Pathogenicity of Escherichia coli strains isolated from calves with diarrhea to mice

Abstract:

[Background] Establishing the diarrhea model is a common method to study the mechanism of bacterial diarrhea and anti-diarrhea mechanism. [Objective] Five strains of Escherichia coli with different serotypes were isolated from the feces of calves with diarrhea. The diarrhea model of mice was established by intragastric administration. [Methods] Kunming mice were randomly divided into six groups:normal control (NC) group, E. coli O1, E. coli O2, E. coli O8, E. coli O78, and E. coli O86 groups, with 12 mice in each group. The number of E. coli in all the challenge groups was 3×10¹³ CFU/mL, and the strain liquid was administrated at 0.2 ml/10 g body weight and twice/day for 7 continuous days. The body weight and diarrhea rate of mice were recorded on days 3, 5 and 7, and the serum samples of mice in each group were collected for the determination of interleukin 1β (IL-1β), IL-6, tumor necrosis factor alpha (TNF-α), immunoglobin G (IgG), IgA, and secretory immunoglobulin A (sIgA) levels. The duodenum of mice in each group was collected on the 7th day to prepare H&E sections, and the cecum contents were collected for 16S rRNA gene analysis. A diarrhea model was established with E. coli O₈ via intragastric or intraperitoneal administration, and ofloxacin was used to treat the mice with diarrhea. [Results] On days 5 and 7, the mice in the E. coli groups showed lower body weight (P<0.05) and higher diarrhea rate than those in the NC group. HE staining results showed that the intestinal villi of mice in E. coli groups showed different degrees of damage, and E. coli O2 and O8 caused the most serious damage. The serum levels of IL-1β, IL-6, and TNF-α significantly elevated after challenge (P<0.05), especially in the E. coli O1 and O8 groups. However, the levels of IgG, IgA, and sIgA in the challenge groups significantly decreased compared with those in the NC group (P<0.05), especially in the group challenged with strain O8. The 16S rRNA gene analysis of cecum contents in mice showed that compared with the NC group, the challenge groups had decreased alpha diversity of bacteria. Specifically, the abundance of Firmicutes decreased while that of Bacteroides, Proteobacteria and Actinobacteria increased in the challenge groups. All the mice died within 12 h after intraperitoneal injection with E. coli O₈, and antibiotic did not reverse this death, while the mice with diarrhea caused by intragastric administration could restore to normal after antibiotic treatment. [Conclusion] The five strains of E. coli from dairy calves with diarrhea could cause different degrees of diarrhea in mice after intragastric administration. Among them, E. coli O2, O8, and O1 caused serious damage to the intestinal villi of mice. The serum levels of IL-1β, IL-6 and TNF-α in mice were higher after administration with E. coli O8. Therefore, intragastric administration of 3×10¹³ CFU/mL E. coli O8 at 0.2 mL/10 g and twice/day for 7 continuous days could well establish the diarrhea model of mice, which can be used to evaluate drug efficacy.