Characterization of a novel insertion of the macrolides-lincosamides-streptogramin B resistance transposon Tn554 in methicillin-resistant Staphylococcus aureus and Staphylococcus epidermidis.

Macrolides-lincosamides-streptogramin B resistance in staphylococci can result from a gene, ermA, that comprises part of transposon Tn554. Tn554 is unusual in (i) its high specificity for a primary chromosomal attachment site, att554, and (ii) the variability of its 3'-terminal six or seven nucleotides, which appear to copy the six or seven chromosomal nucleotides 5' to the parent transposon during transposition. We characterized a novel Tn554 insert in the chromosomes of methicillin-resistant Staphylococcus aureus strains involved in a current outbreak. This insert was found to resemble an insert recently discovered in S. epidermidis in its junctional fragment restriction pattern. Sequence analysis of the junctional regions showed that the attachment site, att155, exhibited 78% similarity to att554 (39 of the 50 nucleotides flanking the insertion sites) for both S. aureus and S. epidermidis inserts and that the 3' hexanucleotide of the S. epidermidis transposon (GACATC) resembled the reverse complement (TACATC) of its commonly occurring S. aureus counterpart (GATGTA). Epidemiologic and molecular data indicated that att155 is harbored by extra DNA characteristic of methicillin-resistant strains and absent from methicillin-susceptible ones. Further, Southern hybridization showed that, even in the absence of Tn554 inserts, some methicillin-resistant strains contain DNA related to att155 and Tn554.