Cath Lab Digest

Given the increasing prevalence of methicillin resistant Staphylococcus aureus (MRSA), these authors discuss the differences between HA-MRSA and CA-MRSA, what the literature reveals about antibiotic therapy and keys to the diagnostic workup of these patients.
To say that methicillin-resistant Staphylococcus aureus (MRSA) is a growing problem in the healthcare setting is an understatement. Indeed, healthcare providers are diagnosing this organism at an alarming rate in severe infections of both healthy people and the immunocompromised. In 1973, the Centers for Disease Control and Prevention (CDC) reported that MRSA accounted for 2 percent of all Staphylococcus infections. In 2004, it accounted for 63 percent.¹

   In both the nondiabetic and diabetic populations, Staphylococcus aureus is the most frequently isolated organism in all lower extremity infections. Some studies have noted that Staphylococcus aureus accounts for up to 76 percent of organisms isolated in the foot with 20 percent being MRSA.^1,2 It now also has the dubious honor of being the most common isolate in infections that occur after vascular graft placements.^3-5

   When researchers first discovered MRSA in the early 1960s, it was believed that a single clone was responsible for all MRSA isolates. The landscape has now been vastly complicated by the emergence of at least five new strains of MRSA. While researchers originally saw MRSA as only a nosocomial pathogen, new strains are emerging as community associated MRSA (CA-MRSA).

   The CDC defines CA-MRSA infections as those acquired by: people who have neither been hospitalized nor had a medical procedure in the past year; people who do not have indwelling catheters or medical devices; people with no history of previous MRSA infections or colonization; and those who are diagnosed in an outpatient setting or within 48 hours of initial hospitalization. The CDC also reports that although 25 to 30 percent of the general population is colonized with Staph aureus, 1 percent of the population is colonized with MRSA.^1,2

What You Should Know About HA-MRSA And CA-MRSA

The difference between hospital-acquired MRSA (HA-MRSA) and CA-MRSA lies in their genetic makeup. All strains of MRSA have a mec-A gene, which is responsible for its drug resistance and is located on an element called staphylococcal cassette chromosome (SCCmec). There are five types of SCCmec with different variations of size and genetic makeup. CA-MRSA strains have a type IV SCCmec gene, which induces its resistance to methicillin, beta-lactams and erythromycin. However, the type IV SCCmec gene is susceptible to other drugs such as clindamycin and trimethoprim-sulfamethoxazole.^6-9

   The CA-MRSA strains also produce Panton-Valentine leukocidin (PVL), a pore-forming exotoxin that researchers have shown is correlated with more febrile days and higher complications of osteomyelitis.⁷ Genestier, et al., discovered that PVLs targeted and caused cell death in neutrophils and lymphocytes by disrupting the mitochondria. This also induced the release of certain neutrophil factors that cause inflammation and tissue loss.

   Researchers have postulated that this mechanism results in the high incidence of CA-MRSA found in skin and soft tissue infections (SST).⁸ Naimi, et al., reported that 75 percent of CA-MRSA isolates were found in SST infections as opposed to 37 percent of HA-MRSA found in SST infections.⁶