Phenotypic Distinction in Enterococcus faeciumand Enterococcus faecalisStrains between Susceptibility and Resistance to Growth-Enhancing Antibiotics

ABSTRACTSusceptibility of Enterococcus faeciumandEnterococcus faecalisstrains from animals and foods to growth-promoting antibiotics used in animal feed was tested by the agar dilution technique. Acquired resistance to bacitracin, narasin, tylosin, and virginiamycin was seen for both species, and for E. faecium, resistance to avilamycin and avoparcin was also seen. Drawing the distinction between susceptibility and resistance based on frequency distributions of MICs was easy with avoparcin, avilamycin, and tylosin but difficult with virginiamycin and to some extent also with bacitracin and narasin.