However, these bacteria are suspected of being reservoirs for resistance genes. In a recent Danish study, isolates were in most cases cross-resistant to tylosin and erythromycin. More macrolide resistant isolates were observed among C.coli isolates from swine than broilers and humans. One study was undertaken in which it was not possible to transfer macrolide resistant plasmids from animal staph to human staph, although this does not mean that it cannot occur. Enterococcal plasmids were readily transferred to a recipient laboratory strain but no human strain has been used as a recipient.
Observations suggest horizontal transfer of genes but are also compatible with a common ancestor.
Transfer of macrolide resistance to humans following their use as growth promoters has not been clearly demonstrated. A temporally related increase in resistance to quinolones has been reported in zoonotic bacteria isolated from animals, food and humans. The significance of this observation would be an important element of a risk assessment of those zoonotic diseases. The potential for selection for resistance in human bacteria by the use of quinolones in human medicine is recognised. The potential for selection for resistance in animal bacteria by the use of quinolones in veterinary medicine is also recognised.