Urinary Tract Infection

The bacterial strains that cause UTIs are the following:

Escherichia (E.) coli is responsible for between 75% and 90% of uncomplicated cystitis cases in younger women and in more than half the cases in older women (over 50). In most cases of UTI, E. coli, which originates as a harmless microorganism in the intestines, spreads to the vaginal passage, where it invades and colonizes the urinary tract. Some bacteria may be able to invade into deeper tissue in the bladder, where they survive to reinfect the patient after resolution of the previous infection.
Staphylococcus saprophyticus accounts for 5% to 15% of UTIs, mostly in younger women. Interestingly, infections caused by this bacterium have a seasonal variation, with a higher incidence in the summer and fall than in the winter and spring.
Klebsiella, Enterococci bacteria, and Proteus mirabilis account for most of remaining bacterial agents that cause UTIs. They are generally found in UTIs in older women.
Rare bacterial causes of UTIs include ureaplasma urealyticum and Mycoplasma hominis, which are generally harmless organisms.

The bacteria that cause kidney infections (pyelonephritis) are generally the same bacteria that cause cystitis. There is some evidence, however, the E. coli strains in pyelonephritis are more virulent (able to spread and cause illness).
Complicated UTIs that are related to physical or structural conditions are apt to be caused by a wider range of organism. E. coli is still the most common organism, but others have also been detected, including Klebsiella, P. mirabilis, and Citrobacter.
Fungal organisms, particularly Candida species. (Candida albicans, for example, causes the so-called "yeast infections" that also occur in the mouth, digestive tract, and vagina.)
Other bacteria associated with complicated or severe infection include Pseudomonas aeruginosa, Enterobacter, and Serratia species; gram-positive organisms, including Enterococcus species; and S. saprophyticus.

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