- Gram-positive cocci in clusters: commonly associated with Staphylococcus species, especially Staphylococcus aureus (i.e. possibly MRSA)
- Gram-positive cocci in chains: indicative of streptococcal species (e.g., Streptococcus pyogenes, Streptococcus pneumoniae…) or enterococcus species (e.g. Enterococcus faecalis, Enterococcus faecium)
- Gram-negative bacilli: associated with enteric bacteria (e.g., E. coli, Klebsiella spp.) or non-fermenters (e.g., Pseudomonas spp.)
Attention
If only aerobic(s) bottle(s) are positive, consider possible non-fermenting bacteria (Pseudomonas, Stenotrophomonas, Acinetobacter…), which never grow in anaerobic bottles (strictly aerobic bacteria).
- Yeasts: typically Candida spp. critical for immuno-compromised or hospitalized patients
- Anaerobic bacteria: suggestive of Clostridium spp. or Bacteroides spp., often linked to severe intra-abdominal infections or deep necrotizing tissue infection.
- Gram-negative diplococci: Neisseria meningitidis, gonorrhoeae: consider meningitidis, purulent arthritis
Culture positivity must be interpreted according to the actual pathogenicity of the bacteria. In some cases, certain germs may be considered contaminants.
Clear pathogen
Never considered as contaminant
- Staphylococcus aureus
- Streptococcus pneumoniae: associated with severe infections such as meningitis or pneumonia, Streptococcus pyogenes
- Enterococcus species: faecalis or faecium, often associated with endocarditis, biliary or digestive tract infections
- Enterobacteriaceae: whatever the antibiotic resistance type
- Pseudomonas aeruginosa and other non-fermenters
- Yeasts or fungi: Candida spp., Cryptococcus neoformans
- Anaerobic bacteria: Clostridium perfringens (can cause fulminant sepsis)
- Uncommon pathogens: Brucella spp., Haemophilus influenzae, Listeria monocytogenes
The presence of any of these bacteria in a blood culture bottle should lead to the prescription of antibiotics, but in a reasoned and considered manner.
Attention
For every positive bottle for clear pathogen, check every time for infection source and for any secondary localization, in order to optimize source control.
Possible contaminant
- Coagulase negative staphylococci (Staphylococcus epidermidis, hominis…)
- Corynebacterium
- Cutibacterium acnes
- Bacillus sp
Interpretation of the positivity of blood cultures for these germs must be considered in relation to the number of positive bottles:
- Only 1 positive bottle: consider probable contaminant; if only one blood culture was performed make a new check with a new pair of blood cultures
- 2 bottles: make a new check with a new pair of blood cultures
- More than 2 bottles: consider infection and possibly an antibiotic treatment, especially in immunocompromised patients or intravascular device, prosthesis
Resistance profiles identified by antibiotic susceptibility testing are critical, particularly with high-risk pathogens:
- MRSA (methicillin-resistant Staphylococcus aureus)
- ESBL
- Carbapenem-resistant Enterobacteriaceae (CRE)
- Multi-drug-resistant Pseudomonas aeruginosa or Acinetobacter baumannii
- Vancomycin-resistant Enterococcus (VRE)
Some pathogens require notification to health authorities due to transmission risks or community management implications:
- Neisseria meningitidis: requires immediate treatment for close contacts
- Salmonella typhi: risk of community transmission
- Listeria monocytogenes: to look for possible epidemic