Papillary necrosis in vitro: A scanning electron microscopic comparison of Escherichia coli and Proteus mirabilis infection

The effect of bacterial infection on excised renal papillae as a model for papillary necrosis and subsequent calcification was investigated. Sterile rat renal papillae were placed in 25 ml aliquots of filter sterilized human urine and then inoculated with one ml suspensions of sterile saline as a control, or 1 x 10⁸/ml Escherichia coli or Proteus mirabilis. After incubation at 37°C for periods of 8 hr, 24 hr, 48 hr, 72 hr, 1 wk, 2 wk and 3 wk, urinary pH was measured, bacterial culture performed and the renal papillae were recovered and examined by scanning electron microscopy (SEM) and energy dispersive spectrophotometry (EDS). In the case of Proteus mirabilis, the sequence of events noted included bacterial-papillary interactions consisting of cell desquamation and strand formation, despite infrequent bacterial attachment. After 10 hr, a rapid, urease induced pH rise resulted in calcium salt deposition on the papillae surface. Organism death was apparent after 72 hr. Escherichia coli infected papillae demonstrated similar cell surface changes after 8 hr as seen in P. mirabilis; however, frequent evidence of bacterial attachment and penetration was apparent. Bacterial attachment was a prominant feature throughout the incubation period with E. coli. After one week, rare areas of degenerating cells and bacteria with increased calcium levels as compared to surrounding areas were noted by EDS analysis. Urinary pH was stable throughout the incubation period. This study suggests varied roles for the organisms most associated with infection induced papillary necrosis (E. coli) and papillary necrosis with subsequent stone formation (P. mirabilis). A role for bacterial calcification in the absence of bacterial urease activity by E. coli is also suggested.