The in vitro Antimicrobial Effects of Hydrolysable and Condensed Tannin Extracts on Escherichia coli Isolated from Pathological Samples of Deceased Poultry

Abstract Colibacillosis, caused by avian pathogenic Escherichia coli (APEC), is one of the most prevalent and economically damaging bacterial diseases affecting poultry globally. Managing colibacillosis is difficult and frequently ineffective because APEC strains have developed widespread resistance to antibiotics, and the strict regulations and public concerns towards using antimicrobial agents in poultry further complicate the situation. This study aimed to evaluate the antibacterial properties of hydrolysable (sweet chestnut extract) and condensed (quebracho extract) tannins on Escherichia coli ( E. coli) isolates from poultry, exploring their potential as antibiotic alternatives in managing colibacillosis. E. coli was isolated from the internal organs of deceased poultry across 18 farms, including layers, broilers, and broiler breeders. Each isolate was assessed for the presence of APEC strain predictors (virulence genes iutA , hlyF , iss , iroN , and ompT ), antimicrobial resistance to 14 antibiotics using the disc diffusion method, and the presence of resistance genes for specific antibiotics (ampicillin, gentamicin, tetracycline, and quinolones). Out of 43 isolates, 27 (62.8%) were classified as APEC, 30 (69.8%) showed resistance to three or more antibiotic classes, and 32 (74.4%) carried at least one AMR gene. The minimum inhibitory concentrations (MICs) for the hydrolysable tannins from sweet chestnut extract ( Castanea sativa Mill.) (SwCh) ranged from 0.5 to 3 mg/mL, while for the condensed tannins from quebracho extract ( Schinopsis lorentzii ) (Que), the MICs ranged from 1.5 to 4.5 mg/mL. The results indicate that both hydrolysable and condensed tannins possess significant in vitro antimicrobial activity against APEC, offering a potentially valuable alternative for controlling colibacillosis in the poultry industry.