URINARY TRACT INFECTION
Category: Urology
Abstract : Urinary tract infection A diagnosis of urinary tract infection (UTI) used to be based on finding >105 bacteria/ml of urine, whether or not there were associated symptoms of infection. However, 20 - 40% of women with symptomatic UTIs present with bacteria counts of 102 - 104 bacteria/ml of urine (the low counts are due to frequent voiding due to the irritation caused by the infection and also
Urinary tract infection
A diagnosis of urinary tract infection (UTI) used to be based on finding >105 bacteria/ml of urine, whether or not there were associated symptoms of infection.
However, 20 - 40% of women with symptomatic UTIs present with bacteria counts of 102 - 104 bacteria/ml of urine (the low counts are due to frequent voiding due to the irritation caused by the infection and also represent the slow doubling time of many bacteria in urine). Furthermore, contamination of urine from perineal bacteria or those on the foreskins of uncircumcised men can often lead to bacterial counts of >105.
UTI is currently defined as the inflammatory response of the urothelium to bacterial invasion. This inflammatory response causes a constellation of symptoms. In bladder infection this is described as cystitis frequent small volume voids, urgency, suprapubic pain or discomfort, urethral burning on voiding (dysuria). In acute kidney infection (acute pyelonephritis) the symptoms are fever, chills, malaise, and loin pain, often with associated LUTS of frequency, urgency, and urethral pain on voiding. The strict requirement for >105 bacteria/ml of urine is no longer required to make a diagnosis of UTI. In symptomatic patients many clinicians will now make a diagnosis of UTI with bacterial counts of >102/ml.
Bacteriuria is the presence of bacteria in the urine. Bacteriuria may be asymptomatic or symptomatic. Bacteriuria without pyuria indicates the presence of bacterial colonization of the urine, rather than the presence of active infection ( active implies an inflammatory response to bacterial invasion of the urothelium).
Pyuria is the presence of white blood cells in the urine (implying an inflammatory response of the urothelium to bacterial infection or, in the absence of bacteriuria, some other pathology such as carcinoma in situ, TB infection, bladder stones, or other inflammatory conditions).
An uncomplicated UTI is one occurring in a patient with a structurally and functionally normal urinary tract. The majority of such patients are women who respond quickly to a short course of antibiotics.
A complicated UTI is one occurring in the presence of an underlying anatomical or functional abnormality (e.g. functional problems causing incomplete bladder emptying, such as BPO, DSD in spinal cord injury), stones in the kidney or bladder, fistula between bladder and bowel, etc. Most UTIs in men occur in association with a structural or functional abnormality and are therefore defined as complicated UTIs.
Complicated UTIs take longer to respond to antibiotic treatment than uncomplicated UTIs, and if there is an underlying anatomical or structural abnormality they will usually recur within days, weeks, or months.
Urinary tract infection may be isolated, recurrent or unresolved.
- Isolated UTI: an interval of at least 6 months between infections.
- Recurrent UTI: >2 infections in 6 months, or 3 within 12 months.Recurrent UTI may be due to reinfection (i.e. infection by a different bacteria) or bacterial persistence (infection by the same organism originating from a focus within the urinary tract). Bacterial persistence is caused by the presence of bacteria within calculi (e.g. struvite calculi), within a chronically infected prostate (chronic bacterial prostatitis), within an obstructed or atrophic infected kidney, or occurs as a result of a bladder fistula (with bowel or vagina) or urethral diverticulum.
- Unresolved infection: implies inadequate therapy and is caused by natural or acquired bacterial resistance to treatment, infection by different organisms, or rapid reinfection.
Risk factors for bacteriuria
Female sex; increasing age; low oestrogen states (menopause); pregnancy; diabetes mellitus; previous UTI; the institutionalized elderly; indwelling catheters; stone disease (kidney, bladder); genitourinary malformation and voiding dysfunction (including obstruction).
Urinary tract infection: microbiology
Most UTIs are caused by faecal-derived bacteria which are facultative anaerobes (i.e. they can grow under both anaerobic and non-anaerobic conditions).
Uncomplicated UTI
Most UTIs are bacterial in origin. The most common cause is Escherichia coli (E. coli), a gram-negative bacillus, which accounts for 85% of community acquired and 50% of hospital acquired infection. Other common causative organisms include Staphylococcus saprophyticus and Enterococcus faecalis (also known as Streptococcus faecalis gram +ve), Proteus mirabilis, and Klebsiella (gram-negative enterobacteriacae).
Complicated UTI
E. coli is responsible for up to 50% of cases. Other causes include Enterococci (e.g. Streptococcus faecalis), Staph. aureus, Staph. epidermidis (gram +ve), Pseudomonas aeruginosa (gram -ve).
Route of infection
- Ascending
The vast majority of UTIs result from infection ascending retrogradely up the urethra. The bacteria, derived from the large bowel, colonize the perineum, vagina, and distal urethra. They ascend along the urethra to the bladder (increased risk in females as urethra shorter) causing cystitis, and from the bladder they may ascend, via the ureters, to involve the kidneys (pyelonephritis). Reflux is not necessary for infection to ascend to the kidneys, but the presence of reflux will encourage ascending infection, as will any process that impairs ureteric peristalsis (e.g. ureteric obstruction, gram-negative organisms and endotoxins, pregnancy). Infection which ascends to involve the kidneys is also more likely where the infecting organism has P pili (filamentous protein appendages also known as fimbriae which allow binding of bacteria to the surface of epithelial cells).
- Haematogenous
Uncommon, but is seen with Staph. aureus, candida fungaemia, and TB.
- Infection via lymphatics
Seen rarely in inflammatory bowel disease, retroperitoneal abscess.
Factors increasing bacterial virulence
- Adhesion factors
Many gram-negative bacteria contain pili on their surface, which aid attachment to urothelial cells of the host. A typical piliated cell may contain 100 - 400 pili. Pili are 5 - 10nm in diameter and up to 2µm long. E. coli produces a number of antigenically and functionally different types of pili on the same cell; other strains may produce only a single type, and, in some isolates, no pili are seen. Pili are defined functionally by their ability to mediate hemagglutination (HA) of specific types of erythrocytes. Mannose-sensitive (type 1) pili are produced by all strains of E. coli. Certain pathogenic types of E. coli also produce mannose-resistant (P) pili (associated with pyelonephritis).
- Avoidance of host defense mechanisms
An extracellular capsule reduces immunogenicity and resists phagocytosis (E. coli). M. tuberculosis resists phagocystosis by preventing phagolysosome fusion.
- Toxins
E. coli species release cytokines which have a direct pathogenic effect on host tissues.
- Enzyme production
Proteus species produce ureases, which causes breakdown of urea in the urine to ammonia, which then contributes to disease processes (struvite stone formation).
- Host defences
Factors which protect against UTI are:
- Mechanical flushing effect of urine through the urinary tract (i.e. antegrade flow of urine).
- A mucopolysaccharide coating of bladder (Tamm - Horsfall protein) helps prevent bacterial attachment.
- Low urine pH and high osmolarity reduces bacterial growth.
- Urinary immunoglobulin (IgA) inhibits bacterial adherence.
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