As mentioned in the course introduction, MRSA infections fall into two general types:
- HA-MRSA Infections that occur in people who are, or have recently been, hospitalized.
- CA-MRSA Infections that are apparently acquired in the community
There are a number of factors that distinguish HA-MRSA from CA-MRSA isolates. These factors are summarized in the table below.
|Origin of strains|
Five isolates associated with healthcare settings: USA100, -200, -500, -600, -800
USA100 is the predominant isolate while USA 200 is the second most common isolate. USA700 has been isolated in both healthcare and community settings.
Evolved from endemic methicillin-susceptible S. aureus (MSSA) strains
Two clones, USA300 and USA400, are associated with the majority of CA-MRSA infections in the United States. USA300 has emerged as the most prominent clone and is not found among hospital strains.
Isolates usually carry large SCCmec types I, II or III (34-67 kb)
The larger size of SCCmec II and III permits the inclusion of other non-beta-lactam resistance genes so that HA-MRSA strains tend to be multi-drug resistant
Isolates carry a smaller SCCmec variant, predominantly type IV (24 kb), less often type V or variant VT.
SCCmec IV (except for mecA) does not permit the inclusion of other non-beta lactam resistance genes so that CA-MRSA isolates exhibit resistance to only methicillin and erythromycin and are more often susceptible to other non-beta lactam antibiotics (e.g., trimethoprim/sulfamethoxazole (SXT) and clindamycin).
|Largely affects older adults and people with weakened immune systems; those who have undergone surgical procedures are at increased risk. ||Healthy persons in the general population without established risk factors for MRSA acquisition|
|Found at multiple sites, most commonly bloodstream infections, urinary tract infections (UTI) and respiratory tract infections|
Predominantly skin and soft tissue infections (SSTIs), such as abscesses, cellulitis, folliculitis and impetigo and a serious form of pneumonia
Genes for Panton-Valentine leukocidin (PVL) are associated with SCCmec IV; the clinical spectrum of infections caused by CA-MRSA is directly related to the presence of PVL genes, coding for the production of a cytotoxin that causes tissue necrosis and leukocyte destruction.