The Roles of the Host and the Pathogens in Urinary Tract Infections Be ´la Ko ¨ves a, *, Bjo ¨rn Wullt b a Department of Urology, South-Pest Teaching Hospital, Budapest, Hungary; b Section of Microbiology, Immunology and Glycobiology, Institute of Laboratory Medicine, Lund, Sweden 1. Introduction Urinary tract infections (UTIs) are among the most frequent infections encountered in the community and also one of the leading causes of hospital-acquired infections with significant consequences of morbidity and cost. UTIs may be acute symptomatic infections of varying severity and localization, but they may also result in the establishment of an asymptomatic carrier state, asymptomatic bacteriuria (ABU) (Fig. 1). The severity of the infection varies depending on the balance between the infecting bacterial strain and the antibacterial host defenses. E U R O P E A N U R O L O G Y S U P P L E M E N T S X X X ( 2 0 1 6 ) X X X – X X X ava ilable at www.sciencedirect.com journa l homepage: www.europea nurology.com Article info Keywords: Urinary tract infections Escherichia coli Virulence factors Host response Innate immunity Abstract The severity of urinary tract infections (UTIs) varies depending on the balance between the virulence of the infecting bacterial strain and the antibacterial host defense. Bacterial virulence is determined by a complex of factors in which bacterial adherence to the uroepithelium is the most important virulence factor, in addition to the production of toxins and the formation of biofilm. In immuno- compromised patients and in patients with severely dysfunctional urinary tracts, however, the importance of bacterial virulence factors to cause symptomatic infection is decreased or nullified. The antibacterial host defense in the urinary tract depends mainly on native immunity and inflammation. Specific immunity, with antigen presentation and antibody production, does not play an important role in acute UTI. Recent research has provided a deeper understanding of the inflammation process in UTI and demonstrated that the individual variation of UTI susceptibility and renal damage not only depends on urinary tract dysfunctions but is also influenced by genetic polymorphisms in innate immune receptors and signaling proteins, crucial for the innate antibacterial defenses. The identification of these molecular mechanisms in UTI pathogenesis is an important focus for future research aimed at the development of novel nonantibiotic therapies. Patient summary: The severity of urinary tract infections (UTIs) varies depending on the balance between the infecting bacterial strain and the antibacterial host defense. Bacterial virulence is determined by different virulence factors that enhance bacterial persistence and tissue damage. The susceptibility to an UTI is influenced by dysfunctions of the urinary tract and by genetic mechanisms that control the innate immune response to infections. # 2016 Published by Elsevier B.V. on behalf of European Association of Urology. * Corresponding author. Department of Urology, South-Pest Teaching Hospital, Ko ¨ ves Str 1, Budapest, 1204-H, Hungary. Tel. +36 12896200; Fax: +36 12856380. E-mail address: bkoves@gmail.com (B. Ko ¨ ves). EURSUP-711; No. of Pages 7 Please cite this article in press as: Ko ¨ ves B, Wullt B. The Roles of the Host and the Pathogens in Urinary Tract Infections. Eur Urol Suppl (2016), http://dx.doi.org/10.1016/j.eursup.2016.04.005 http://dx.doi.org/10.1016/j.eursup.2016.04.005 1569-9056/# 2016 Published by Elsevier B.V. on behalf of European Association of Urology.