erm genes in erythromycin‐resistant Staphylococcus aureus and coagulase‐negative staphylococci

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Abstract
Erythromycin‐resistant S. aureus from general practice and a dermatology ward with a very high consumption of erythromycin was studied for ermA and ermC content by Southern blot analysis. The prevalence of these erm genes was also studied in coagulase‐negative staphylococci from the same dermatology ward and in a collection of 15 S. aureus and 18 coagulase‐negative staphylococci found in the same specimen from 15 different patients. ermA was only found as a chromosomal insert and ermC only on small plasmids. In erythromycin‐resistant S. aureus from general practice ermC was responsible for 84% of erythromycin resistance, while 16% of the strains contained ermA. In 17 of 18 S. aureus strains from the dermatology ward a 2.5 kb plasmid contained ermC. Among 58 erythromycin‐resistant coagulase‐negative staphylococci only 2 (3%) had ermA, whereas 44 strains (76%) had ermC. ermA or ermC genes were common in S. epidermidis (36/38). However, in 20 isolates of erythromycin‐resistant coagulase‐negative staphylococci of other species, 10 had neither ermA nor ermC. ermC was the most common erm gene in both coagulase‐negative staphylococci and S. aureus. In 11 of 15 patients with erythromycin‐resistant S. aureus the co‐isolated erythromycin‐resistant coagulase‐negative staphylococcus had another resistance mechanism, either another gene or a different pheno‐typic expression of the same gene. Resistance to pristinamycin, a streptogramin antibiotic only used in animals, was not found in S. aureus but was found in 14% of erythromycin‐resistant coagulase‐negative staphylococci.

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