Molecular Detection of Methicillin Resistance among Staphylococci Isolated from Clinical Specimen

Abstract

Staphylococcus aureus (S. aureus) strains that produce the enzyme -lactamase and, thus, are resistant to penicillin emerged as early as in the 1940s and 1950s. In response, various -lactamase-stable penicillins were developed to treat infections caused by penicillin-resistant staphylococci. The most notable of these drugs is methicillin, and for a time it seemed to solve the problem of effectively treating penicillin-resistant S. aureus strains. In the mid-1980s, however, S. aureus strains that were resistant to methicillin and other -lactamase-stable penicillins began to emerge. These strains were termed Methicillin-Resistant Staphylococcus g_ureus, or MRSA.(J)

MRSA is considered one of the most important pathogens causing considerable morbidity and mortality' in hospitals. Owing to expression of an additional penicillin binding protein, PBP2a, with decreased affinity to -lactam agents, MRSA is resistant to all -lactam antibiotics, including cephalosporins and staphylococcal penicillins.(2)

Today, only 5% of S. aureus isolates are susceptible to penicillins. Although most S. aureus strains remain susceptible to -lactamase-stable penicillins, those that are resistant to these drugs usually are resistant to other classes of antibiotics as well. In addition to their resistance to cephalosporins and other P-lactams, MRSA strains usually are resistant to clindamycin, erythromycin, and tetracycline; and they are often resistant to fluoroquinolones, gentamycin, trimethoprim/sulfamethoxazole, or rifampin. For this reason, vancomycin is often the only drug· with which to treaf systemic MRSA infections.(!)

The epidemiology of MRSA is changing constantly. Two new stages of MRSA evolution have occurred during recent years: emergence of MRSA strains with reduced susceptibility or with resistance to glycopeptide antibiotics (Glycopeptide Intermediate resistant S. aureus "GISA" or Glycopeptide Resistant S. aureus "GRSA" strains, respectively)< 3l, and community-acquisition of MRSA by persons without known risk factors.<4J

MRSA has long been recognized as a dangerous pathogen, but it has historically been confined to the healthcare setting. Today, however, that epidemiological pattern is changing. In recent years, MRSA has begun to appear outside the healthcare environment. Community-associated MRSA (CA-MRSA) has been reported among aboriginals in Canada, New Zealand, and Australia; Native Americans in the United States; prison inmates; players of close-contact sports; children attending day care; men who have sex with men; and injecting drug users. In the healthcare setting, MRSA remains a major cause of nosocomial infections. Persons in the healthcare environment who are at risk include those with a recent history of surgery, dialysis, or admission to a healthcare facility; and those with indwelling permanent catheters or other devices that puncture the skin.< 1 l

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Community-acquired MRSA have been isolated from persons with risk factors for MRSA, such as intravenous drug use or previous hospital stays. However, between 1997 and 1999, CA-MRSA caused fatal infections in four children without any known risk factors for MRSA. Although it is not known whether the community-acquired isolates