Incomplete Cross Resistance Against Ionophores in Enterococcus faecium and Enterococcus faecalis Strains from Pigs and Poultry

Thirty-two Enterococcus faecium strains and 33 Enterococcus faecalis strains were tested for their susceptibility to the ionophore antibiotics salinomycin, narasin, monensin, and lasalocid. Enterococcal strains originated from poultry in which these products are in use as coccidiostats, and from pigs in which these products are allowed as growth promoters. Resistance against salinomycin and narasin in enterococci was frequent among poultry strains, whereas in pig strains, resistance was less common. No resistance was found against monensin and lasalocid. Full cross resistance between salinomycin and narasin was evident. There was no cross resistance between these two ionophores and monensin and lasalocid.