From the Guidelines
Repeat blood cultures are necessary for anaerobic gram negative bacteremia as part of the minimum evaluation, which includes a thorough history and exam, and a transthoracic echocardiogram (TTE) 1. This approach is supported by a 2023 study published in Clinical Infectious Diseases, which emphasizes the importance of repeat blood cultures in the management of bacteremia 1. Although the study focuses on Staphylococcus aureus bacteremia, the principle of repeat blood cultures can be applied to anaerobic gram-negative bacteremia as well.
- Key points to consider in the management of anaerobic gram-negative bacteremia include:
- Initial blood cultures should be collected before starting antibiotics
- Appropriate antimicrobial therapy, such as metronidazole, carbapenems, or beta-lactam/beta-lactamase inhibitor combinations, should be initiated promptly
- Clinical improvement should be monitored closely, and follow-up cultures may be indicated if the patient remains febrile or shows signs of clinical deterioration despite 48-72 hours of appropriate antibiotic therapy
- The duration of antibiotic therapy may be extended for complicated infections or if there's an undrainable focus
- It is essential to note that anaerobic bacteria can grow slowly in culture media, often taking 2-5 days for identification, so initial empiric therapy should cover these organisms when suspected 1.
From the Research
Anaerobic Gram-Negative Bacteremia
- Anaerobic gram-negative bacteremia is typically caused by bacteria such as Bacteroides fragilis group, Peptostreptococcus, Clostridium spp., and Fusobacterium spp. 2
- The majority of anaerobic bacteremia cases are due to Gram-negative bacilli, mostly Bacteroides fragilis group 2
Repeat Blood Cultures
- Repeat blood cultures may not be necessary for gram-negative bacteremia, as they add little value in the management of the condition 3
- However, repeat blood cultures can be useful in differentiating true bacteremia from pseudobacteremia, especially in cases with potential contaminants in blood cultures 4
- Repeating 2 sets of blood cultures might be useful in the clinical differentiation of true bacteremia and pseudobacteremia, since the contamination rate of patients with potential contaminants in 1 set of blood cultures declined from 95% to 87% with such a strategy 4
Treatment of Anaerobic Infections
- Treatment of anaerobic infections is complicated by their slow growth in culture, by their polymicrobial nature and by their growing resistance to antimicrobials 5
- The most effective antimicrobials against anaerobes are: metronidazole, the carbapenems (imipenem, meropenem, doripenem, ertapenem), chloramphenicol, the combinations of a penicillin and a beta-lactamase inhibitors (ampicillin or ticarcillin plus clavulanate, amoxicillin plus sulbactam, piperacillin plus tazobactam), tigecycline, cefoxitin and clindamycin 5
Clinical Significance
- True bloodstream infections had a significantly higher all-cause mortality rate at 14 days than those with pseudobacteremia (23.8% vs 7.3%, p=0.028), suggesting the validity of the clinical criteria 4
- Early recognition and appropriate treatment of anaerobic bacteremia are of great clinical importance 2