Is ampicillin and ceftriaxone (Ceftriaxone) sufficient for a patient with blood cultures positive for Gram-positive cocci in pairs and chains?

Treatment for Blood Cultures with Gram-Positive Cocci in Pairs and Chains

The combination of ampicillin and ceftriaxone is an appropriate treatment regimen for blood cultures positive for Gram-positive cocci in pairs and chains, which likely represents either streptococcal or enterococcal bacteremia. 1

Identification of the Organism

Gram-positive cocci in pairs and chains typically represent:

• Streptococci (including viridans group streptococci)
• Enterococci
• Rarely, other streptococcal species

Treatment Considerations

For Streptococcal Infections:

If the organism is a streptococcus with low-level penicillin resistance (MIC ≥0.5 μg/mL):

• Ampicillin plus ceftriaxone is a reasonable combination therapy 1
• The American Heart Association guidelines state that ceftriaxone combined with gentamicin may be a reasonable alternative treatment option for viridans group streptococci (VGS) isolates that are susceptible to ceftriaxone and resistant to penicillin 1

For Enterococcal Infections:

If the organism is an enterococcus:

• Ampicillin plus ceftriaxone is specifically recommended for aminoglycoside-resistant enterococcal strains 1
• This combination has shown similar success rates to ampicillin-gentamicin regimens with fewer cases of renal failure 1
• The major advantages of the ampicillin-ceftriaxone regimen are lower risk of nephrotoxicity and no need for measuring aminoglycoside serum concentrations 1

Duration of Therapy

• For native valve endocarditis (NVE): 4-6 weeks depending on the organism and duration of symptoms before therapy initiation 1
• For prosthetic valve endocarditis (PVE): 6 weeks of antimicrobial therapy is reasonable 1
• If ampicillin plus ceftriaxone is selected as the treatment regimen, 6 weeks of therapy is reasonable regardless of symptom duration 1

Potential Pitfalls and Caveats

1. Organism identification is crucial: Final identification and susceptibility testing should guide definitive therapy.

2. Potential hypersensitivity reactions: Using two β-lactams (ampicillin and ceftriaxone) carries a risk of hypersensitivity reactions. If a reaction occurs, both drugs might need to be discontinued and replaced with vancomycin-gentamicin therapy 1.

3. Susceptibility considerations: For streptococci with higher MICs (≥0.5 μg/mL), the efficacy of the combination may be reduced. Recent research suggests that E. faecalis isolates with penicillin MICs of 4 μg/mL less frequently demonstrated synergy with penicillin/ceftriaxone combinations 2.

4. Monitoring: Regular clinical assessment for treatment response is essential.

Alternative Regimens

If the patient cannot tolerate ampicillin or ceftriaxone:

• For streptococcal infections: Vancomycin monotherapy is reasonable for patients unable to tolerate penicillin or ceftriaxone therapy 1
• For enterococcal infections: Vancomycin plus gentamicin, though this combination has potential increased risk of ototoxicity and nephrotoxicity 1

Conclusion

The current combination of ampicillin and ceftriaxone is an appropriate empiric therapy for blood cultures showing Gram-positive cocci in pairs and chains. This regimen provides coverage for both streptococci and enterococci, including aminoglycoside-resistant strains, with a favorable safety profile compared to aminoglycoside-containing regimens.