From the Guidelines
Micrococcus and Bacillus species are typically considered contaminants when isolated from blood cultures, and their presence usually represents contamination rather than true bacteremia, as supported by the guidelines for the diagnosis and management of intravascular catheter-related infection 1. These organisms are commonly found in the environment and on human skin, making them frequent contaminants during the blood collection process. When these bacteria appear in blood cultures, especially in a single bottle or after prolonged incubation, they usually represent contamination rather than true bacteremia.
Key Considerations
- Clinical context is essential for interpretation, with factors such as:
- Multiple positive cultures with the same organism
- Presence of intravascular devices
- Immunocompromised status
- Matching clinical symptoms may occasionally suggest true infection.
- However, in most cases (over 90% for Micrococcus and approximately 75% for Bacillus species excluding B. anthracis), these represent false positives, as stated in the guidelines for the prevention of intravascular catheter-related infections 1.
- Laboratory technicians and clinicians should evaluate these findings carefully, considering the patient's clinical presentation, the number of positive culture bottles, time to positivity, and other laboratory findings before determining clinical significance.
- Proper skin antisepsis during blood collection and careful venipuncture technique can help reduce contamination rates, as emphasized in the clinical practice guidelines for the diagnosis and management of intravascular catheter-related infection 1.
Diagnosis and Management
- Diagnosis of CRBSI due to Corynebacterium, Bacillus, and Micrococcus species requires at least 2 positive results of blood cultures performed on samples obtained from different sites, as recommended by the Infectious Diseases Society of America 1.
- For the management of these infections, catheter removal is indicated for patients with a short-term CVC, and it is also indicated for patients with an infected long-term catheter or implanted port, unless there are no alternative intravascular access sites, as stated in the guidelines 1.
From the Research
Blood Culture Contaminants
- Micrococcus and Bacillus species are considered potential blood culture contaminants 2, 3, 4
- These species are part of the skin flora and can be introduced into blood cultures during the collection process 4
- The contamination rate of blood cultures with these species can be high, with one study reporting a contamination rate of 83.9% 2
Clinical Significance
- The clinical significance of Micrococcus and Bacillus species in blood cultures can be difficult to determine 2, 3
- Repeating blood cultures can help to differentiate between true bacteremia and pseudobacteremia 2
- The growth time of microorganisms in blood cultures can also be used to estimate whether the isolated microorganism is a pathogen or a contaminant 5
Reduction of Contaminants
- Implementing a selection criterion for processing potential blood culture contaminants can reduce the laboratory workload and the number of contaminants processed 4
- This can be achieved by only processing potential contaminants if they are present in more than one blood culture pair drawn from the same patient within the same day 4