Staphylococcus aureus is a common bacteria often found harmlessly on the skin and in the nose of about 30% of people. When Staph enters the body through a cut, wound, or medical device, it can cause infections ranging from mild skin abscesses to severe conditions like pneumonia or endocarditis. Treating these infections is challenging because the bacteria rapidly develop resistance to antibiotics. Since no single drug is universally effective, selecting the correct treatment requires guidance from laboratory results.
How Testing Determines the Right Antibiotic
The specific vulnerability of a Staphylococcus aureus strain must be determined in a laboratory before treatment begins. This is done through Antibiotic Susceptibility Testing (AST), which involves growing the bacteria from a sample (such as blood or tissue) and exposing it to various antibiotics. The results classify the strain as either Methicillin-Sensitive S. aureus (MSSA) or Methicillin-Resistant S. aureus (MRSA).
If the strain is susceptible to methicillin and related drugs, it is classified as MSSA, meaning standard antibiotics will be effective. Resistance to methicillin classifies the infection as MRSA, requiring a specialized treatment approach. The test results are reported as Susceptible, Intermediate, or Resistant, directly guiding the clinician’s choice of medication. Identifying this resistance pattern is the first step in ensuring the patient receives a drug that can successfully clear the infection.
Standard Treatment for Sensitive Strains
If testing confirms the infection is MSSA, treatment relies on beta-lactam antibiotics. The standard approach for severe MSSA infections uses drugs like nafcillin, oxacillin, or dicloxacillin. These drugs are highly effective because MSSA lacks the resistance factor that would neutralize them. Nafcillin and oxacillin are typically given intravenously for serious infections, such as bacteremia or endocarditis. For less severe infections, an oral agent like dicloxacillin may be prescribed.
Alternatively, a first-generation cephalosporin, such as cefazolin, is frequently used intravenously. Cefazolin is an excellent option for patients with a mild allergy to penicillin. These standard antibiotics are preferred for MSSA because they actively kill the bacteria and often have fewer side effects compared to the drugs required for MRSA. However, these drugs are ineffective if the Staph strain possesses the methicillin-resistance mechanism.
Specialized Treatment for Resistant Strains
MRSA resists most standard antibiotics, requiring specialized drugs that target the bacteria through different mechanisms. Vancomycin is the traditional first-line treatment for severe MRSA infections, particularly those involving the bloodstream or deep tissues. Vancomycin works by interfering with the construction of the bacterial cell wall.
Vancomycin is usually given intravenously and requires careful monitoring to maintain adequate concentrations without causing kidney damage. However, Vancomycin is less effective for MRSA pneumonia due to poor penetration into lung tissue. In these situations, or if a patient cannot tolerate Vancomycin, alternative agents are used.
Linezolid is a valuable alternative because it penetrates the lungs well, making it a preferred choice for MRSA pneumonia. Linezolid inhibits protein synthesis and is available in both intravenous and oral formulations. Another potent option is daptomycin, which works by disrupting the bacterial cell membrane, causing rapid cell death.
Daptomycin is highly effective for MRSA bacteremia and right-sided endocarditis. However, it must be avoided for pneumonia because it is inactivated by pulmonary surfactant in the lungs. Other specialized drugs include ceftaroline, a newer cephalosporin active against MRSA, and oral agents like trimethoprim-sulfamethoxazole (TMP-SMX) or doxycycline for simple skin infections.
Factors Influencing Successful Recovery
Successful treatment of a Staphylococcus aureus infection depends on several non-drug factors. Source control is one of the most important interventions, involving physically removing the focus of the infection. For localized infections like skin abscesses, this means performing an incision and drainage (I&D) procedure to evacuate the pus.
If the infection is associated with a medical device, such as a catheter or prosthetic joint, the device must often be removed to achieve a cure. The route of administration also matters; severe systemic infections like endocarditis require intravenous therapy for weeks. Less complicated infections can be managed with oral antibiotics once the patient is stable.
The required duration of treatment varies significantly based on the type and extent of the infection. Simple skin infections may require 5 to 10 days of therapy. However, deep-seated infections, such as bone infections (osteomyelitis) or endocarditis, often demand four to eight weeks of continuous treatment. Patient adherence to the full prescribed course is necessary to prevent recurrence and limit the opportunity for the bacteria to develop new resistance.