PREVALENCE OF ERM GENE CLASSES IN ERYTHROMYCIN-RESISTANT STAPHYLOCOCCUS-AUREUS STRAINS ISOLATED BETWEEN 1959 AND 1988

The epidemiology of the two common erythromycin resistance methylase ( erm) genes ermA and ermC was analyzed by Southern blotting in 428 eryt hromycin-resistant Staphylococcus aureus strains isolated from blood b etween 1959 and 1988 in Denmark, ermA and/or ermC was present in 98% o f the erythromycin-resistant strains tested, ermA was found only as a chromosomal insert and was solely responsible for erythromycin resista nce in these strains until about 1971. ermA was the only erm gene foun d in 337 strains and was a single insert in 61% of these strains, two inserts were seen in 37%, and three inserts were found in 2%. Thirteen different ermA EcoRI restriction fragment length polymorphisms were i dentified, ermA was not found in strains of phage type patterns group II and type 95, which are very common today. ermC was found on a plasm id in 77 strains, ermC was first seen in 1971 and spread rapidly in th e S. aureus population, with a 5- to 10-fold increase every 5 years, a nd in 1984 to 1988, it was responsible for erythromycin resistance in 72% of the strains. The predominant plasmid carrying ermC was 2.5 kb, while four plasmids were smaller and three were larger, ermC has been found in all phage type patterns. Eight strains contained combinations of ermA and ermC, and no erm gene was detected in six strains.