High Biofilm-Forming Ability and Clonal Dissemination among Colistin-Resistant <i>Escherichia coli</i> Isolates Recovered from Cows with Mastitis, Diarrheic Calves, and Chickens with Colibacillosis in Tunisia

Background: Escherichia coli (E. coli) is one of the main etiological agents responsible for bovine mastitis (BM), neonatal calf diarrhea (NCD), and avian colibacillosis (AC). This study aimed to assess resistance and virulence genes content, biofilm-forming ability, phylogenetic groups, and genetic relatedness in E. coli isolates recovered from clinical cases of BM, NCD, and AC. Materials/Methods: A total of 120 samples including samples of milk (n = 70) and feces (n = 50) from cows with BM and calves with NCD, respectively, were collected from different farms in Northern Tunisia. Bacterial isolation and identification were performed. Then, E. coli isolates were examined by disk diffusion and broth microdilution method for their antimicrobial susceptibility and biofilm-forming ability. PCR was used to detect antimicrobial resistance genes (ARGs), virulence genes (VGs), phylogenetic groups, and Enterobacterial repetitive intergenic consensus PCR (ERIC-PCR) for their clonal relationship. Results: Among the 120 samples, 67 E. coli isolates (25 from BM, 22 from AC, and 20 from NCD) were collected. Overall, 83.6% of isolates were multidrug resistant. Thirty-six (53.73%) isolates were phenotypically colistin-resistant (CREC), 28.3% (19/67) were ESBL producers (ESBL-EC), and forty-nine (73.1%) formed biofilm. The blaTEM gene was found in 73.7% (14/19) of isolates from the three diseases, whilst the blaCTXM-g-1 gene was detected in 47.3% (9/19) of isolates, all from AC. The most common VG was the fimA gene (26/36, 72.2%), followed by aer (12/36, 33.3%), cnf1 (6/36, 16.6%), papC (4/36, 11.1%), and stx1 and stx2 genes (2/36; 5.5% for each). Phylogenetic analysis showed that isolates belonged to three groups: A (20/36; 55.5%), B2 (7/36; 19.4%), and D (6/36; 16.6%). Molecular typing by ERIC-PCR showed high genetic diversity of CREC and ESBL E. coli isolates from the three animal diseases and gave evidence of their clonal dissemination within farms in Tunisia. Conclusion: The present study sheds new light on the biofilm-forming ability and clonality within CREC and ESBL-EC isolated from three different animal diseases in Tunisian farm animals