Aetiology
Escherichia coli is the cause in 70% to 95% of uncomplicated cases, and Staphylococcus saprophyticus is the cause in 5% to 20% of cases.[12] Other causative pathogens in uncomplicated UTIs include Enterobacteriaceae such as Proteus mirabilis and Klebsiella species, enterococci, group B streptococci, Pseudomonas aeruginosa, and Citrobacter genus.
A broad range of bacteria can cause complicated UTIs, and many are resistant to multiple antimicrobial agents. Citrobacter and Enterobacter genera, P aeruginosa, enterococci, and Staphylococcus aureus account for a relatively high proportion of cases compared with uncomplicated UTI.[13]
Worldwide, infections caused by gram-negative strains (e.g., E coli, Enterobacteriaceae, Ps aeruginosa, or Acinetobacter genus infection) have increasing rates of resistance to the main antibiotic classes.[14] Localised knowledge of the common causative pathogens of UTIs, including local susceptibility patterns, is essential for the judicious use of antibiotics and ongoing antimicrobial stewardship.[15]
Methicillin-resistant S aureus remains a very uncommon cause of uncomplicated cystitis or pyelonephritis.[16]
Pathophysiology
The most common route of infection in females is via an ascending pathway. Colonisation of the vagina may occur first, then ascends into the urinary tract.[17] Ascending UTI is amplified by factors that promote the introduction of bacteria at the urethral meatus and by iatrogenic means. Stasis of bladder urine impairs the defence against infection provided by bladder emptying.[18] While the mechanical model of ascending infection explains the means of onset of bacteriuria, host and bacterial factors explain the variability of risk for UTI among women.
Type 1 pili may enhance bacterial adherence and seem to be instrumental in the pathogenesis of bacterial cystitis.[19] Type 1 piliated Escherichia coli bind in greater numbers to vaginal fluid from women with E coli vaginal colonisation. Alkalinisation of vaginal fluid (as occurs post-menopausally) results in augmented binding. Conversely, acidification of vaginal fluid pH by application of topical oestrogen compounds may reduce recurrence of UTI in post-menopausal women.[20]
Classification
Uncomplicated or complicated
Uncomplicated UTIs include acute cystitis occurring in otherwise healthy, non-pregnant women without functional or anatomical urinary tract abnormalities or comorbidities.[1][2]
Complicated UTIs include infections in patients with functional or structural impairments that reduce the efficacy of antimicrobial therapy, e.g., abnormalities of the genitourinary tract, the presence of urological obstruction, an underlying condition that interferes with host defence, or recent urological intervention.[1][3][4][2] The involvement of the kidneys (pyelonephritis) or UTIs occurring in pregnancy are also considered complicated UTIs.[1][2]
Acute or recurrent
A UTI can be acute or recurrent. Acute UTIs are infections causing acute symptoms in the presence of infected urine. Some women may have infrequent or isolated UTIs, while other women may have frequent recurrent infections. Recurrent UTI is defined as at least two separate culture-proven episodes of acute UTIs and associated symptoms within 6 months, or at least three UTIs in 12 months.[1]
Re-infection/bacterial persistence
Re-infection can occur with varying time intervals or causative organisms.
Bacterial persistence is persistent infection with the same organism, usually because the nidus of infection has not been eradicated.
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