Last updated on June 21st, 2021
The microbial etiology of urinary tract infections (UTI) has been regarded as well-established and reasonably consistent.
Polymicrobial UTI in the absence of underlying anatomic abnormalities, foreign bodies or trauma are rare. The presence of three different organisms even in large quantities suggest a contaminated urine specimen.
- Escherichia coli: The majority of community-acquired symptomatic UTIs in elderly women are caused by E coli. E. coli remains the predominant uropathogen (80%) isolated in acute community-acquired uncomplicated Urinary Tract infections.
- Staphylococcus saprophyticus (10% to 15%)
- Proteus species
Other pathogens (comparatively rare)
- Streptococcus agalactiae
- Enterococci infrequently cause uncomplicated cystitis and pyelonephritis.
The etiology of UTI is also affected by underlying host factors that complicate UTI, such as
- Age: The most common organisms isolated in children with uncomplicated UTI are Enterobacteriaceae. Gram-positive organisms are common, and polymicrobial infections account for up to 1 in 3 infections in the elderly.
- Gender: Women are affected more often than men (about 40 to 50 times), because of the shorter female urethra (4 cm) compared with the male urethra (20 cm) i.e. infectious agents reach the bladder more easily in females.
- Diabetes: Etiologic pathogens associated with UTI among patients with diabetes include Klebsiella spp., Group B streptococci, and Enterococcus spp., as well as E. coli.
- Spinal cord injury or urinary catheterization: Patients with spinal cord injuries commonly have E. coli infections.
Complicated vs. Uncomplicated UTI:
Complicated UTI has a more diverse etiology than uncomplicated UTI, and organisms that rarely cause disease in healthy patients can cause significant disease in hosts with anatomic, metabolic, or immunologic underlying disease.
Treatment of Urinary Tract Infections
Choice of the antibiotics for the treatment of any infections depends on patients’ specific factors (age, underlying diseases/abnormalities) local resistance patterns of the etiological agents, and cost & availability of the drugs).
There are several national and international guidelines that help to choose an empiric regimen for UTI. You can refer to 2010 IDSA/ESCMID guidelines (they preferred nitrofurantoin, trimethoprim-sulfamethoxazole, fosfomycin, and pivmecillinam for uncomplicated cystitis in women) or local guidelines of your country/hospital regarding the choice of antibiotics for the treatment of UTI.
Drugs used for the empiric treatment of uncomplicated UTI are:
- Avoid fluoroquinolones for uncomplicated UTI when alternative antibiotics are possible.
- Fluoroquinolones should not be used if the local prevalence of resistance of the uropathogen exceeds 10%.
- In patients with reduced renal function, Nitrofurantoin should only be used if local resistance data suggests a high level of resistance to alternative agents.
- Due to lower drug levels in the renal parenchyma both Nitrofurantoin and fosfomycin should be avoided in cases of suspected pyelonephritis.
- Trimethoprim-sulfamethoxazole (co-trimoxazole): should not be used if the local prevalence of resistance of uropathogen (E.coli) exceeds 20%.
In the cases of suspected pyelonephritis (loin pain and fever), suspected UTI in men, recurrent UTI, pregnancy, and failed antibiotics treatment or persistent symptoms urine should be sent for the culture.