Pitfalls of antimicrobial susceptibility testing of enterococci isolated from farming broilers by the disk diffusion method

Enterococci are gram-positive, facultative anaerobic bacteria bacteria that belong to the commensal microbiota that colonises the intestinal tract of many animal species, including mammals and birds. They are usually considered non pathogenic, but in the last decade they have been frequently associated with several infections in humans, such as nosocomial infections [1, 2, 3], endocarditis, urinary and genital tract infections, meningitis and septicaemia [2]. As members of the normal intestinal microbiota, they are exposed to the vast majority of the antimicrobial compounds administered to their host, which can lead to a high probability of acquiring and transferring antimicrobial resistance genes [2, 4, 5], and also of transferring those genes to other bacteria present in the faecal environment [5]. These features render enterococci ideal bacteria for controlling parameters related to antimicrobial susceptibility studies. Antimicrobial susceptibility testing is essential for the adequate control of antimicrobial resistance transfer from enterococci to other bacteria genera. Although the minimum inhibitory concentration (MIC) determination is the reference method for antimicrobial susceptibility testing, the disk diffusion method (DD) is often chosen, as it provides rapid results at reduced costs, being easy to perform and interpret, allowing for the evaluation of the bacterial culture purity, and presenting a good inter-laboratory reproducibility [6]. Nevertheless, some studies refer to the occurrence of discrepancies between the susceptibility results obtained by DD and MIC testing [2, 7, 8], and between in vitro susceptibility testing methods and drug performance in vivo [8]. To address these shortcomings, the Clinical Laboratory Standards Institute (CLSI) frequently evaluates and modifies the interpretive criteria for both methods, including adaptation of MIC breakpoints, revision of the categories, determination of species’ breakpoints, establishing new screening test concentrations and methods [8]. Antibiotics are widely used in veterinary medicine, as therapeutic agents or growth promoters [5], rendering veterinary field isolates a useful tool for studies concerning the comparison of susceptibility testing methods. This study aimed at comparing DD susceptibility results of enterococci isolated from slaughtered broilers with MIC values of four antimicrobials commonly used in veterinary medicine, and also of vancomycin, in order to evaluate its adequacy for enterococci field isolates testing.