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Methicillin-resistant ''Staphylococcus aureus'' (MRSA) ( or ) is a bacterium responsible for several difficult-to-treat infections in humans. It is also called oxacillin-resistant ''Staphylococcus aureus'' (ORSA). MRSA is any strain of ''Staphylococcus aureus'' that has developed, through the process of natural selection, resistance to beta-lactam antibiotics, which include the penicillins (methicillin, dicloxacillin, nafcillin, oxacillin, etc.) and the cephalosporins. Strains unable to resist these antibiotics are classified as methicillin-sensitive ''Staphylococcus aureus'', or MSSA. The evolution of such resistance does not cause the organism to be more intrinsically virulent than strains of ''S. aureus'' that have no antibiotic resistance, but resistance does make MRSA infection more difficult to treat with standard types of antibiotics and thus more dangerous. MRSA is especially troublesome in hospitals, prisons, and nursing homes, where patients with open wounds, invasive devices, and weakened immune systems are at greater risk of nosocomial infection than the general public. MRSA began as a hospital-acquired infection, but has developed limited endemic status and is now sometimes community-acquired. The terms HA-MRSA (healthcare-associated MRSA) and CA-MRSA (community-associated MRSA) reflect this distinction. == Signs and symptoms == ''S. aureus'' most commonly colonizes under the anterior nares (the nostrils). The rest of the respiratory tract, open wounds, intravenous catheters, and the urinary tract are also potential sites for infection. Healthy individuals may carry MRSA asymptomatically for periods ranging from a few weeks to many years. Patients with compromised immune systems are at a significantly greater risk of symptomatic secondary infection. In most patients, MRSA can be detected by swabbing the nostrils and isolating the bacteria found inside the nostrils. Combined with extra sanitary measures for those in contact with infected patients, swab screening patients admitted to hospitals has been found to be effective in minimizing the spread of MRSA in hospitals in the United States, Denmark, Finland, and the Netherlands. MRSA may progress substantially within 24–48 hours of initial topical symptoms. After 72 hours, MRSA can take hold in human tissues and eventually become resistant to treatment. The initial presentation of MRSA is small red bumps that resemble pimples, spider bites, or boils; they may be accompanied by fever and, occasionally, rashes. Within a few days, the bumps become larger and more painful; they eventually open into deep, pus-filled boils.〔(【引用サイトリンク】url=http://www.mayoclinic.org/diseases-conditions/mrsa/basics/symptoms/con-20024479 )〕 About 75 percent of community-associated (CA-) MRSA infections are localized to skin and soft tissue and usually can be treated effectively. Some CA-MRSA strains display enhanced virulence, spreading more rapidly and causing illness much more severe than traditional HA-MRSA infections, and they can affect vital organs and lead to widespread infection (sepsis), toxic shock syndrome, and necrotizing pneumonia. This is thought to be due to toxins carried by CA-MRSA strains, such as PVL and PSM, though PVL was recently found not to be a factor in a study by the National Institute of Allergy and Infectious Diseases at the National Institutes of Health. It is not known why some healthy people develop CA-MRSA skin infections that are treatable while others infected with the same strain develop severe infections or die. People are occasionally colonized with CA-MRSA and are completely asymptomatic. The most common manifestations of CA-MRSA are simple skin infections, such as impetigo, boils, abscesses, folliculitis, and cellulitis. Rarer, but more serious, manifestations can occur, such as necrotizing fasciitis and pyomyositis (most commonly found in the tropics), necrotizing pneumonia, and infective endocarditis (which affects the valves of the heart), and bone and joint infections.〔 CA-MRSA often results in abscess formation that requires incision and drainage. Before the spread of MRSA into the community, abscesses were not considered contagious, because infection was assumed to require violation of skin integrity and the introduction of staphylococci from normal skin colonization. However, newly emerging CA-MRSA is transmissible (similar, but with very important differences) from HA-MRSA. CA-MRSA is less likely than other forms of MRSA to cause cellulitis. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Methicillin-resistant Staphylococcus aureus」の詳細全文を読む スポンサード リンク
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