Molecular analysis of naturally occurring erm C‐encoding plasmids in staphylococci isolated from animals with and without previous contact with macrolide/lincosamide antibiotics

A total of 16 epidemiologically unrelated macrolide‐resistant staphylococcal isolates of various animal origins were investigated for the molecular basis of macrolide resistance with respect to previous contact of their host animals with macrolides and lincosamides. All isolates carried erm C‐encoding plasmids of 2.3–4.0 kbp. The eight plasmids of staphylococci from animals which had not received macrolides or lincosamides showed inducible erm C gene expression and did not exhibit alterations in the erm C regulatory region. The remaining eight plasmids expressed the erm C gene constitutively. Six of these plasmids were from staphylococci from animals which had received tylosin or spiramycin as feed additives or lincomycin for therapeutic purposes. All constitutively expressed erm C genes revealed either sequence deletions or sequence duplications in their erm C regulatory region, as detected by a PCR assay and by sequence analysis. These sequence deletions and duplications found in naturally occurring plasmids corresponded closely to the mutations seen in the erm C‐encoding plasmids after growth of an inducibly resistant strain in the presence of non‐inducing macrolides or lincosamides under in vitro conditions.