Determination of Epidemiological Cutoff Values for Antimicrobial Resistance of Enterococcus cecorum

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 cutoff (ECOFF) values are unknown. To determine tentative ECOFF (CO ) values for 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 ( = 208) to 29 antimicrobials by the disc diffusion (DD) method. We also determined the MICs of 23 antimicrobials by the broth microdilution method. To detect chromosomal mutations conferring antimicrobial resistance, we investigated the genomes of 118 isolates obtained mainly from infectious sites and described previously in the literature. We determined the CO values for more than 20 antimicrobials and identified two chromosomal mutations explaining fluoroquinolone resistance. The DD method appears better suited for detecting antimicrobial resistance. Although tetracycline and erythromycin resistances were persistent in clinical and nonclinical isolates, we found little or no resistance to medically important antimicrobials.