Rarity of gene transfer between animal and human isolates of Staphylococcus aureus in vitro.

Antibiotic-resistant Staphylococcus aureus strains of animal origin were studied for transfer of resistance to human strains in vitro, with special reference to tylosin, an antibiotic which is used only in animals. Resistance to tylosin in animal and human strains was part of a constitutive resistance to all macrolide antibiotics. Resistance to tylosin could not be transferred from animal to human strains in vitro. Transfer of other resistances occurred between few strains and at low frequency. Resistance determinants from only seven animal cultures (out of 196 tested) could donate resistance to human strains in mixed cultures. The pattern of metal ion resistance was markedly different in animal and human strains: this supports the conclusion that spontaneous transfer between them is rare in nature.