Bacterial Pathogens of the Urinary Tract

Infections of the genitourinary tract are usually caused by a single organism. In around 20% of cases, however, two or more bacterial species are involved. The majority of infections is caused by aerobic bacteria of intestinal origin, gaining access to the bladder via the urethra. E. coli is by far the organism most commonly isolated from the urinary tract of dogs and cats, but other pathogens, such as Staphylococcus spp., Proteus, Klebsiella, and Pseudomonas, are involved as well (23).

Urinary tract pathogens


Bacterial Invasion Strategies

Pathogenic bacteria have developed different strategies to circumvent the host's defense mechanisms.

A number of pathogens attach to the uroepithelium by fimbriae to avoid washout during micturition. Some produce endotoxins decreasing urethral peristalsis in order to faciliate adhesion to the mucosa (O-antigens). Others produce substances like K-antigens, which interfere with opsonization and phagocytosis, or they suppress the resident flora with bacteriocins, hemolysins, and iron chelators (23).

Categories of Urinary Tract Infections

UTI basically are divided into two subcategories: uncomplicated and complicated forms (23).
Uncomplicated UTI is defined as a simple infection with no underlying primary disease. This condition rapidly responds to therapy when an appropriate antibiotic is given.
Complicated UTI is associated with defects in the host's immune defense, such as micturition disorders, anatomic defects, damage of the mucosal barrier, as well as alteration in urine volume or composition. This is the reason why patients with decreased voiding frequency or low urine volume, bladder atony with residual urine, urolithiasis, hyperadrenocorticism, or diabetes mellitus are predisposed to an infection of the genitourinary system.

In older male dogs, chronic prostatitis with purulent or hematogenous discharge and formation of intraprostatic abscesses is frequent. As prostatic fluid also refluxes to the bladder, intact male dogs with prostatitis often have cystitis and vice versa (24).

(4) Aucoin DP: TARGET, the antimicrobial reference guide to effective treatment . North American Compendiums Inc., 1993.
(23) Grauer GF: Urinary Tract Infections, in Allen DG (ed): Small Animal Medicine, Lippincott, Philadelphia: 625–655, 1991.
(24) Barsanti JA: Prostatic infections: diagnosis and therapy. Selected proceedings from the scientific program of The North
American Veterinary Conference, Orlando: 40–49, 1998.