Diploma Thesis from the year 2003 in the subject Biology - Micro- and Molecular Biology, grade: 1 (A), University of Innsbruck (Institute for Microbiology), 22 entries in the bibliography, language: English, abstract: In 1885 Theodor Escherich described the gram negative bacterium Escherichia coli (E. coli) (Escherich T., Rev.1989). The gram negative rod belongs to the family of the enterobacteriaceae. It is a natural inhabitant of the human and animal intestine. E. coli can also cause diseases like diarrhea, inflammation of the urinary tract or the gall bladder. Urinary tract infections (UTIs) are one of the most common reasons for antibiotic therapy worldwide (Burman and Olsson-Liljequist, 2001). People with UTIs suffer inflammation of the urinary tract, and frequently the kidneys. Two-thirds of patients with UTIs are women (Canbaz et al. 2002). This is related in part to the shortness of the urethra, which makes colonization of the bladder by bacteria more likely. The elderly and those who undergo genitourinary operations and catheterisation are also frequent sufferers of UTIs (Orenstein and Wong, 1999). The leading causative agent of UTIs is E. coli (60-80 %), usually originating from the patients own faecal flora, followed by Staphylococcus saprophyticus (10 %), Klebsiella sp., other Gram negative bacteria and enterococci (Burman and Olsson-Liljequist, 2001). The antibiotic class most frequently prescribed to treat UTIs in Western Europe and North America is the fluoroquinolones. Fluoroquinolones are synthetic antibiotics derived from nalidixic acid. Resistance to the synthetic fluoroquinolone antibiotics is increasing among the organisms that cause UTIs.