Blood Cultures and Gram-Positive Blood Isolates

A positive blood culture begins two linked investigations. The laboratory identifies the organism and asks whether the recovery represents bloodstream infection, contamination during collection, or growth from an intravascular device. The answer depends on blood volume, the number and source of the sets, the bottle pattern, the time to detection, the Gram stain, and the final organism identification.¹

Blood culture is the reference method for diagnosing bacteremia and common fungemia and remains central to the laboratory evaluation of endocarditis and other endovascular infections. Molecular methods can shorten identification after a bottle becomes positive, while culture supplies the isolated organism needed for full identification and susceptibility testing.^1,3

General specimen transport, direct stains, media, incubation, and staged reporting are covered in Specimen Collection, Transport, and Primary Processing in Microbiology. Blood cultures add collection, monitoring, and interpretation decisions that depend on the source, set, bottle pattern, and organism recovered.

Collection for bloodstream infection

Blood cultures should be collected before antimicrobial therapy when the patient’s condition permits. Routine adult sets can be collected sequentially from separate peripheral venipunctures. Volume and aseptic collection are more important than a routine interval between sets. A separate protocol may specify timing when infective endocarditis or another continuous endovascular infection is being evaluated.^2,3

Prepare the venipuncture site with an appropriate alcohol-containing skin antiseptic and disinfect each bottle septum with 70% isopropyl alcohol. Collect each set from a separately prepared site and document the date, time, anatomic site, collection method, and bottle fill volume.²

An adult blood-culture set contains 20 to 30 mL of blood, divided among the bottles required by the validated culture system. A common two-bottle set places about 10 mL into an aerobic bottle and 10 mL into an anaerobic bottle. At least two sets, totaling 40 to 60 mL, should be collected within a 24-hour septic episode; CDC describes two to four sets as the standard range. Blood volume has the largest controllable effect on adult recovery, because many bloodstream infections contain few organisms per milliliter.^2,3

The bottle manufacturer defines the accepted fill range. Underfilling lowers sensitivity. Severe overfilling can disturb the blood-to-broth ratio and may cause instrument or interpretation problems. The laboratory should monitor the measured fill volume for each bottle as a collection quality indicator.^1,2

Pediatric volume follows the child’s weight, the total blood volume that can be drawn safely, and the instructions for the pediatric bottle. Small-volume collections commonly use one pediatric aerobic bottle. Older children who can supply an adult volume use the adult collection plan. The laboratory’s weight-based table defines the volume for each pediatric range.³

Peripheral venipuncture is the preferred source. When catheter-related bloodstream infection is being evaluated, collect a peripheral set and a catheter-drawn set at the same time and label the source of each. Comparing the bottle patterns and time to detection from these paired sources helps distinguish line colonization, collection contamination, and bloodstream infection.^1,3

Bottle monitoring and positive-culture workup

Modern blood-culture bottles use enriched broth, an anticoagulant such as sodium polyanethol sulfonate, and formulation-specific systems that may bind or neutralize antimicrobial agents. A validated blood-to-broth ratio, commonly between about 1:5 and 1:10, dilutes complement, antibodies, phagocytes, and residual antimicrobial activity. Some bottles add adsorbing resins for further antimicrobial neutralization. Automated instruments incubate the bottles and repeatedly measure a microbial metabolic signal, usually carbon dioxide production or a change in headspace gas pressure.¹

The laboratory follows the incubation period validated for its bottle and instrument combination. A routine period of about 5 days recovers most bacteria and yeasts, including most HACEK organisms (Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, and Kingella) and Brucella species. Suspected mycobacteremia, selected fungi, Cutibacterium acnes, and other unusual organisms may require a different bottle, system, or incubation plan. Suspected Brucella activates the laboratory’s biosafety and public-health referral procedure.^1,3

Most nonfastidious bloodstream isolates signal during the first 24 to 36 hours. Time to positivity is affected by organism burden, collected volume, bottle atmosphere, antimicrobial exposure, and transport delay. An early signal can support a high organism burden, and earlier growth from a catheter bottle can support a catheter source when compared with the paired peripheral bottle. Time to positivity remains one part of the interpretation. Facultative organisms may signal in either an aerobic or anaerobic bottle, so bottle atmosphere alone is an unreliable predictor of detection time.^1,3

When a bottle signals positive, the laboratory performs the blood-specific workup in this order:

1. Mix and sample the bottle with an aerosol-controlled procedure.
2. Prepare a Gram stain and communicate the result through the laboratory’s urgent-result policy.
3. Inoculate solid media appropriate for the Gram-stain finding and bottle atmosphere.
4. Apply a validated rapid identification method when available. A positive-bottle molecular panel can identify its listed targets and selected resistance genes; validated mass-spectrometry workflows can shorten identification after adequate organism preparation.
5. Confirm the identification and complete susceptibility testing through the laboratory’s validated pathway, which may begin directly from positive broth or from an isolated colony. Resolve a disagreement between a resistance-gene result and the phenotype under the laboratory’s validated policy.

Rapid identification is most useful when the laboratory communicates the result promptly and explains its limits. Each panel covers a fixed target list, so an unlisted organism or resistance mechanism remains possible. The Gram stain is available first and continues to guide culture work while a rapid assay is running.⁴

A flagged bottle with an initially negative Gram stain still requires repeat staining and subculture. Organisms such as Brucella may be faint or sparse, while a false instrument signal can occur without viable growth. A suspected high-risk organism activates the laboratory’s biosafety and public-health referral procedure.^1,3

Contamination and clinical significance

Coagulase-negative staphylococci, Corynebacterium species, Bacillus species other than B. anthracis, Micrococcus species, Cutibacterium acnes, and some viridans-group streptococci are common skin commensals. Each can also cause true infection, especially in a patient with an intravascular catheter, prosthetic valve, other implanted material, immunosuppression, or repeated recovery of the same organism.⁵

The number of positive bottles within one set is weaker evidence than the number of independently collected sets. Bottle composition and organism oxygen preference influence which bottle becomes positive. The laboratory should report the exact set and bottle pattern and use a qualified comment that names a possible skin contaminant and calls for clinical correlation.^2,5

Contamination rate is a microbiology collection-quality measure. Laboratories track the percentage of eligible culture sets classified as contaminated and investigate a rising rate through collection location, collector, and technique data.⁵

Initial identification

The positive-bottle Gram stain determines where identification begins. Colony morphology and validated rapid reactions narrow the possibilities. Mass spectrometry, an automated system, or a molecular method provides the final identification.⁶

Catalase testing requires a colony taken without blood-agar carryover because erythrocytes contain catalase and can produce a false-positive reaction.⁶

Staphylococcus from blood

Staphylococci are catalase-positive, facultatively anaerobic gram-positive cocci that usually form clusters. Mannitol salt agar selects salt-tolerant staphylococci and differentiates mannitol fermentation. It provides a screening pattern; final identification uses a validated method.

Coagulase testing separates the principal phenotypic groups. The slide reaction detects bound coagulase or clumping factor, and the tube reaction detects free coagulase. Autoagglutinating strains require a saline control, and each procedure follows its reagent and laboratory validation. Staphylococcus lugdunensis can produce clumping factor and appear slide-coagulase positive while remaining tube-coagulase negative.⁶

These findings provide a presumptive identification. The final report comes from a validated identification system.⁶

Methicillin resistance in S. aureus most often results from mecA, which encodes the altered penicillin-binding protein PBP2a. mecC and uncommon phenotypes can produce a disagreement with an assay limited to mecA. Cefoxitin phenotypic testing is a strong inducer of mecA and gives clear endpoints. Validated PBP2a immunoassays and mecA/mecC molecular assays are other methods for detecting resistance. The organism-specific method and breakpoint must match the current corrected CLSI M100 edition and the laboratory’s validated system.^7,8

An erythromycin-resistant, clindamycin-susceptible staphylococcal isolate requires assessment for inducible macrolide-lincosamide-streptogramin B resistance under the current CLSI method. The D-zone test reveals induction by flattening the clindamycin inhibition zone beside the erythromycin disk. Disk placement and interpretation follow the adopted M100 procedure.⁸

Vancomycin-intermediate S. aureus (VISA) has a vancomycin minimum inhibitory concentration (MIC) of 4 to 8 µg/mL. Vancomycin-resistant S. aureus (VRSA) has an MIC of at least 16 µg/mL. When an isolate has an MIC of at least 8 µg/mL, CDC directs the laboratory to verify culture purity, repeat species identification and MIC testing with a validated method, preserve the isolate, notify infection prevention and public health, and send the isolate through the health department for confirmatory testing. VISA and VRSA detection requires a validated MIC method because disk diffusion misses part of this range.⁹

Streptococcus and Enterococcus from blood

Streptococci and enterococci are catalase-negative gram-positive cocci that usually appear in pairs or chains. Hemolysis on sheep blood agar supplies the first presumptive classification. Lancefield carbohydrate grouping further classifies chiefly beta-hemolytic streptococci and selected group D organisms. Species identification is needed because hemolysis varies and a Lancefield antigen can occur in more than one species.⁶

The historical name S. bovis covers several organisms with different clinical associations. Current reporting uses the S. gallolyticus and S. infantarius group names when the identification method can resolve them. Database content controls the accuracy of mass-spectrometry and biochemical systems, and 16S ribosomal RNA sequencing alone often lacks subspecies resolution.¹⁰

Several less common catalase-negative cocci can be misidentified:

These uncommon cocci require species confirmation before a clinically important sterile-site report.⁶

Pneumococcal penicillin susceptibility can begin with the current 1-µg oxacillin screen. A zone of at least 20 mm supports penicillin susceptibility; a smaller zone proceeds to MIC testing with meningitis or nonmeningitis breakpoints as appropriate. The exact zone and MIC criteria come from the laboratory’s adopted, corrected CLSI M100 edition.⁸

Enterococcal vancomycin resistance has two major laboratory patterns. Acquired vanA and vanB resistance has important transmission and infection-prevention implications. Chromosomal vanC produces characteristic low-level intrinsic resistance in Enterococcus gallinarum and E. casseliflavus. Both species are motile; E. casseliflavus is commonly yellow-pigmented, while E. gallinarum is usually nonpigmented. Species identification and the complete susceptibility pattern separate these findings.⁸

Coryneform rods and Listeria

Irregular gram-positive rods recovered from blood range from common skin commensals to invasive and public-health pathogens. Gram-stain shape provides the first clue. Mass spectrometry, sequencing, or a validated biochemical system provides the reportable species identification.^6,11-13

The clinical context determines the workup for nondiphtherial Corynebacterium species. Recent clinical evidence identifies C. jeikeium and C. striatum as important causes of catheter-associated and other bloodstream infections, especially in patients with hematologic malignancy, neutropenia, or an implanted device. Repeated recovery from independent sets supports species-level identification. Phenotypic schemes can misidentify these organisms, so mass spectrometry or reference-laboratory testing is preferred when the local method cannot resolve them.¹¹

Suspected diphtheria follows a three-part confirmation sequence: isolate and identify the organism, detect the tox gene, and demonstrate toxin production. In the United States, CDC currently performs the Elek immunoprecipitation test used to confirm toxin production. State and local public health laboratories coordinate submission. Diphtheria confirmation requires demonstrated toxin production.¹²

Listeria monocytogenes can resemble group B streptococcus because both may form small colonies with a narrow zone of beta hemolysis. The Gram stain identifies a rod, and the positive catalase reaction separates it from group B streptococcus. Room-temperature motility and a validated identification system complete the workup. Cold enrichment belongs chiefly to food and environmental investigation. Routine blood cultures use the laboratory’s validated clinical system.¹³

L. monocytogenes has intrinsic clinical resistance to cephalosporins. An apparently susceptible cephalosporin result is suppressed under the laboratory’s organism-specific reporting rules. Susceptibility testing for Listeria and other infrequently isolated bacteria follows the current method and criteria adopted from CLSI M45 and related standards.¹⁴

References

1. Clinical and Laboratory Standards Institute. Principles and Procedures for Blood Cultures. 2nd ed. CLSI guideline M47. Clinical and Laboratory Standards Institute; 2022. Accessed August 29, 2026.
2. Centers for Disease Control and Prevention. Collect adult blood culture sets. Updated March 31, 2026. Accessed August 29, 2026.
3. Miller JM, Binnicker MJ, Campbell S, et al. Guide to utilization of the microbiology laboratory for diagnosis of infectious diseases: 2024 update by the Infectious Diseases Society of America and the American Society for Microbiology. Clin Infect Dis. Published online March 5, 2024. doi:10.1093/cid/ciae104
4. Wolk DM, Parrott JS, Babady NE, et al. The American Society for Microbiology's evidence-based laboratory medicine practice guidelines for the diagnosis of bloodstream infections using rapid tests: a systematic review and meta-analysis. Clin Microbiol Rev. 2025;38(3):e00137-24. doi:10.1128/cmr.00137-24
5. Sautter RL, Parrott JS, Nachamkin I, et al. American Society for Microbiology evidence-based laboratory medicine practice guidelines to reduce blood culture contamination rates: a systematic review and meta-analysis. Clin Microbiol Rev. 2024;37:e00087-24. doi:10.1128/cmr.00087-24
6. Carroll KC, Pfaller MA, Landry ML, McAdam AJ, Karlowsky JA, Patel R, Pritt BS, eds. Manual of Clinical Microbiology. 13th ed. ASM Press; 2023.
7. Centers for Disease Control and Prevention. Laboratory testing for methicillin (oxacillin)-resistant Staphylococcus aureus. Updated June 27, 2025. Accessed August 29, 2026.
8. Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing. 36th ed. CLSI supplement M100. Clinical and Laboratory Standards Institute; 2026. Accessed August 29, 2026.
9. Centers for Disease Control and Prevention. Laboratory testing for vancomycin-resistant Staphylococcus aureus. Updated April 15, 2024. Accessed August 29, 2026.
10. Putnam NE, Youn J, Wallace MA, et al. Comparative evaluation of current biochemical-, sequencing-, and proteomic-based identification methods for the Streptococcus bovis group. J Clin Microbiol. 2023;61(4):e01712-22. doi:10.1128/jcm.01712-22
11. Mitchell BI, Markantonis JE. An underestimated pathogen: Corynebacterium species. J Clin Microbiol. 2025;63(10):e01552-24. doi:10.1128/jcm.01552-24
12. Centers for Disease Control and Prevention. Laboratory testing for diphtheria. Updated July 31, 2026. Accessed August 29, 2026.
13. Centers for Disease Control and Prevention. Listeria monocytogenes identification chart. Accessed August 29, 2026.
14. Clinical and Laboratory Standards Institute. Methods for Antimicrobial Dilution and Disk Susceptibility Testing of Infrequently Isolated or Fastidious Bacteria. 3rd ed. CLSI guideline M45. Clinical and Laboratory Standards Institute; 2016. Accessed August 29, 2026.