Staphylococcus aureus, often referred to simply as "staph," are bacteria commonly carried on the skin or in the nose of healthy people. Approximately 25% to 30% of the population is colonized (when bacteria are present, but not causing an infection) in the nose with staph bacteria. Sometimes, staph can cause an infection. Staph bacteria are one of the most common causes of skin infections in the United States. Most of these skin infections are minor (such as pimples and boils) and can be treated without antibiotics (also known as antimicrobials or antibacterials). However, staph bacteria also can cause serious infections (such as surgical wound infections, bloodstream infections, and pneumonia). MRSA infections that are acquired by persons who have not been recently (within the past year) hospitalized or had a medical procedure (such as dialysis, surgery, catheters) are know as CA-MRSA infections. Staph or MRSA infections in the community are usually manifested as skin infections, such as pimples and boils, and occur in otherwise healthy people.
Methicillin-resistant Staphylococcus aureus (MRSA) is a type of bacteria that is resistant to certain antibiotics. These antibiotics include methicillin and other more common antibiotics such as oxacillin, penicillin and amoxicillin. Staph infections, including MRSA, occur most frequently among persons in hospitals and healthcare facilities (such as nursing homes and dialysis centers) who have weakened immune systems. The Centers for Disease Control reports that MRSA accounts for over 60% of all Staphylococcus aureus infections in the US.
MRSA infections can occur in wounds of the skin, burns or IV sites and other places where intravenous tubes enter the body, as well as in the eyes, bones, heart, and blood.
Most infections caused by MRSA are skin infections, but MRSA can also invade the blood and cause potentially serious complications such as blood stream infections, infections of the joints and pneumonia. Organ failure and death may result from untreated MRSA infections. There is a 23% mortality rate among patients with MRSA bactereremia (bacteerial infections of the bloodstream).
Community-associated (CA) methicillin-resistant Staphylococcus aureus (CA-MRSA) is a global emerging threat.
Staph and MRSA can also cause illness in persons outside of hospitals and healthcare facilities. MRSA infections that are acquired by persons who have not been recently (within the past year) hospitalized or had a medical procedure (such as dialysis, surgery, catheters) are know as CA-MRSA infections. Staph or MRSA infections in the community are usually manifested as skin infections, such as pimples and boils, and occur in otherwise healthy people.
Established risk factors for MRSA infection include recent hospitalization or surgery, residence in a long-term–care facility, dialysis, and indwelling percutaneous medical devices and catheters. Recently, however, cases of MRSA have been documented in healthy community-dwelling persons without established risk factors for MRSA acquisition. Because they are apparently acquired in the community, these infections are referred to as community-acquired (CA)-MRSA. CA-MRSA infections have been reported in North America, Europe, Australia, and New Zealand. The recent genomic sequence of a CA-MRSA isolate indicated the presence not only of a novel smaller variant of the methicillin-resistance locus (SCCmec IVa, according to Baba et al. designation), but also that of the locus for the Panton-Valentine leukocidin (PVL). The PVL locus is carried on a bacteriophage and is present in only a small percentage of S. aureus isolates from France, where this locus is associated with skin infections, and occasionally, severe necrotizing pneumonia.
In a recent study, it was found that CA-MRSA infections in France are caused by a single clone producing the PVL. Analysis of a set of CA-MRSA strains from the United States and Australia confirmed the presence of SCCmec IVa in most of them, and genetic comparison of the CA-MRSA by multi-locus sequence typing (MLST) indicated that they belonged to five clonal complexes, two of which predominated. This finding suggested that CA-MRSA have arisen from diverse genetic backgrounds rather than the worldwide spread of a single clone.
CA-MRSA infections appear to be an emerging phenomenon worldwide. The PVL locus represents a stable genetic marker of these CA-MRSA strains, which explains the frequency of primary skin infections and occasionally necrotizing pneumonia associated with these strains. Although the selective advantage conferred by the combination of genetic traits (i.e., PVL locus and SSCmec IV in an agr3 background) is not clear, the spread of a limited number of clones in each continent suggests that these CA-MRSA strains are particularly suited to be successful community-based pathogens.
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