Fig. 2: In vitro and in vivo assessments of the pathogenicity of K. pneumoniae from humans, dogs, and pigs.

a Bacterial adherence and invasion to respiratory and intestinal epithelial cells derived from humans and/or pigs. b Bacterial invasion of respiratory and intestinal epithelial cells derived from humans and/or pigs. Each experiment was performed with four technical replicates. c Clinical sign scores of pigs challenged with K. pneumoniae strains originating from humans, dogs, and pigs. d The number of K. pneumoniae strains recovered from different organs of challenged pigs at 48 hours post-challenge. e The number of K. pneumoniae strains recovered from different organs of challenged pigs at 7 days post-challenge. The error bar represents the standard deviation. The significance level was set at p < 0.05 (*), or p < 0.01 (**), non-significant comparisons (p > 0.05) are not labeled. The error bar represents the standard deviation. N = 5 biologically independent animals per group. f Histological examinations of different organs from challenged pigs are shown. (Scale bars = 50 μm) The lungs of challenged pigs exhibit extensive thickening of the alveolar walls, with unclear alveolar wall structure. A small amount of inflammatory cell infiltration is observed on the alveolar walls. The bronchial structures appeared normal without any evident abnormalities. There were no significant interstitial proliferations or other notable abnormalities observed in the interstitium. The livers showed hepatocellular focal necrosis with hemorrhage, accompanied by a small amount of inflammatory cell infiltration within the necrotic lesion. Additionally, there was a small amount of hepatocellular hydropic degeneration, cellular swelling, and pale staining of the cytoplasm. The spleens of challenged pigs exhibited a decrease in white pulp volume, occasionally accompanied by mild hemorrhage within the white pulp, along with a small amount of extravasated red blood cells. There was no significant abnormality in the number of parenchymal cells within the red pulp, and the splenic sinuses do not show significant dilation. However, there was notable infiltration of granulocytes. The renal tissue of the kidneys showed widespread tubular necrosis, with necrosis and dissolution of the renal tubular epithelial cells, resulting in an unclear tubular structure. Acidophilic material can be seen within the glomerular capillaries. Focal hemorrhage was observed in multiple areas of the interstitium, along with scattered inflammatory cell infiltration. The lymph nodes exhibited abundant and well-defined lymph follicles within the cortex, without any evident abnormalities. Focal hemorrhage is observed in multiple areas of the medulla.