Staphylococcus aureus causes a variety of manifestations and diseases. The treatment of choice for S. aureus infection is penicillin. In most countries, S. aureus strains have developed a resistance to penicillin due to production of an enzyme by the bacteria called penicillinase.

The first line therapy is penicillinase-resistant penicillins like oxacillin or flucloxacillin. Therapy is often given in combination with aminoglycosides like gentamicin for more serious infections. The duration of treatment depends on the site of infection and on severity.

Antibiotic resistance of S. aureus

S. aureus strains may become resistant to penicillin by producing enzymes like penicillinase that destroys the antibiotic. This is a form of β-lactamase which breaks down the β-lactam ring of the penicillin molecule. To overcome this molecules resistant to penicillinase have been developed. These include:

• methicillin
• nafcillin
• oxacillin
• cloxacillin
• dicloxacillin
• flucloxacillin

Genetic mutation and modification

Genetic mutation and modification is said to be the mechanisms that makesS. aureusresistant to methicillin to produce Methicillin Resistant S. aureus or MRSA. The modification in the mecA gene of the bacteria which codes for an altered penicillin-binding protein leads to a lower affinity for binding β-lactams (penicillins, cephalosporins and carbapenems). This allows for resistance to all β-lactam antibiotics.

MRSA infections in both the hospital and community setting are commonly treated with non-β-lactam antibiotics such as clindamycin (a lincosamine) and co-trimoxazole (also commonly known as trimethoprim/sulfamethoxazole). In severe cases vancomycin is used.

Enzyme modification and other methods of resistance

Aminoglycosides like gentamicin, Amikacin, streptomycin and Kanamycin were once effective against Staphylococcal infections. They have developed resistance by modifying enzymes, changing the ribosomal attachment sites and by actively pushing out the drug from the bacteria.

Antibiotic therapy

• Treatment for bacteremia or blood infection with S. aureus or infection from a medical device – the medical device or the foci of the infection needs to be removed after identification. Of antibiotics β-lactams, oxacillin, nafcillin, cefazolin etc. are preferred. For MRSA vancomycin, daptomycin, linezolid, Quinupristin/dalfopristin, Cotrimoxazole,  Ceftaroline,  Telavancin etc. are chosen.
• Treatment of infection of the heart or its valves (Endocarditis) – the foci is removed when possible. Choice of antibiotics includes oxacillin, cefazolin, nafcillin, gentamycin etc. for Methicillin sensitive strains (MSSA). Others include Ciprofloxacin, rifampin, vancomycin, daptomycin etc.
• Infections of soft tissues and skin – the primary treatment is removal of foci of infection by drainage of pus from abscesses, cellulitis etc. Choice of antibiotics for MSSA include Cephalexin, Dicloxacillin, Clindamycin, Amoxicillin/clavulanate etc. For MRSA antibiotics like Cotrimoxazole, Clindamycin, tetracyclines, Doxycycline, Minocycline, Linezolid etc. may be used. For skin infections local application of antibiotics like Mupirocin 2% ointment are also prescribed.
• Lung infections or pneumonia – for MRSA cases Linezolid, Vancomycin, Clindamycin etc. may be used.
• Bone and joint infections – for MSSA oxacillin, cefazolin, nafcillin, gentamycin etc. may be used. For MRSA cases Linezolid, Vancomycin, Clindamycin, Daptomycin, Coptrimoxazole etc. may be used.
• Brain and meninges infection (meningitis) - for MSSA oxacillin, cefazolin, nafcillin, gentamycin etc. may be used. For MRSA cases Linezolid, Vancomycin, Clindamycin, Daptomycin, Cotrimoxazole etc. may be used.
• Toxic Shock Syndrome - for MSSA oxacillin, nafcillin, clindamycin etc. may be used. For MRSA cases Linezolid, Vancomycin, Clindamycin etc. may be used.