Biochemical and histologic findings in experimental pyelonephritis due to Ureaplasma urealyticum

Different inflammatory responses are associated with Ureaplasma parvum-induced UTI and urolith formation

Background

Epidemiologic studies show a strong association between Ureaplasmas and urogenital tract disease in humans. Since healthy humans can be colonized with Ureaplasmas, its role as a pathogen remains controversial. In order to begin to define the role of the host in disease, we developed a rodent model of urinary tract infection (UTI) using Fischer 344 (F344) rats. Animals were inoculated with sterile broth, 10¹, 10³, 10⁵, 10⁷, or 10⁹ log CFU of a rat-adapted strain of Ureaplasma parvum.

Results

Infected animals exhibited two distinct profiles, asymptomatic UTI and UTI complicated with struvite urolithiasis. Inoculum dose of U. parvum affected the incidence of UTI, and 50% to 57% of animals inoculated with ≥ 10⁷ CFU of U. parvum remained infected (p < 0.04). However, inoculum dose did not influence immune response to U. parvum. Asymptomatic UTI was characterized by a minimal immune response that was predominantly monocytic and lymphocytic, with limited lesions, and elevated urinary levels of IFN-γ, IL-18 and MCP-1 (P ≤ 0.02). UTI complicated with struvite formation was characterized by an exaggerated immune response that was mostly neutrophilic (P ≤ 0.0001), with lesions that showed extensive uroepithelial hyperplasia (P ≤ 0.0001), and a predominance of IL-1α, IL-1β, and GRO/KC in the urine (P ≤ 0.02). Animals with asymptomatic UTI also had a significantly high rate of kidney infection (P ≤ 0.0005).

Conclusion

Complications associated with U. parvum infection are primarily dependent upon host-specific factors rather than Ureaplasma microbial load. The immune response in F344 rats is similar to that which occurs in humans with ureaplasmal associated disease. Therefore, this model of infection is a useful tool for elucidating U. parvum-host interactions that confer UTI and disease.

Rat strains differ in susceptibility to Ureaplasma parvum-induced urinary tract infection and struvite stone formation

Individuals with struvite uroliths are susceptible to recurrent urinary tract infections (UTI), sepsis, and renal disease. Unfortunately, little is known about the host-specific factors that predispose to this disease. In order to develop a rodent model that can address this problem, we inoculated female Fischer 344 (F344), Lewis (LEW), Sprague-Dawley (SD), and Wistar (WIS) rats with a host-adapted strain of Ureaplasma parvum. Animals were necropsied at 2 weeks postinoculation; 100% of F344, 42% of SD, 10% of LEW, and 10% of WIS rats remained infected. Severe bladder lesions and struvite calculi were seen in 64% of F344 rats; in other rat strains, bladder lesions were mild or absent. F344 rats with struvite uroliths had the highest urinary levels of proinflammatory cytokines, such as GRO/KC, interleukin-1alpha (IL-1alpha), and IL-1beta. F344 rats without struvite stones at necropsy had milder bladder lesions and significantly lower urinary levels of proinflammatory cytokines but a more prominent inflammatory response than did other rat strains. Based on our results, struvite stone formation is linked to a robust inflammatory response that does not resolve UTI but instead promotes damage to surrounding tissues.

Ureaplasma urealyticum-induced urinary tract stones in rats

This study investigated the possible role of Ureaplasma urealyticum, which is predominantly located in the urogenital tract, in the formation of infectious stones. A standardized Ureaplasma urealyticum broth culture isolated from a human urogenital specimen was inoculated into the renal medulla of five male rats (Rattus norvegicus L., Wistar C, weighing 170 +/- 10 g) and the same amount of culture media was used for five identical control rats. Five days after the inoculation, the rats were killed and fresh preparations from the bladders and the inoculated kidneys of both groups were prepared. At the same time biochemical and histopathological analysis of the contents of the bladders and the inoculated kidneys of both groups was performed. Crystal formation within the bladders of the inoculated rats was demonstrated and biochemical analysis of the crystals showed calcium, magnesium and phosphate, which indicated the existence of infection-induced crystals. These findings were absent in the control rats. The role of Ureaplasma in the production of urinary tract infectious stones was thus demonstrated in vivo.