Environmental Fitness and Human Pathogenicity Characteristics of Extended-Spectrum Cephalosporin-Resistant Escherichia coli Isolated from Danish Poultry Production

The occurrence of extended-spectrum cephalosporin (ESC)-resistant Escherichia coli in poultry could threaten human health. This study aimed to evaluate environmental fitness (the ability to grow at low temperatures and form biofilms) and pathogenicity potential of 30 ESC-resistant E. coli, isolated from the Danish poultry production chain and one human case, to understand the risk of foodborne transmission. The whole genome sequenced isolates were selected among 162 isolates to represent different origins, sequence types (STs), and ESC-resistance genes and characterized in terms of their growth at low temperatures (4, 7, 10, and 14 °C), resistance to antimicrobials, formation of biofilm, invasion of human intestinal epithelial cells, and possession of antibiotic resistance and virulence-associated genes. All isolates exhibited resistance to ≥2 antibiotics, with 47% being resistant to ≥ three classes. None of the isolates grew at low temperatures (4 °C and 7 °C), while all grew at 10 and 14 °C. Biofilm formation at 15 °C and 37 °C was observed in all isolates, as determined by the crystal violet assay. Testing of metabolic activity in biofilms revealed that ST155 and ST162 isolates exhibited the highest activity at both 15 °C and 37 °C. Invasion rates varied with ST429 and ST162 scoring highest. Two ST155 isolates were strong biofilm producers but lacked the ability to invade human INT-407 cells. This study revealed that three STs (ST155, ST162, and ST429) showed the highest growth rates at 10 °C, biofilm formation, and/or invasion rates, indicating their environmental fitness and pathogenicity potential. Future research into their role in human disease burden is needed before inclusion in risk-based surveillance programs for antibiotic-resistant E. coli in poultry production.