Corynebacterium in a Single Blood Culture Bottle: Contamination vs. True Infection
A single blood culture bottle growing Corynebacterium species in this clinical context almost certainly represents contamination and does not require any change in antimicrobial therapy directed at MRSA pneumonia. 1
Rationale for Contamination
Corynebacterium species are classic oropharyngeal commensals and skin flora that frequently contaminate blood cultures. 1 The American Thoracic Society/Infectious Diseases Society of America guidelines explicitly categorize Corynebacterium species alongside coagulase-negative staphylococci, viridans group streptococci, and Neisseria species as organisms whose "significant growth from distal bronchial specimens is difficult to interpret" and typically represent contamination rather than true infection in immunocompetent hosts. 1
Key Features Supporting Contamination:
- Single bottle positivity: Growth in only one of multiple blood culture bottles is the hallmark of contamination rather than true bacteremia 1
- Clinical context: The patient's presentation is entirely consistent with MRSA pneumonia (sepsis, acute hypoxemic respiratory failure, probable MRSA pneumonia) without features suggesting Corynebacterium infection 1
- Typical contaminant: Corynebacterium is listed specifically in guidelines as a common blood culture contaminant 1
When Corynebacterium Might Be Pathogenic
While contamination is overwhelmingly likely, Corynebacterium can produce true infection in specific circumstances that do not apply to this patient:
- Immunocompromised hosts (not mentioned in this case) 1
- Presence of prosthetic devices or implanted hardware (not mentioned) 1
- Growth from multiple blood culture bottles or sets 1
- Clinical features consistent with endocarditis or device-related infection 2
Recommended Management Approach
Continue current MRSA-directed therapy without modification. 1, 3 The patient's clinical presentation—sepsis and acute hypoxemic respiratory failure secondary to probable MRSA pneumonia—requires aggressive anti-MRSA coverage as the priority. 3, 4
Specific Actions:
- Do not add or change antibiotics based on the single Corynebacterium-positive bottle 1
- Ensure appropriate MRSA coverage is maintained (vancomycin 15-20 mg/kg IV q8-12h or linezolid 600 mg IV q12h) 3, 4
- Repeat blood cultures if clinically indicated to document clearance of any true bacteremia 1
- Document the Corynebacterium isolate as likely contamination in the medical record to prevent unnecessary antibiotic escalation 1
Critical Pitfall to Avoid
The most common error is treating blood culture contaminants as true pathogens, leading to unnecessary antibiotic exposure, prolonged hospitalization, and increased costs (estimated at $1,000-$5,000 per contaminated culture). 1 In this elderly patient with multiple comorbidities (seizure disorder, GAD, MDD) and severe sepsis from MRSA pneumonia, adding unnecessary antibiotics for a contaminant increases the risk of adverse drug reactions, Clostridioides difficile infection, and selection of multidrug-resistant organisms. 1
Focus on the True Pathogen:
The patient's mortality risk is driven by the MRSA pneumonia and septic shock, not by Corynebacterium. 3, 4 MRSA pneumonia carries a case fatality rate of 15-30% even with appropriate therapy, and each hour of delay in appropriate antimicrobial therapy reduces survival by approximately 7.6%. 3, 5 Maintaining aggressive, appropriate anti-MRSA therapy is the priority for reducing morbidity and mortality. 3, 4, 5