Urinary tract infections (UTIs) are common, and most are uncomplicated. Bacteria are the most common organisms implicated, most often the Enterobacteriaceae, and the majority are due to E. coli.

Though many species of bacteria can be implicated in UTIs, the most common are given above.

Uncomplicated UTIs can be treated empirically. Infections that are recurrent or more severe can be assessed with microbiologic culture and antibiotic sensitivity to choose the best treatment.

Since collection of urine for culture by voiding is inherently a nonsterile method with contamination by perineal bacteria, then one criterion for assessment of a true infection is a "colony count" performed using a calibrated loop, with 0.001 mL of the urine sample plated onto culture media and incubated. The number of colonies present is multiplied by 1000. In this example, there are over 100,000 colonies/mL. This number, >10⁵ for one organism, is considered significant. A blood agar plate for colony counting is shown below:

Antibiotic sensitivity testing with a Kirby-Bauer plate us shown below illustrating the diffusion disk method in which antibiotic-impregnated disks are placed on a culture plate with the organism, and the zone of inhibition of growth is measured, compared to controls. In general, less than 2 mm of inhibition defines resistance, while a large zone of inhibition suggests sensitivity. Smaller zones imply intermediate sensitivity. In the example below, the organism is sensitive to Amikacin and Penicillin, with intermediate sensitivity to Gentamicin and Cefotaxime, with resistance to Methicillin, Vancomycin, and Nafcillin.