Serious S. aureus infections require aggressive treatment, including incision and drainage of localized lesions, as well as systemic anti biotics. Choice of antibiotics is complicated by the frequent presence of acquired antibiotic resistance determinants. Virtually all community and hospital-acquired S. aureus infections are now resistant to penicillin G due to penicillinase-encoding plasmids or transposons. This has required the replacement of the initial agent of choice, penicillin G, by β-lactamase-resistant penicillins, such as methicillin or oxacillin. However, increased use of methicillin and related antibiotics has resulted in S. aureus that is resistant to a number of β-lactam antibiotics, such as methicillin, oxacillin and amoxicillin (Figure 1). These strains are known as methicillin-resistant S. aureus.

Fig1. Trends in the prevalence of methicillin resistant strains of Staphylococcus aureus.

 1. Hospital-acquired methicillin-resistant S. aureus (MRSA): In recent decades, a high percentage (often in the range of 50 percent) of hospital S. aureus isolates has been found to be also resistant to methicillin or oxacillin. Antibiotic resistance is caused by chromosomal acquisition of the gene for a distinct penicillin binding protein (PBP), PBP-2a. This protein codes for a new peptidoglycan transpeptidase with a low affinity for all currently available β-lactam antibiotics, and thus renders infections with MRSA unresponsive to β-lactam therapy. Compared with methicilllin-sensitive S. aureus, MRSA infections are associated with worse outcomes, including longer hospital and intensive care unit stays, longer durations of mechanical ventilation, and higher mortality rates. MRSA strains are also frequently resistant to many other antibiotics, some being sensitive only to glycopeptides such as vancomycin.

2. Community-acquired MRSA (CA-MRSA): Community acquired MRSA infections were documented in the mid-1990s, occurring in individuals who had no previous risk factors for MRSA infections, such as exposure to hospital. The most common clinical manifestations of CA-MRSA are skin and soft tissue infections such as abscesses or cellulitis (Figure 2). Less commonly, CA-MRSA can also cause severe diseases such as necrotizing pneumonia, osteomyelitis, and septicemia. Community-acquired MRSA has a number of characteristics that help distinguish it from hospital associated MRSA. For example, CA-MRSA has a characteristic pattern of DNA fragments obtained upon enzymic cleavage and electrophoresis, and it produces specific toxins. CA-MRSA also exhibits a unique antibiotic resistance pattern, that is, CA-MRSA is sensitive to many antibiotics that do not show much activity against hospital-associated MRSA. These antibiotics include ciprofloxacin and clindamycin, with some CA-MRSA even sensitive to erythromycin, gentamicin, rifampin, tetracycline, and/or trimethoprim -sulfamethoxazole. Emerging antibiotic-resistant strains of S. aureus that infect otherwise healthy individuals (community- acquired infections) are often more virulent than the more common strains that originate in hospitals.

Fig2. Comparison of hospital-acquired methicillin-resistant Staphylococcus aureus (HA-MRSA) with community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA). PVL = Panton-Valentine leukocidin.

3. Vancomycin resistance: Vancomycin has been the agent of choice for empiric treatment of life-threatening MRSA S. aureus infections. Unfortunately, in 1997, several MRSAs were isolated that had also acquired low-level vancomycin resistance. The incidence of vancomycin resistance has increased steadily, prompting the use of alternative drugs such as quinupristin- dalfopristin, linezolid, and daptomycin. These agents have good in vitro activity against MRSA and most other clinically important gram-positive bacterial pathogens.

Prevention

 There is no effective vaccine against S. aureus. Infection control procedures, such as barrier precautions and disinfection of hands and fomites, are important in the control of nosocomial S. aureus epidemics.

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عباس.. طفل فقد والده فوجد في خدمة زائري عاشوراء وطنًا آمنًا

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الآخبار التكنلوجية

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المزيد

مواضيع ذات صلة

Pathogenesis of Streptococcus pneumoniae

Pathogenesis of Staphylococcus aureus

Direct Detection Methods for Erysipelothrix, Lactobacillus, and Similar Organisms

Pathogenesis and Spectrum of Disease Erysipelothrix, Lactobacillus, and Similar Organisms

Antimicrobial Susceptibility Testing and Therapy for Listeria, Corynebacterium, and Similar Organisms

Approach to Identification for Listeria, Corynebacterium, and Similar Organisms

GROWTH AND METABOLISM

SPORES AND SPORULATION

Susceptibility Testing

Immunologic Detection of Microorganisms

Identification of Bacteria

Growing Bacteria in Culture