Antimicrobial resistance of Enterococcus cecorum: ECOFF determination

Enterococcus cecorum, a commensal Gram-positive bacterium of the chicken gut, has emerged as a worldwide cause of lameness in poultry, particularly in fast-growing broilers. It is responsible for osteomyelitis, spondylitis and femoral head necrosis, causing animal suffering, mortality and antimicrobial use. Research on the antimicrobial resistance of E. cecorum clinical isolates in France is scarce, and epidemiological cut-off (ECOFF) values unknown. To determine tentative ECOFF (COWT) values for E. cecorum and to investigate the antimicrobial resistance patterns of isolates from mainly French broilers, we tested the susceptibility of a collection of commensal and clinical isolates (n=208) to 29 antimicrobials by the disc diffusion (DD) method. We also determined the minimum inhibitory concentrations (MICs) of 23 antimicrobials by the broth micro-dilution method. To detect chromosomal mutations conferring antimicrobial resistance, we investigated the genomes of 118 E. cecorum isolates mainly obtained from infectious sites and previously described in the literature. We determined the COWT values for more than 20 antimicrobials and identified two chromosomal mutations explaining fluoroquinolone resistance. The DD method appears better suited for detecting E. cecorum antimicrobial resistance. Although tetracycline and erythromycin resistances were persistent in clinical and non-clinical isolates, we found little or no resistance to medically important antimicrobials.