CA-MRSA strains have been distinguished from their health care-associated MRSA (HA-MRSA) counterparts by molecu- lar means. HA-MRSA strains carry a relatively large staphy- lococcal chromosomal cassette mec (SCCmec) belonging to type I, II, or III. These cassettes all contain the signature mecA gene, which is nearly universal among MRSA isolates. They are often resistant to many classes of non-?-lactam antimicro- bials. HA-MRSA strains seldom carry the genes for the Pan- ton-Valentine leukocidin (PVL). In contrast, CA-MRSA iso- lates carry smaller SCCmec elements, most commonly SCCmec
type IV or type V. These smaller elements also carry the mecA gene and are presumably more mobile, although few explicit data support this notion (61). They are resistant to fewer non- ?-lactam classes of antimicrobials and frequently carry PVL genes. In addition to these genotypic characteristics, CA-MRSA
strains affect a population distinct from those affected by HA- MRSA and cause distinct clinical syndromes. CA-MRSA in- fections tend to occur in previously healthy younger patients. They have been associated predominantly with SSTIs (105, 642, 654) but have also been linked to several severe clinical syndromes such as necrotizing pneumonia and severe sepsis. In contrast, HA-MRSA strains have been isolated largely from people who are exposed to the health care setting; the patients are older and have one or more comorbid conditions. HA- MRSA strains tend to cause pneumonia, bacteremia, and in- vasive infections.
Community-associated methicillin-resistant Staphylococcus aureus: Epidemiology and clinical consequences of an emerging epidemic