Urinary tract infection (UTI) – inflammatory response of the urothelium to bacterial invasion, associated with bacteriuria and pyuria
Bacteriuria – presence of bacteria in the urine
Cutoff for ‘significance’ is debated – initially thought to be 105 cfu / mL, but may be as low as 102 cfu / mL in symptomatic patients
Pyuria – presence of white blood cells in the urine, typically representing infection or inflammation
Bacteriuria without pyuria may represent colonisation
Sterile pyuria warrants further investigation if no known cause
Uncomplicated UTI – non pregnant women, with no known anatomical or functional urinary tract abnormality, without co-morbidities
Complicated UTI – all UTIs not fitting the above definition -> higher risk of complicated course
- Men
- Pregnant women
- Underlying functional or anatomical abnormalities
- Immunosuppression including diabetics
Normal host defences to urinary tract infection:
- Antegrade flow of urine
- Commensal flora of females – e.g. lactobacillus lowers vaginal pH
- Oestrogen (reduces pH, maintains skin integrity)
- Physical and chemical characteristics of urine (osmolality, pH, organic acid concentration, urea concentration)
- Normal excretion/exfoliation of urothelial cells
- Intact GAG layer
- Tamm-Horsfall protein (binds E.coli, preventing adherence)
Pathogenicity: ability of an organism to cause disease
Virulence: the degree of pathogenicity
Virulence factors: factors which help bacteria avoid host defences, and assist in colonisation
Common organisms causing UTI:
- E.coli – > 80 % of UTIs in some series – UPEC (uropathogenic E.coli) differentiated from normal gut colonisers
- Klebsiella
- Proteus
- Enterococcus
- Pseudomonas
- Staph. Saprophyticus
- Serratia
Pathogenic factors promoting infection / evading host response
- Extracellular capsule – to avoid phagocytosis and reduce immunogenic response
- Production of toxins and enzymes (gram negative sepsis commonly mediated by endotoxins)
- Mechanisms to reduce efficacy of antibiotics
- Intrinsic resistance – with these species selected for and replicated
- Biofilms
- Acquired – often plasmid mediated – e.g. production of beta-lactamase
- Adhesion mechanisms to attach to the host
- UPEC has a number of adhesins
- Type 1 (mannose sensitive) pili
- P pili (mannose resistant, p for pyelo)
Routes of infection:
- Ascending
- From bowel and skin -> ascend into bladder via urethra
- Bladder -> kidney
- Haematogenous
- Uncommon – usually staph seeded
- Lymphatic
- Unusual and rare
