|Methicillin-resistant Staphylococcus aureus (MRSA) is a specific strain of Staphylococcus aureus bacteria that is resistant to methicillin, a penicillin-like antibiotic; it often is resistant to other antibiotics as well. Antibiotic resistant organisms do not die as intended when exposed to an antibiotic that normally would be expected to kill them. MRSA is called a "superbug" because of its strong antibiotic resistance and its ability to complicate treatment of staphylococcal infections.|
Staphylococci are gram-positive organisms; that is, they test positive when a sample of infected material is cultured and stained with Gram stain. They are also aerobic organisms, indicating that they are dependent on oxygen to fuel their metabolism. Staphylococcal organisms are responsible for causing a variety of diseases and conditions, including abscesses, gastroenteritis, endocarditis, hospital-acquired bacteremia associated with the use of catheters and IV equipment, skin infections, wound and burn infections, pneumonia, and meningitis. Staphylococci also produce bacterial toxins (i.e., exotoxins, enterotoxins, exfoliative toxins, toxic-shock-syndrome toxin or TSST-1) that can cause both local infection and serious systemic infection that may result in shock (toxic shock syndrome), organ failure, and death.
MRSA has two sub-types based on the origins of the infection. Hospital-acquired MRSA (HA-MRSA) is found in hospitalized individuals and in those who have had recent surgery or have been in a hospital or other healthcare facility (e.g., rehabilitation or long-term care facilities) within a year prior to developing infection. Community-acquired MRSA (CA-MRSA) is found in relatively healthy individuals who have not recently been in a healthcare facility. Infection spreads within the community among individuals who share personal items (e.g., drinking glasses, utensils, towels, razors) or sports equipment, or who are enclosed for extended periods within the same physical space (e.g., schools or daycare facilities).
Colonies of Staphylococcus aureus are found in the nose, armpits, groin, and on the skin of about 20% to 30% of healthy adults; about 80% of all people may be colonized with the organism at various times but not consistently (Herchline). Hospitalized individuals, hospital personnel, and healthcare workers generally have higher rates of Staphylococcus aureus colonization (Herchline). Although the presence of the organism does not result in disease in the majority of individuals, it may more easily overwhelm the immune system and cause infection in those who have an existing skin injury, other significant injury, chronic illness such as diabetes, or whose immune system function is compromised (immunosuppression). MRSA is the most pathogenic of all staphylococci, most often causing skin infections, pneumonia, infection of the heart valves (endocarditis), and bone marrow infection (osteomyelitis). Staphylococcus aureus also has the ability to clot (coagulate) blood (coagulase-positive staphylococci), contributing to its virulence and antibiotic resistance.
Risk: Risk of MRSA infection is highest in individuals with chronic lung disease (e.g., chronic bronchitis, emphysema, cystic fibrosis), influenza, leukemia, malignant tumors, transplanted organs, implanted prostheses or devices (e.g., prosthetic heart valves), burns, chronic skin disorders (e.g., psoriasis, eczema), diabetes, surgical incisions that have not yet healed, urinary catheters, or indwelling intravenous catheters. Individuals undergoing kidney dialysis are at increased risk as well. Those who are immunocompromised as a result of receiving chemotherapy, immunosuppressant drugs, radiation, or corticosteroids also are at increased risk. Newborn infants and mothers who are nursing infants are particularly susceptible to staphylococcal infections. Groups within the community who are at increased risk for staphylococcal infection include athletes who share equipment, military personnel, hospital personnel, healthcare workers, children in daycare facilities, and individuals who get tattoos. Elderly individuals have increased risk due to the prevalence of chronic disease and immune system dysfunction.
Incidence and Prevalence: From 20% to 30% of individuals are colonized with Staphylococcus aureus consistently, and up to 80% are colonized at some time (Herchline). Over half (59%) develop skin and soft-tissue infection, and more serious systemic infection develops in about 94,000 individuals annually (Davis). H-MRSA affects about 64% of all individuals in intensive care units (ICU); another 126,000 individuals are admitted to hospitals with CA-MRSA (Davis).
MRSA is found worldwide. Estimates of colonization range from 11% to 40% in specific populations; more than 50% of these are estimated to develop infection (Davis).