Blood Cultures and Gram-Positive Blood Isolates

Collection for bloodstream infection

Collect blood cultures before antimicrobial therapy when the patient's condition permits, because the drug can suppress growth in the bottle. 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.^{3,23 Centers for Disease Control and Prevention. Collect adult blood culture sets. 2026.2 Miller JM, et al. Guide to utilization of the microbiology laboratory for… 2024.}

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.^{33 Centers for Disease Control and Prevention. Collect adult blood culture sets. 2026.}

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. Collect at least two sets, totaling 40 to 60 mL, within a 24-hour septic episode. The Centers for Disease Control and Prevention (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.^{3,23 Centers for Disease Control and Prevention. Collect adult blood culture sets. 2026.2 Miller JM, et al. Guide to utilization of the microbiology laboratory for… 2024.}

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. Monitor the measured fill volume for each bottle as a collection quality indicator.^{1,31 Clinical and Laboratory Standards Institute. Principles and Procedures for Blood Cultures. 2022.3 Centers for Disease Control and Prevention. Collect adult blood culture sets. 2026.}

Pediatric volume is based on 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.^{22 Miller JM, et al. Guide to utilization of the microbiology laboratory for… 2024.}

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.^{1,21 Clinical and Laboratory Standards Institute. Principles and Procedures for Blood Cultures. 2022.2 Miller JM, et al. Guide to utilization of the microbiology laboratory for… 2024.}

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 contain 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.^{11 Clinical and Laboratory Standards Institute. Principles and Procedures for Blood Cultures. 2022.}

Follow the incubation period validated for your bottle and instrument combination. A routine period of about 5 days recovers most bacteria and yeasts, including the HACEK organisms (Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, and Kingella) and Brucella species on a validated routine protocol without extended incubation. Suspected mycobacteremia, selected fungi, Cutibacterium acnes, and other unusual organisms may require a different bottle, system, or incubation plan.^{1,21 Clinical and Laboratory Standards Institute. Principles and Procedures for Blood Cultures. 2022.2 Miller JM, et al. Guide to utilization of the microbiology laboratory for… 2024.} When brucellosis is clinically suspected, current American Society for Microbiology and Association of Public Health Laboratories sentinel-laboratory guidance extends incubation in automated systems to about 10 days. Consider a blind or terminal subculture when brucellosis remains suspected after negative bottles.^{44 American Society for Microbiology and… Brucellosis-causing Brucella species (BBS). 2025.} Brucella is a biosafety level 3, laboratory-acquired-infection risk organism. When brucellosis is suspected, follow the laboratory's biosafety and public-health referral procedure.^{1,21 Clinical and Laboratory Standards Institute. Principles and Procedures for Blood Cultures. 2022.2 Miller JM, et al. Guide to utilization of the microbiology laboratory for… 2024.}

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 is 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.^{1,21 Clinical and Laboratory Standards Institute. Principles and Procedures for Blood Cultures. 2022.2 Miller JM, et al. Guide to utilization of the microbiology laboratory for… 2024.}

When a bottle signals positive, work it up in this order:^{1,21 Clinical and Laboratory Standards Institute. Principles and Procedures for Blood Cultures. 2022.2 Miller JM, et al. Guide to utilization of the microbiology laboratory for… 2024.}

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.^{55 Wolk DM, et al. The American Society for Microbiology's evidence-based… 2025.}

A flagged bottle with an initially negative Gram stain requires repeat staining and subculture. Organisms such as Brucella may stain faintly or be sparse. A false instrument signal can also occur without viable growth. When a high-risk organism is suspected, follow the laboratory's biosafety and public-health referral procedure.^{1,21 Clinical and Laboratory Standards Institute. Principles and Procedures for Blood Cultures. 2022.2 Miller JM, et al. Guide to utilization of the microbiology laboratory for… 2024.}

Contamination and clinical significance

The most common interpretive problem in blood culture is a single bottle growing a skin organism, which may represent infection or contamination. 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.^{66 Sautter RL, et al. American Society for Microbiology evidence-based laboratory… 2024.}

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. Report the exact set and bottle pattern, and add a qualified comment that names a possible skin contaminant and calls for clinical correlation.^{3,63 Centers for Disease Control and Prevention. Collect adult blood culture sets. 2026.6 Sautter RL, et al. American Society for Microbiology evidence-based laboratory… 2024.}

The blood-culture contamination rate is a collection quality indicator. Laboratories track the percentage of eligible culture sets classified as contaminated and investigate a rising rate through collection location, collector, and technique data.^{66 Sautter RL, et al. American Society for Microbiology evidence-based laboratory… 2024.}

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.^{77 Carroll KC, et al. Manual of Clinical Microbiology. 2023.}

Catalase testing requires a colony taken without blood-agar carryover because erythrocytes contain catalase and can produce a false-positive reaction.^{77 Carroll KC, et al. Manual of Clinical Microbiology. 2023.}

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 gives a presumptive 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. Perform each procedure as validated for the reagent and the laboratory. Staphylococcus lugdunensis can produce clumping factor and can be slide-coagulase positive and tube-coagulase negative.^{77 Carroll KC, et al. Manual of Clinical Microbiology. 2023.}

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.^{77 Carroll KC, et al. Manual of Clinical Microbiology. 2023.}

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.^{7,12-137 Carroll KC, et al. Manual of Clinical Microbiology. 2023.12 Mitchell BI, et al. An underestimated pathogen. 2025.13 Centers for Disease Control and Prevention. Listeria monocytogenes identification chart.}

Practice