Evolution of sporadic isolates of methicillin-resistant Staphylococcus aureus (MRSA) in hospitals and their similarities to isolates of community-acquired MRSA

Abstract

Forty-one methicillin-resistant Staphylococcus aureus (MRSA) hospital isolates that clearly differed from the six major pandemic clones of MRSA in pulsed-field gel electrophoresis type, mecA and Tn554 polymorphism, and epidemic behavior were selected from an international strain collection for more detailed characterization. SpaA typing, multilocus sequence typing, and SCCmec (staphylococcal cassette chromosome mec) typing demonstrated extensive diversity among these sporadic isolates both in genetic background and also in the structure of the associated SCCmec elements. Nevertheless, the isolates could be grouped into restricted clonal complexes by using the BURST (i.e., based upon related sequence types) program algorithm, which predicted that most sporadic MRSA isolates evolved from pandemic MRSA clones. Several of the sporadic MRSA resembled community-acquired MRSA isolates in properties that included a relatively limited multiresistance pattern, faster growth rates, diversity of genetic backgrounds, and a frequent association with SCCmec type IV.

FIG. 1.

Diagram of CCs defined with the algorithm BURST. Each number represents an MLST-derived ST. The ST of the predicted ancestral genotype is placed in the central circle, SLVs are within the second circle, and DLVs within the outer dotted circle. One ST (ST59) for which an ancestral genotype cannot be inferred and one ST (ST80) that is not member of any CC (singletons) are also shown. CC30 and CC45 are not graphically represented since they are represented by unique STs in the present study. Blue numbers denote STs found in the present study; black numbers denote STs in the database associated with a particular CC. The numbers of isolates with a particular ST deposited in the database, followed by the numbers of isolates with that ST found in our study, e.g., “(73/3),” are given in parentheses.

FIG. 2.

Evolutionary origins and patterns of descent within different CCs. Alterations in the allelic profile, the locus that has changed (in red), and the acquisition of the SCCmec types are shown. The STs present in our collection are in blue. Dashed arrows indicate unclear pathways. (A) Evolutionary origins and patterns of descent within CC8; (B) evolutionary origins and patterns of descent within CC5; (C) evolutionary origins and patterns of descent within CC91 and CC22.