevent a UTI? 
How to Manage Urinary Tract Infection (UTI) in Children?