Causes

Bacteria cause a UTI, most commonly Escherichia coli (E.Coli), which has overcome the body’s defenses in the urinary tract. This is most commonly caused by colonization of the vaginal introitus by fecal flora.  This flora ascends the urethra (which is shorter in women than in men) into the bladder and establishes an infection. Though several other microbes can cause a UTI, Gram-negative E.coli accounts for 75% to 95% of uncomplicated UTI infections. Staphylococcus saprophyticus is the offending pathogen in 5% to 15% of UTI diagnoses in younger women, and other prevalent UTI pathogens include Klebsiella spp, Proteus Mirabilis, and Enterobacteriaceae.  Organisms such as lactobacilli, enterococci, Group B streptococci, and coagulase-negative staphylococci other than S. saprophyticus from voided midstream urine most commonly represent contamination of the urine specimen and not an actual infection.  High counts of these bacteria (>105 bacteria counts) or if they grow in isolation can indicate infection rather than colonization.5

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