Are Gram-negative bacilli in blood cultures a contaminant?

From the Guidelines

Gram-negative bacilli in blood cultures should not be considered a contaminant and requires immediate attention. These organisms are rarely contaminants and almost always represent true bacteremia, which can lead to sepsis if not promptly treated. Initial empiric antibiotic therapy should include broad-spectrum coverage such as piperacillin-tazobactam 4.5g IV every 6 hours, meropenem 1g IV every 8 hours, or cefepime 2g IV every 8 hours, with dosage adjustments needed for renal impairment 1. Blood cultures should be repeated to confirm clearance of bacteremia, and antibiotic therapy should be narrowed based on identification and susceptibility results, typically continuing for 7-14 days depending on the source and clinical response. The source of infection must be identified and controlled, which may require removal of infected devices, drainage of abscesses, or surgical intervention. Gram-negative bacteremia is serious because these bacteria contain endotoxins in their cell walls that can trigger inflammatory cascades leading to septic shock, with mortality rates of 12-38% depending on the pathogen and patient factors.

Key Considerations

• The incidence of gram-negative bacillary intravascular device infection and secondary bacteremia has decreased, but infections due to antibiotic-resistant gram-negative pathogens have increased 1.
• Risk factors for infection due to MDR gram-negative bacilli include being critically ill, being neutropenic, having received prior antibiotic therapy, and having a femoral catheter 1.
• Empirical antibiotic therapy should include vancomycin and coverage for gram-negative bacilli, based on the local antibiogram 1.
• The infected catheter should always be removed for patients with hemodialysis CRBSI due to S. aureus, Pseudomonas species, or Candida species 1.

Management Recommendations

• Initial empiric antibiotic therapy should include broad-spectrum coverage such as piperacillin-tazobactam, meropenem, or cefepime 1.
• Blood cultures should be repeated to confirm clearance of bacteremia, and antibiotic therapy should be narrowed based on identification and susceptibility results 1.
• The source of infection must be identified and controlled, which may require removal of infected devices, drainage of abscesses, or surgical intervention 1.
• Antibiotic therapy should be continued for 7-14 days depending on the source and clinical response 1.

From the Research

Gram-Negative Bacilli in Blood Cultures

• Gram-negative bacilli (GNB) bacteremia is typically transient and usually resolves rapidly after the initiation of appropriate antibiotic therapy and source control 2
• The optimal duration of treatment and utility of follow-up blood cultures (FUBC) have not been studied in detail, and the management of gram-negative bacteremia is determined by clinical judgment 2

Follow-up Blood Cultures

• FUBC added little value in the management of GNB bacteremia, and unrestrained use of blood cultures has serious implications for patients, including increased healthcare costs, longer hospital stays, unnecessary consultations, and inappropriate use of antibiotics 2
• However, other studies suggest that FUBCs are associated with improved outcome of patients with gram-negative bloodstream infections, particularly in severe cases or when the infection is caused by difficult-to-treat pathogens 3, 4
• FUBCs were performed in more severely ill patients, with nonurinary sources, difficult-to-treat pathogens, and receipt of initial inappropriate therapy, and were associated with longer treatment duration and lower mortality 3

Risk Factors for Positive Follow-up Blood Cultures

• Risk factors for positive FUBCs included multidrug-resistant GN infection and vascular catheter source 5
• In neutropenic patients with GN-BSI, the prevalence of positive FUBCs was low (2.8%) and the negative predictive value was 92% among patients lacking these risk factors 5
• Positive FUBCs were associated with prolonged hospitalization and longer duration of antimicrobial therapy, but did not significantly impact mortality or microbiologic relapse 5

Clinical Significance of Persistent Bacteremia

• Persistent bacteremia was found in 38.5% of patients with GNB bacteremia, and was often associated with the presence of septic thrombus infection (STI) 4
• FUBCs seem useful for the optimal management of GNB in critically ill patients, particularly in the presence of STI or other complicated infections 4