Introduction

Urinary tract infections (UTIs) are among the most prevalent microbial diseases, and their financial burden on society is substantial. A large part of antibiotic treatment is therefore allotted to UTIs. UTIs accounted for more than 100,000 hospital admissions annually in the USA, most often for pyelonephritis.1,2 They also account for at least 40% of all hospital-acquired infections which are in the majority of cases catheter associated.3-5

UTIs can be classified into uncomplicated and complicated UTI. Uncomplicated UTI are those where complicating factors are not present. Complicated UTI in contrast is a very heterogenous entity, with a common pattern of complicating factors:

•Anatomical, structural, or functional alterations of the urinary tract (e.g.,stents,urine transport disturbances, instrumentation of the urinary tract, stones, tumors, neurological disorders)

•Impaired renal function, by parenchymal diseases, or pre-, intra, or post-renal nephropathies (e.g., acute, chronic renal insufficiencies, heart insufficiency)

•Accompanying diseases that impair the patient’s immune status (e.g., diabetes mellitus, liver insufficiency, immunosuppression, cancer, AIDS, hypothermia)

In uncomplicated UTI the virulence properties of the causative bacteria are the predominant factors leading to the infection, whereas in complicated UTI the host immune deficiency in its various forms is the leading cause.

Pathophysiology

Bacteria can reach the urinary tract by hematogenous or lymphatic spread, but there is abundant clinical and experimental evidence to show that the ascent of microorganisms from the urethra is the most common pathway leading to a UTI, especially organisms of enteric origin (i.e., Escherichia coli and other Enterobacteriaceae, enterococci). Therefore, UTI are more frequent in women than in men and there is an increased risk of infection following bladder catheterization or instrumentation. A single insertion of a catheter into the urinary bladder in ambulatory patients, for example, results in urinary infection in 1–2% of cases. It is thought that in those cases bacteria migrate within the mucopurulent space between the urethra and catheter leading to the development of bacteriuria in almost all patients within about 4 weeks.

The concept of bacterial virulence or pathogenicity in the urinary tract infers that not all bacterial species are equally capable of inducing infection. The more compromised the natural defense mechanisms (e.g., obstruction,