The bacterial strains that cause UTIs include:

• Escherichia (E.) coli is responsible for 75 - 90% of uncomplicated cystitis cases in younger women and in more than half the cases in older women (over age 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 - 15% of UTIs, mostly in younger women. Infections caused by this bacterium tend to 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 organisms 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.

Organisms in Severe or Complicated Infections

• 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.

Bacterial Strains in Recurrent UTIs

Recurring infections are often caused by different bacteria than those that caused a previous or first infection.

Even if the reinfecting bacterium is still E. coli, it may be a variant of the original infecting E. coli strain. Such strains produce substances, such as one called P fimbriae, which tend to make the bacteria more infectious. Uncommon causes of reinfection include Ureaplasma and Mycoplasma hominis, which are sometimes associated with acute urethral syndrome.

Factors in Overcoming the Bacterial Defense Systems

Changes in the amount or type of acid within the genital and urinary tracts are certainly major contributors to lowering the resistance to infection. For example, beneficial organisms called lactobacilli increase the acidic environment in the urinary tract. Reductions in their number (which, for example, occur with estrogen loss after menopause), increase pH and therefore the risk of infection.