Antibiotic Resistance among Gastrointestinal Bacteria in Broilers: A Review Focused on Enterococcus spp. and Escherichia coli

SIMPLE SUMMARY: Chicken meat has become one of the most consumed meats worldwide, and antibiotics have been used to ensure high levels of production. However, antibiotic usage in animal production has contributed to the development of antibiotic-resistant bacteria, largely among intestinal microbiota. Enterococcus spp. and Escherichia coli are frequently found in the gastrointestinal tract of chickens, and the presence of resistant strains has been revealed by several studies. Enterococcus spp. isolated from broilers have shown resistance to at least seven classes of antibiotics, while E. coli have shown resistance to at least four. Furthermore, some clonal lineages, such as ST16, ST194, and ST195 in Enterococcus spp. and ST117 in E. coli, have been identified in broilers and humans. These data suggest that bacteria can be transmitted through the consumption of contaminated animal-source food, direct contact with animals, or environmental exposure. Therefore, the main goal of this review was to highlight the existing literature on the gastrointestinal microbiota in broilers and antibiotic-resistant Enterococcus spp. and E. coli of broiler origin. ABSTRACT: Chickens can acquire bacteria at different stages, and bacterial diversity can occur due to production practices, diet, and environment. The changes in consumer trends have led to increased animal production, and chicken meat is one of the most consumed meats. To ensure high levels of production, antimicrobials have been used in livestock for therapeutic purposes, disease prevention, and growth promotion, contributing to the development of antimicrobial resistance across the resident microbiota. Enterococcus spp. and Escherichia coli are normal inhabitants of the gastrointestinal microbiota of chickens that can develop strains capable of causing a wide range of diseases, i.e., opportunistic pathogens. Enterococcus spp. isolated from broilers have shown resistance to at least seven classes of antibiotics, while E. coli have shown resistance to at least four. Furthermore, some clonal lineages, such as ST16, ST194, and ST195 in Enterococcus spp. and ST117 in E. coli, have been identified in humans and animals. These data suggest that consuming contaminated animal-source food, direct contact with animals, or environmental exposure can lead to the transmission of antimicrobial-resistant bacteria. Therefore, this review focused on Enterococcus spp. and E. coli from the broiler industry to better understand how antibiotic-resistant strains have emerged, which antibiotic-resistant genes are most common, what clonal lineages are shared between broilers and humans, and their impact through a One Health perspective.