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 acquired in the community.
Table 1 summarizes a number of factors that distinguish HA-MRSA from CA-MRSA isolates.
Table 1. HA-MRSA vs. CA-MRSA.| Factor | HA-MRSA | CA-MRSA |
| Origin of strains | Nosocomial infections 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. |
| Genetic lineage | 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). |
| Affected population | 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. |
| Clinical syndromes | 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. |
