Bloods
Although blood is normally sterile, microorganisms may be introduced during specimen collection. To improve the detection of bloodstream infections and aid in interpretation, most hospitals obtain at least two blood culture sets from separate venipuncture sites (spaced 15 minutes apart if same site). Each set typically contains one aerobic and one anaerobic bottle.
The significance of a positive blood culture should never be based on culture results alone. Interpretation requires consideration of the patient’s signs and symptoms, the identity of the organism, and the number of blood culture sets that yield the same isolate.
Organisms recovered from blood cultures should generally be considered potential pathogens; however, some species are more commonly associated with contamination. Isolation of the same organism from two or more separately collected blood culture sets increases the likelihood that it represents true bacteremia rather than contamination and may warrant identification and antimicrobial susceptibility testing (AST) in conjunction with the patient’s clinical presentation.
| Organism | Gram Stain | Colony Morphology | Biochems | Plate Growth |
|---|---|---|---|---|
| Micrococcus spp. | GPC in tetrads and pairs | Bright yellow (“super yellow”), can be smeary; variable yellow-white colonies | Catalase + Microdase+ | Typically grows better on chocolate agar than BAP |
| Bacillus spp. | Large GPR, can observe spores in older cultures | Dry, gray colonies; may swarm | Variable biochems | Grows on BAP; some species are β-hemolytic (e.g., B. cereus). If β-hemolytic, not B. anthracis. |
| Paenibacillus spp. | GPR, can stain variable | Gray, slightly mucoid colonies | Variable biochems | Non-hemolytic on BAP |
| Coagulase-Negative Staphylococci (CNS) | GPC in clusters | White to gray colonies; morphology varies by species | Catalase + Coagulase− | Growth on routine media; evaluate different morphologies separately by MALDI-TOF if isolated from multiple blood culture sets |
| Diphtheroids (Coryneform bacteria) | Short GPR, often clubbed ends; may stack/palisade | White to gray colonies | Catalase + | Growth on BAP; commonly considered skin contaminant |
| Brevibacterium spp. | GPR | Usually pigmented colonies | Catalase + | Growth on BAP; resembles diphtheroids but with more pigmentation |
