What antibiotics are effective against Methicillin-Sensitive Staphylococcus aureus (MSSA)?

From the Guidelines

Methicillin-sensitive Staphylococcus aureus (MSSA) can be effectively treated with nafcillin or oxacillin as the parenteral drugs of choice, or dicloxacillin as the oral agent of choice, according to the practice guidelines for the diagnosis and management of skin and soft-tissue infections 1.

Key Treatment Options

• Nafcillin or oxacillin: 1-2 g every 4 h IV, as the parenteral drugs of choice for MSSA infections 1
• Dicloxacillin: 500 mg 4 times per day PO, as the oral agent of choice for methicillin-susceptible strains 1
• Cephalexin: 500 mg 4 times per day PO, for penicillin-allergic patients, except those with immediate hypersensitivity reactions 1
• Clindamycin: 600 mg every 8 h IV or 300-450 mg 3 times per day PO, as an alternative option for patients with penicillin allergies, although it has potential for cross-resistance and emergence of resistance in erythromycin-resistant strains 1

Considerations

• The choice of antibiotic should be based on the severity of the infection, patient allergies, and local resistance patterns 1
• Treatment duration typically ranges from 7-14 days for uncomplicated infections, but may extend to 4-6 weeks for complicated infections like endocarditis or osteomyelitis
• Clinical response should be assessed within 48-72 hours of starting therapy to ensure the selected antibiotic is effective

Additional Options

• Amoxicillin-clavulanate: 875/125 mg twice per day PO, as an oral option for less severe infections
• Doxycycline: 100 mg twice per day PO, as an alternative option for patients with penicillin allergies, although it has limited recent clinical experience
• Trimethoprim-sulfamethoxazole: 1 or 2 double-strength tablets twice per day PO, as an alternative option, although its efficacy is poorly documented 1

From the FDA Drug Label

MICROBIOLOGY Penicillinase-resistant penicillins exert a bactericidal action against penicillin-susceptible microorganisms during the state of active multiplication. All penicillins inhibit the biosynthesis of the bacterial cell wall. Nafcillin sodium has been shown to be active against most isolates of the following microorganism, both in vitroand in clinical infections as described in the INDICATIONS AND USAGEsection. Gram-Positive Bacteria Staphylococcus aureus(Methicillin-susceptible isolates only)

Mechanism of Action Clindamycin inhibits bacterial protein synthesis by binding to the 23S RNA of the 50S subunit of the ribosome. Clindamycin is bacteriostatic. Resistance Resistance to clindamycin is most often caused by modification of specific bases of the 23S ribosomal RNA. Cross-resistance between clindamycin and lincomycin is complete Because the binding sites for these antibacterial drugs overlap, cross-resistance is sometimes observed among lincosamides, macrolides and streptogramin B. Macrolide-inducible resistance to clindamycin occurs in some isolates of macrolide-resistant bacteria Macrolide-resistant isolates of staphylococci and beta-hemolytic streptococci should be screened for induction of clindamycin resistance using the D-zone test Antimicrobial Activity Clindamycin has been shown to be active against most of the isolates of the following microorganisms, both in vitro and in clinical infections [see Indications and Usage (1)]: Gram-positive bacteria Staphylococcus aureus (methicillin-susceptible strains)

Microbiology Mode of Action Penicillinase-resistant penicillins exert a bactericidal action against penicillin susceptible microorganisms during the state of active multiplication All penicillins inhibit the biosynthesis of the bacterial cell wall. Mechanism of Resistance Resistance to penicillins may be mediated by destruction of the beta-lactam ring by a beta-lactamase, altered affinity of penicillin for target, or decreased penetration of the antibiotic to reach the target site Cross Resistance Resistance to oxacillin (or cefoxitin) implies resistance to all other beta-lactam agents, except newer agents with activity against methicillin-resistant Staphylococcus aureus

The antibiotics that cover Staph aureus methicillin sensitive are:

• Nafcillin (2)
• Oxacillin (3)
• Clindamycin (4)

These antibiotics have been shown to be active against methicillin-susceptible Staphylococcus aureus.

From the Research

Antibiotics for Methicillin-Sensitive Staphylococcus aureus (MSSA)

• The following antibiotics are effective against MSSA:
  • Penicillinase-resistant penicillins (flucloxacillin, dicloxacillin) 5
  • First generation cephalosporins (cefazolin, cephalothin and cephalexin) 5, 6, 7, 8
  • Clindamycin 5, 6
  • Lincomycin 5
  • Erythromycin 5
  • Antistaphylococcal penicillins (nafcillin, oxacillin) 6, 7, 8

Treatment Considerations

• For serious MSSA infections, penicillinase-resistant penicillins or first generation cephalosporins are recommended 5, 7, 8
• For patients with penicillin hypersensitivity, clindamycin, lincomycin, or erythromycin may be used 5
• Cefazolin is considered a suitable alternative to antistaphylococcal penicillins for patients with non-severe penicillin allergies 8
• Vancomycin is not recommended for MSSA infections unless the patient has a severe penicillin allergy 6, 7, 8

Specific Antibiotic Options

• Cefazolin is a first-line treatment option for MSSA bacteremia, especially for patients with non-severe penicillin allergies 6, 7, 8
• Antistaphylococcal penicillins (nafcillin, oxacillin) are also effective against MSSA bacteremia 6, 7, 8
• Clindamycin and lincomycin may be used for less serious MSSA infections, such as skin and soft tissue infections 5