Pyometra is the most common reproductive disease in bitches and it is characterized by a bacterial infection in the uterus with clinical manifestations ranging from purulent vulvar discharges to life-threatening systemic manifestation. Escherichia coli stands out as the most frequent pathogen involved, being reported in up to 90% of cases. Despite its great clinical relevance, the pathogenesis of the disease is still poorly understood. Therefore, the aim of this study were: (1) to characterize E. coli isolates from uterine contents and feces of bitches affected by pyometra and feces from healthy dogs consuming two different diets; (2) determine the genetic similarity of E. coli isolates from the uterine contents and feces of two cohabiting bitches that were diagnosed with pyometra in the same period; (3) assess whether there is an association between bacterial pathogenicity, endometrial histological changes and clinical prognosis in canine pyometra. E. coli strains belonging to the B2 phylogroup and positive for virulence factor genes associated mostly with adhesion predominated in the uterine content and rectal swabs of dogs with E. coli pyometra. Interestingly, a lower growth rate of E. coli from the B2 phylogroup was observed in dogs fed a raw-meat based diet compared to those fed commercial dry feed. These results suggests most cases of E. coli pyometra are caused by strains from phylogroup B2, and also indicates that diet can influence intestinal colonization by such strains. The simultaneous occurrence of pyometra in two cohabitant bitches underwent a depth investigation due to the hypothesis of transmission between these animals. Both whole-genome-multilocus sequence typing and single-nucleotide polymorphism analysis supported the hypothesis that the isolates from the uterine content of both animals and from the rectum of one were clonal. This finding confirmed, for the first time, the transmission of E. coli associated with pyometra between two animals. Clinical data, histopathological alterations and microbiological findings of dogs with pyometra (n=39) were analyzed in order to assess possible associations. There was an association between the detection of papC in E. coli isolates and higher necrosis scores. Additionally, the score of necrosis was positively associated with the length of hospitalization, with each point increase in the necrosis score leading to two more days of hospitalization. These results suggest that papC-positive E. coli plays an important role in the severity of pyometra in dogs.

Keywords: Escherichia coli, ExPEC, transmission of pyometra, bacterial pathogenicity.