Does ampicillin cover Methicillin-Sensitive Staphylococcus aureus (MSSA)?

From the Guidelines

Ampicillin is not the preferred choice for treating Methicillin-Sensitive Staphylococcus aureus (MSSA) infections, as nafcillin or oxacillin are the recommended parenteral drugs of choice, and dicloxacillin is the recommended oral agent, according to the 2014 IDSA guidelines 1. The typical dosing for adults with MSSA infections for nafcillin or oxacillin is 1-2 g every 4 hours IV, and for dicloxacillin, it is 500 mg four times a day orally. For patients with penicillin allergies, alternative antibiotics such as cephalosporins (for non-severe allergies) or clindamycin, trimethoprim-sulfamethoxazole, or fluoroquinolones may be considered. Some key points to consider when treating MSSA infections include:

• The importance of susceptibility testing to ensure the chosen antibiotic is effective against the specific strain of MSSA
• The need to consider local epidemiology and resistance patterns when selecting an antibiotic
• The potential for beta-lactamase enzymes to affect the efficacy of certain antibiotics, such as ampicillin
• The availability of alternative antibiotics for patients with penicillin allergies or other contraindications. It's also worth noting that the 2011 IDSA guidelines for MRSA infections provide additional context and recommendations for treating skin and soft tissue infections, including the use of vancomycin, linezolid, and daptomycin for complicated infections 1. However, these guidelines are focused on MRSA infections, and the treatment of MSSA infections should be guided by the most recent and relevant evidence, such as the 2014 IDSA guidelines for skin and soft tissue infections 1.

From the FDA Drug Label

Microbiology: ...those strains of staphylococci, which do not produce penicillinase... NOTE: Ampicillin is inactivated by penicillinase and therefore is ineffective against penicillinase-producing organisms including certain strains at staphylococci... Ampicillin covers Methicillin-Sensitive Staphylococcus aureus (MSSA) because MSSA does not produce penicillinase. Key points:

• Ampicillin is effective against staphylococci that do not produce penicillinase
• MSSA does not produce penicillinase, it is sensitive to beta-lactam antibiotics, including ampicillin 2

From the Research

Antibiotic Coverage for Methicillin-Sensitive Staphylococcus aureus (MSSA)

• Ampicillin is not typically considered a first-line treatment for MSSA infections, as most strains of Staphylococcus aureus are resistant to penicillin 3.
• Penicillinase-resistant penicillins, such as flucloxacillin and dicloxacillin, are generally the antibiotics of choice for managing serious MSSA infections 3.
• Cefazolin, a first-generation cephalosporin, has been shown to be an effective alternative to oxacillin for treating deep-seated MSSA infections, including bacteremia 4, 5.
• Ampicillin/sulbactam may be considered as a therapeutic option for MSSA bacteremia in cases where cefazolin is not available or in patients with allergies to penicillinase-stable penicillins 6.
• The choice of antibiotic for MSSA infections should be guided by susceptibility testing and clinical judgment, taking into account factors such as the severity of the infection, patient allergies, and potential adverse effects 3, 5, 7.

Specific Considerations for Ampicillin

• There is limited evidence to support the use of ampicillin as a primary treatment for MSSA infections, as it is not effective against beta-lactamase-producing strains 3, 6.
• Ampicillin/sulbactam may be effective against some strains of MSSA, but its use should be guided by susceptibility testing and clinical experience 6.
• Cefazolin and antistaphylococcal penicillins are generally preferred over ampicillin for treating MSSA infections due to their broader spectrum of activity and better efficacy 4, 5, 7.