Staph vs strep: catalase, coagulase and haemolysis
Two tests do most of the work in staph vs strep identification. Catalase separates the genera, then each genus has its own fork: coagulase and novobiocin, or haemolysis and a susceptibility disc.
Use catalase, coagulase, haemolysis pattern and key susceptibility tests to identify staphylococci and streptococci.
The short version
- Catalase splits the genera: bubbles in hydrogen peroxide mean Staphylococcus, no bubbles mean Streptococcus.
- For staphylococci, coagulase separates S. aureus (positive) from the coagulase-negative species, and novobiocin separates S. epidermidis (sensitive) from S. saprophyticus (resistant).
- For streptococci, read the haemolysis first: beta is complete clearing, alpha is partial and greenish, gamma is none.
- Among beta-haemolytic streptococci, bacitracin sorts group A (sensitive) from group B (resistant); among alpha-haemolytic ones, optochin sorts S. pneumoniae (sensitive) from viridans streptococci (resistant).
- A simple order: Gram stain, then catalase, then coagulase for staphylococci or haemolysis for streptococci.
Staph vs strep: clusters, chains and the catalase test
A blood culture bottle flags positive and the Gram stain shows purple cocci. The lab has hydrogen peroxide and a few agar plates, but no sequencer. Can you name the organism in a few steps? That is the job of staph vs strep lab identification.
Both groups are gram-positive cocci. Staphylococci sit in clusters; streptococci form chains or pairs.[7]
The plan for this lesson is to sort staphylococci and streptococci with catalase, coagulase, haemolysis and a few susceptibility tests. Each test removes one branch of the possibilities, so the identification is a sequence of short decisions rather than one big recall task.
Step one is the catalase test. Touch a colony to hydrogen peroxide. Catalase is an enzyme that splits hydrogen peroxide into water and oxygen, so the bubbles are oxygen gas.[1][7]
Bubbles are O₂ gas: catalase positive
Bubbles mean catalase positive, which points to Staphylococcus. No bubbles points to Streptococcus.[1][7]
That one result decides which fork you take next. Catalase positive sends you to the staphylococcal tests (coagulase, then novobiocin). Catalase negative sends you to the streptococcal tests (haemolysis, then a disc).
Staphylococci: the coagulase test and mannitol salt agar
For a catalase-positive coccus, step two is the coagulase test. Coagulase converts fibrinogen to fibrin, so in a positive test the plasma clots.[1][7]
Coagulase positive points to Staphylococcus aureus, which also ferments mannitol, giving yellow colonies on mannitol salt agar.[1][7]
Coagulase negative means a coagulase-negative staphylococcus. Two of these matter for the exam.
Mannitol salt agar gives a second clue. S. aureus colonies ferment mannitol and turn yellow, with yellow zones around them, while most coagulase-negative staphylococci form small pink or red colonies. Coagulase, not mannitol, is the deciding test.[7]
| Organism | Novobiocin | Note |
|---|---|---|
| Staphylococcus epidermidis | Sensitive | |
| Staphylococcus saprophyticus | Resistant | A common cause of community-acquired urinary infection |
Novobiocin is the test that separates the two.[2][7]
In short: coagulase positive is S. aureus. Coagulase negative is one of the coagulase-negative staphylococci, and novobiocin names which one. Sensitive is S. epidermidis; resistant is S. saprophyticus.
Streptococci: haemolysis on blood agar
Streptococci are catalase negative. Step two is the haemolysis pattern on blood agar.[4][7]
| Pattern | What the plate shows |
|---|---|
| Beta | Complete clearing around the colony |
| Alpha | Partial clearing, with a greenish zone |
| Gamma | No change: no haemolysis |
Lancefield grouping names streptococci by a carbohydrate antigen in the cell wall, which is why we say group A and group B.[7]
So the sequence for a catalase-negative coccus is: read the haemolysis, then choose the disc. Bacitracin is used for the beta-haemolytic group, and optochin for the alpha-haemolytic group. The gamma group is handled by different tests, covered below.
Beta-haemolytic streptococci: bacitracin, PYR and CAMP
Take the beta-haemolytic ones first. A bacitracin disc sorts them.[7][8]
| Group and species | Bacitracin | Other tests |
|---|---|---|
| Group A, Streptococcus pyogenes (strep throat) | Sensitive | PYR positive |
| Group B, Streptococcus agalactiae | Resistant | CAMP test positive; hydrolyses hippurate |
The Lancefield groups are the labels behind the names: group A is S. pyogenes and group B is S. agalactiae. Remember the pairing of letter, species and bacitracin result: A is sensitive, B is resistant.[7][8]
Alpha-haemolytic and gamma streptococci: optochin, bile and bile esculin
Now the alpha-haemolytic ones. An optochin disc sorts them. Streptococcus pneumoniae is optochin sensitive, and its colonies dissolve in bile. The viridans streptococci are optochin resistant.[3][7]
| Organism | Optochin | Bile |
|---|---|---|
| Streptococcus pneumoniae | Sensitive | Colonies dissolve (bile soluble) |
| Viridans streptococci | Resistant | Do not dissolve (bile insoluble) |
Last, the non-haemolytic, or gamma, group. By exam convention enterococci are placed here, although their haemolysis can vary. Enterococci are bile esculin positive and grow in 6.5% sodium chloride.[1][5]
Enterococci are therefore identified by growth conditions and bile esculin rather than by a haemolysis disc.[5]
The order to remember for staph vs strep identification
A simple order to remember: Gram stain, then catalase, then coagulase for staphylococci or haemolysis for streptococci.
- Gram stainPurple cocci: clusters point to staphylococci, chains or pairs to streptococci.
- CatalaseBubbles: Staphylococcus. No bubbles: Streptococcus.
- Staphylococci: coagulase, then novobiocinCoagulase positive is S. aureus. Coagulase negative: sensitive is S. epidermidis, resistant is S. saprophyticus.
- Streptococci: haemolysis, then a discBeta: bacitracin (sensitive is group A, resistant is group B). Alpha: optochin (sensitive is S. pneumoniae, resistant is viridans). Gamma: bile esculin and 6.5% NaCl for enterococci.
Three things to keep. One: catalase splits the genera; coagulase and novobiocin split the staphylococci. Two: for streptococci, read the haemolysis, then bacitracin for beta, and optochin for alpha. Three: group B is bacitracin resistant and CAMP positive; pneumococcus is optochin sensitive.
Worked isolates: running the tests in order
- Cocci in clusters: Staphylococcus likely
- Catalase positive: Staphylococcus
- Coagulase negative: a coagulase-negative staphylococcus
- Resistant to novobiocin: Staphylococcus saprophyticus
That is Staphylococcus saprophyticus, a common cause of community-acquired urinary infection.[2][7]
- Gram-positive cocci in chains: Streptococcus likely
- Catalase negative: Streptococcus
- Beta haemolytic on blood agar
- Clear zone of inhibition around the bacitracin disc: sensitive
Chains point to Streptococcus, and beta haemolysis plus bacitracin sensitivity gives group A, Streptococcus pyogenes.[7][8]
Practice question
Original · USMLE styleA 3-day-old infant is febrile and lethargic. A blood culture grows gram-positive cocci in chains. On sheep blood agar the colonies show a narrow zone of complete haemolysis. The catalase test is negative, and there is no zone of inhibition around a bacitracin disc.
Common questions
How do you tell staph from strep in the lab?
Use the catalase test. Staphylococci are catalase positive (bubbles in hydrogen peroxide) and streptococci are catalase negative.
How do you tell Staphylococcus aureus from other staphylococci?
S. aureus is coagulase positive and ferments mannitol, giving yellow colonies on mannitol salt agar. Coagulase-negative staphylococci do not clot plasma.
What separates Staphylococcus epidermidis from Staphylococcus saprophyticus?
Novobiocin. S. epidermidis is sensitive and S. saprophyticus is resistant; the latter is a common cause of community-acquired urinary infection.
What do alpha, beta and gamma haemolysis mean?
Beta is complete clearing around the colony, alpha is partial clearing with a greenish zone, and gamma is no haemolysis.
How do you separate Streptococcus pneumoniae from viridans streptococci?
Both are alpha haemolytic. S. pneumoniae is optochin sensitive and its colonies dissolve in bile; the viridans streptococci are optochin resistant.
Transcript
Read the full narration, by chapter609 words
0:00A positive blood culture and today's plan
A blood culture bottle flags positive, and the Gram stain shows purple cocci. The clinic is waiting, and your lab has no sequencer, only a drop of hydrogen peroxide and a few agar plates. Can you name the organism in a few steps? Today: sort staphylococci and streptococci using catalase, coagulase, haemolysis and a few susceptibility tests.
0:23Clusters vs chains, then the catalase test
Both are gram-positive cocci. Staphylococci sit in clusters; streptococci form chains or pairs. Step one is the catalase test. Touch a colony to hydrogen peroxide. Catalase is an enzyme that splits hydrogen peroxide into water and oxygen, so the bubbles are oxygen gas. Bubbles mean catalase positive, which points to Staphylococcus. No bubbles points to Streptococcus.
0:52Staphylococci: coagulase and mannitol salt agar
Now the staphylococci. Step two is the coagulase test. Coagulase converts fibrinogen to fibrin, so in a positive test the plasma clots. Coagulase positive points to Staphylococcus aureus, which also ferments mannitol, giving yellow colonies on mannitol salt agar. Coagulase negative means a coagulase-negative staphylococcus, and two of these matter for the exam.
1:20Coagulase-negative staph: novobiocin
Staphylococcus epidermidis is sensitive to novobiocin. Staphylococcus saprophyticus is resistant, and it is a common cause of community-acquired urinary infection.
1:31Streptococci: haemolysis on blood agar
Now the streptococci, which are catalase negative. Step two is the haemolysis pattern on blood agar. Beta haemolysis is complete clearing around the colony. Alpha is partial, with a greenish zone. Gamma is none.
1:47Lancefield groups and beta haemolysis: bacitracin, PYR, CAMP
Lancefield grouping names streptococci by a carbohydrate antigen in the cell wall, which is why we say group A and group B. Take the beta haemolytic ones first. A bacitracin disc sorts them. Group A, Streptococcus pyogenes, the cause of strep throat, is sensitive to bacitracin, and its PYR test is positive. Group B, Streptococcus agalactiae, is resistant, and it is CAMP test positive and hydrolyses hippurate. Now the alpha haemolytic ones.
2:19Alpha haemolysis: optochin and bile solubility
An optochin disc sorts them. Streptococcus pneumoniae is optochin sensitive, and its colonies dissolve in bile. The viridans streptococci are optochin resistant.
2:33Enterococci, the full flowchart and two isolates
Last, the non-haemolytic, or gamma, group. Enterococci are bile esculin positive and grow in 6.5% sodium chloride. Let us run two isolates. First: cocci in clusters, catalase positive, coagulase negative, resistant to novobiocin. That is Staphylococcus saprophyticus, the likely cause of a community-acquired urinary infection. Second: gram-positive cocci in chains, catalase negative, beta haemolytic. A bacitracin disc shows a clear zone of inhibition around it. Chains point to Streptococcus, and beta haemolysis plus bacitracin sensitivity gives group A, Streptococcus pyogenes.
3:21Practice question
Your turn. A three-day-old infant is febrile and lethargic. Blood culture grows gram-positive cocci in chains. On sheep blood agar the colonies show a narrow zone of complete haemolysis. The catalase test is negative, and there is no zone of inhibition around a bacitracin disc. Which of the following organisms is the most likely cause? A, Streptococcus agalactiae. B, Staphylococcus aureus. C, Staphylococcus epidermidis. D, Streptococcus pneumoniae. E, Streptococcus pyogenes. Pause here and decide.
4:04Answer and explanation
B is out: Staphylococcus aureus is catalase positive and grows in clusters. C is out: Staphylococcus epidermidis is also catalase positive, and it is not haemolytic. D is out: Streptococcus pneumoniae is alpha haemolytic and optochin sensitive. E is out: Streptococcus pyogenes is beta haemolytic, but it is sensitive to bacitracin. So the answer is A. Catalase negative chains with beta haemolysis and bacitracin resistance are group B, Streptococcus agalactiae.
4:42Recap and next lesson
Three things to keep. One: catalase splits the genera; coagulase and novobiocin split the staphylococci. Two: for streptococci, read the haemolysis, then bacitracin for beta, and optochin for alpha. Three: group B is bacitracin resistant and CAMP positive; pneumococcus is optochin sensitive. A simple order to remember: Gram stain, then catalase, then coagulase for staphylococci or haemolysis for streptococci. Next, why people without a spleen are so vulnerable to encapsulated bacteria.
Sources
Numbers in the text, e.g. [1], point here- Europe PMC, Evolving approaches to bacterial identification (PMC13256776): europepmc.org/article/PMC/PMC13256776
- Europe PMC, coagulase-negative staphylococci classification (PMC13669825): europepmc.org/article/PMC/PMC13669825
- Europe PMC, optochin and bile solubility for S. pneumoniae (PMC5403922): europepmc.org/article/PMC/PMC5403922
- Europe PMC, alpha, beta and gamma haemolysis on sheep blood agar (PMC10569484): europepmc.org/article/PMC/PMC10569484
- Europe PMC, Enterococcus identification (PMC12744406; PMC13707596): europepmc.org/article/PMC/PMC12744406 · europepmc.org/article/PMC/PMC13707596
- Europe PMC, group B streptococcus and early-onset neonatal disease (PMC5850457): europepmc.org/article/PMC/PMC5850457
- Wikipedia: Catalase, Coagulase, Mannitol salt agar, Hemolysis (microbiology), Staphylococcus epidermidis, Staphylococcus saprophyticus, Streptococcus pyogenes, Streptococcus agalactiae, Viridans streptococci ( and the pages named) en.wikipedia.org/wiki/Streptococcus_pyogenes
- Microbe Notes, Bacitracin susceptibility test: microbenotes.com/bacitracin-susceptibility-test/
The plates, tubes and flowchart on screen are schematic teaching drawings, not photographs.
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined): Petri dish. NIAID NIH BioArt Source (bioart.niaid.nih.gov/bioart/773), public domain: Staphylococcus aureus. Health Icons (healthicons.org), CC0: bladder. The blood culture bottle, Gram stain field, tubes, discs, zones, CAMP streaks, the flowchart, question card, pocket card and all handwriting are drawn in code (original). No AI-generated anatomy, labels or graphs.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-MICRO-004.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01Gram-positive vs gram-negative cell walls5:39
- EP 02Acid-fast stain and the mycobacterial cell wall5:01
- EP 03Endotoxin vs exotoxin: lipid A, A-B toxins and toxoids5:46
- EP 04Bacterial toxins that raise cAMP: cholera, pertussis, anthrax5:54
- EP 05Clostridial toxins compared: tetanus, botulinum, alpha, C. difficile5:22
- EP 06Staph vs strep: catalase, coagulase and haemolysis5:23
- EP 07Encapsulated organisms and the spleen: asplenia and conjugate vaccines5:08
- EP 08Bacterial genetics: how bacteria swap genes5:13
- EP 09Beta-lactamase and antibiotic resistance: three ways bacteria win5:13
- EP 10DNA vs RNA virus replication: which enzyme each genome needs5:48
- EP 11HIV life cycle and drug targets, step by step5:39
- EP 12Hepatitis B serology: reading the panel5:17
- EP 13Antifungal drugs explained: mechanisms of action5:26
- EP 14Bacterial meningitis organisms by age and risk group5:19