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Gram-positive isolates from blood

16 min

Skills you'll build3 skills
  • Tell S. aureus from S. lugdunensis and other staphylococci in blood
  • Tell streptococci from enterococci and their vancomycin-resistant look-alikes in blood
  • Recognize a catalase-positive blood rod that needs a Listeria workup

Try first

Try first

A blood bottle grows gram-positive cocci in clusters. The colony is catalase positive, and the slide coagulase test is positive. What do you report now?

Keep going to see the reasoning.

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Gram stain and catalase first

The positive bottle's Gram stain determines where identification starts, and catalase separates the gram-positive organisms into groups. Pick the colony for catalase without any blood agar, because red cells contain catalase and give a false-positive reaction.1

Gram stain and colonyCatalaseGroup
Cocci in clustersPositiveStaphylococcus
Cocci in pairs or chainsNegativeStreptococcus, Enterococcus and look-alikes
Club-shaped or palisading rodsUsually positiveCorynebacterium and other coryneforms
Small rods or coccobacilli, narrow beta hemolysisPositiveListeria monocytogenes

Staphylococci

The slide coagulase test detects clumping factor. The tube test detects free coagulase. S. lugdunensis can be slide positive and tube negative, and it is more invasive than most coagulase-negative staphylococci.1 S. aureus in blood is usually significant, even in one set. A validated system gives the final species.1

Catalase-negative cocci

These cocci are grouped by hemolysis, Lancefield group and species identification.1

  • Enterococcus is pyrrolidonyl arylamidase (PYR) positive and bile-esculin positive, and most strains grow in 6.5% NaCl.
  • The S. gallolyticus group is often group D and bile-esculin positive, is PYR negative, and does not grow in 6.5% NaCl. S. gallolyticus subsp. gallolyticus bacteremia is strongly linked to endocarditis and colorectal neoplasia, so the isolate is identified to species.2
  • Leuconostoc and Pediococcus are intrinsically vancomycin resistant. A vancomycin-resistant catalase-negative coccus needs species confirmation before it is reported as an enterococcus.1

Coryneform rods and Listeria

A single set growing a coryneform rod commonly reflects skin contamination.1 Listeria monocytogenes forms small rods or coccobacilli with a narrow zone of beta hemolysis, so it can be mistaken for a diphtheroid or group B streptococcus. The Gram stain shows a rod, and a positive catalase separates it from group B streptococcus. Tumbling motility at room temperature, umbrella motility in semisolid medium and a validated identification system complete the workup.1,3 A Listeria isolate from blood or CSF is significant.1

Follow the bench demonstration

Sources for this step
References
  1. Carroll KC, Pfaller MA, Landry ML, McAdam AJ, Karlowsky JA, Patel R, Pritt BS, eds. Manual of Clinical Microbiology. 13th ed. ASM Press; 2023.
  2. 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
  3. Centers for Disease Control and Prevention. Listeria monocytogenes identification chart. Accessed September 27, 2026. https://reach.cdc.gov/sites/default/files/job-aids-resources/Listeria_monocytogenes_ID_Chart_Branded_508.pdf

Watch one

Read each decision and its explanation.

A blood bottle grows gram-positive cocci in pairs and short chains. The colonies are small and nonhemolytic, and catalase is negative. The susceptibility panel reports a vancomycin MIC of 64 µg/mL. A colleague wants to report vancomycin-resistant Enterococcus (VRE).

What do you do before reporting?

  1. Read the Gram stain and catalase: catalase-negative cocci in pairs and chains fit streptococci, enterococci and their look-alikes.

    Catalase and arrangement place the isolate among the catalase-negative cocci.

  2. Interpret the vancomycin result: it fits VRE and also fits Leuconostoc or Pediococcus.

    An intrinsically resistant organism gives the same high MIC as acquired resistance.

  3. Identify the species: the results so far do not identify it, so send the colony for identification by MALDI-TOF or another validated system.

    A VRE report places the patient on contact precautions, so the species must be correct.

  4. Read the result: MALDI-TOF identifies Leuconostoc mesenteroides.

    The confirmed species determines how the resistance is interpreted.

Report Leuconostoc mesenteroides. Its vancomycin resistance is intrinsic, and the isolate is not reported as VRE.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

Staphylococcus aureus, confirmed by tube coagulase and MALDI-TOF, grows in the aerobic bottle of one of two sets. The other set has no growth. How is it reported?

The contaminant comment is used for skin commensals such as coagulase-negative staphylococci. S. aureus in blood is usually significant.

S. aureus in blood usually represents a clinically important bloodstream pathogen, even in one set. The report gives the pattern and the full workup follows.

A confirmed S. aureus identification is reported when it is ready. Repeat cultures are a clinical decision and do not delay the report.

Hint
  1. Check which blood isolates raise the question of contamination.
  2. Ask whether S. aureus in blood is usually significant.

Review Contamination and clinical significance

Problem 2 of 3 · Hints available

A blood bottle grows gram-positive rods. Which finding calls for a Listeria workup?

This fits a nondiphtherial Corynebacterium, which in one set commonly reflects skin contamination.

This fits Arcanobacterium haemolyticum. Listeria is catalase positive.

Catalase positive with tumbling motility at room temperature is the Listeria pattern. A validated identification system confirms it.

This fits Erysipelothrix rhusiopathiae, which these reactions separate from Listeria.

Hint
  1. Several blood isolates are gram-positive rods like Listeria. Look for the features specific to Listeria.
  2. Check each option's catalase and motility against the Listeria pattern.

Review Coryneform rods and Listeria

Problem 3 of 3 · On your own

A blood isolate is a catalase-negative coccus in chains. It is nonhemolytic, group D and bile-esculin positive, PYR negative, and does not grow in 6.5% NaCl. A colleague suggests Enterococcus. Which group fits?

Enterococcus is PYR positive, and most strains grow in 6.5% NaCl. This isolate is negative for both.

Group D, bile-esculin positive, PYR negative and no growth in salt is the S. gallolyticus pattern. A validated system identifies the species and subspecies, which are important because of their links to endocarditis and colorectal neoplasia.

A group D antigen with a positive bile-esculin reaction and no growth in salt suggests the S. gallolyticus group. Reporting it as a contaminant would dismiss a finding linked to endocarditis.

S. pneumoniae is alpha hemolytic and forms lancet-shaped diplococci. This isolate is nonhemolytic, in chains and group D.

Review Streptococcus and Enterococcus from blood

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Priya Pelletier, 31, is 30 weeks pregnant with fever and muscle aches.
  • Two blood-culture sets were drawn, and both aerobic bottles signal positive at 18 hours.
  • The Gram stain shows small gram-positive rods and coccobacilli.
  • On blood agar the colonies are small with a narrow zone of beta hemolysis.
Decision 1 of 2

A colleague reads the plate as group B streptococcus. Which test distinguishes the two first?

Group B streptococcus is catalase negative, and Listeria is catalase positive. Picking the colony without blood agar avoids a false positive from red cells.

The latex test detects one antigen. The Gram stain already shows rods, and catalase separates the two immediately.

CAMP supports a group B streptococcus identification. It does not explain the rods seen on the Gram stain.

Review Coryneform rods and Listeria

Decision 2 of 2

The colony is catalase positive and shows tumbling motility at room temperature. How do you handle it?

Catalase positive with tumbling motility is the Listeria pattern, and it grew in both sets. A contaminant comment could delay care.

Dismissed Listeria as a diphtheroid contaminant

Listeria monocytogenes forms small rods or coccobacilli with narrow beta hemolysis that can pass for a diphtheroid or group B streptococcus. A catalase-positive rod with tumbling motility at room temperature points to Listeria, and a blood or CSF isolate is significant.

The pattern suggests Listeria monocytogenes, which is significant in blood. The preliminary report states that Listeria is suspected, and a validated system then confirms the species.

Both bottles are positive now, and the pattern already suggests a significant organism. Waiting delays results the care team needs.

Review Coryneform rods and Listeria

The key result is the positive catalase on a small rod with narrow beta hemolysis. It separates Listeria from group B streptococcus, and tumbling motility separates it from a diphtheroid. MALDI-TOF confirmed L. monocytogenes from both sets.

For reference

Sources checked