Staph vs Strep | Bone Wizardry

The Staph Family · Coagulase Sorts Them

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You know it's Staph (catalase positive, clusters). Now what? The next test is coagulase.

🔥 CoagulaseCoagulase converts fibrinogen → fibrin, forming a clot around the bacteria. This is a virulence factor · the clot protects the bacteria from phagocytosis. Only S. aureus is coagulase positive. It literally shields itself with your own clotting system. positive? That's S. aureus · the only coagulase-positive Staph. Everything else is CoNSCoagulase-Negative Staphylococci. The main two are S. epidermidis and S. saprophyticus. Less virulent than aureus, but still clinically important. (coagulase-negative staph).

Meet the family:

Organism	Coagulase	Key Feature	Diseases
S. aureus	POSITIVE	Golden pigment, most virulent	Skin infections, endocarditis, osteomyelitis, food poisoning, MRSA
S. epidermidis	Negative	Biofilm on prosthetics, novobiocin sensitiveThis is how you tell epidermidis from saprophyticus. Epidermidis is SENSITIVE to novobiocin (killed by it). Saprophyticus is RESISTANT. This is a pure board fact · novobiocin is rarely used clinically.	Prosthetic valve endocarditis, catheter infections, joint replacements
S. saprophyticus	Negative	Novobiocin RESISTANT	UTIs in young sexually active women (2nd after E. coli)

🔑 S. aureus = gold standard of infection. Aureus = Au (gold on the periodic table). Golden color, golden virulence · it does everything and does it aggressively.

⚠ Board Trap: S. epidermidis vs S. saprophyticus

Both are coagulase-negative. Both are low-virulence. The board question will give you a young woman with a UTI and a coagulase-negative staph · that's S. saprophyticus. Or it'll give you a prosthetic valve infection with a coagulase-negative staph · that's S. epidermidis.

Lab differentiator: Novobiocin. Epidermidis = Sensitive. Saprophyticus = Resistant.
Mnemonic: Saprophyticus = Resistant. Epidermidis = Sensitive. Short form: SR, ES. The UTI one (Saprophyticus) resists.

A 22-year-old sexually active woman has dysuria and frequency. Urine culture grows coagulase-negative, novobiocin-resistant gram-positive cocci in clusters. What's the organism?

The Strep Family · Hemolysis Sorts Them

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Catalase negative, chains on Gram stain. It's Strep. Now you look at hemolysis pattern on a blood agar plate.

💡 Three patterns: Beta (complete lysis · clear zone), Alpha (partial lysis · green zone), Gamma (no lysis · nothing happens).

BETA-hemolytic Strep (complete destruction):

Group A · S. pyogenes

Hemolysis: Beta

Key test: Bacitracin SENSITIVE

Diseases: Pharyngitis, scarlet fever, necrotizing fasciitis"Flesh-eating bacteria." S. pyogenes produces exotoxins and proteases that destroy tissue rapidly. Surgical emergency · antibiotics alone won't save the patient., rheumatic feverAutoimmune reaction weeks after strep pharyngitis. Anti-streptococcal antibodies cross-react with cardiac tissue (molecular mimicry). Causes pancarditis. Jones criteria for diagnosis. This does NOT happen after skin infections · only pharyngitis., post-strep glomerulonephritisImmune complex deposition in glomeruli 1-3 weeks after strep infection (pharyngitis OR skin). Type III hypersensitivity. Hematuria, proteinuria, edema, hypertension. Unlike rheumatic fever, PSGN CAN follow skin infections.

Group B · S. agalactiae

Hemolysis: Beta

Key test: Bacitracin RESISTANT

Diseases: Neonatal meningitis/sepsisGBS colonizes the vaginal canal. During delivery, the baby gets exposed. Neonates have immature immune systems → meningitis, pneumonia, sepsis. This is why we screen pregnant women at 35-37 weeks and give intrapartum penicillin if positive., pregnant women screened at 35-37 weeks

⚠ Board Trap: Group A vs Group B · Both are Beta!

Both are beta-hemolytic. The board WILL try to confuse you. The differentiator is bacitracin.

• Group A = bacitracin sensitive (A is Affected by bacitracin)
• Group B = bacitracin resistant (B says "Bye, that doesn't work on me")

Clinical context matters too: pharyngitis + scarlet fever = A. Neonatal sepsis + pregnant woman = B.

ALPHA-hemolytic Strep (partial · green zone):

S. pneumoniae

Hemolysis: Alpha (green zone)

Key test: Optochin SENSITIVE

Morphology: Lancet-shaped diplococci

Other: Quellung reaction positiveAdd anticapsular antibodies to the bacteria. If it has a capsule (pneumococcus does), the capsule swells visibly under microscopy. "Quellung" = German for "swelling." The capsule is its main virulence factor · prevents phagocytosis., bile soluble

Diseases: #1 bacterial meningitis (adults), #1 bacterial pneumonia, otitis media, sinusitis

Viridans Group Strep

Hemolysis: Alpha (green zone)

Key test: Optochin RESISTANT

Morphology: No special shape

Other: Normal dental flora, bile insoluble

Diseases: Subacute endocarditisViridans strep are dental flora. After dental procedures, they enter the bloodstream and settle on PREVIOUSLY DAMAGED valves (rheumatic, prosthetic). Subacute onset over weeks-months with low-grade fever, unlike the acute fulminant endocarditis caused by S. aureus on NORMAL valves. (damaged valves)

GAMMA-hemolytic (no hemolysis):

💡 Enterococcus (formerly Group D Strep) · gamma-hemolytic (no hemolysis). Grows in 6.5% NaCl and is bile esculin positiveEnterococcus can hydrolyze esculin in the presence of bile (40% bile). This produces a black precipitate on the agar. Group D strep (including Enterococcus) are bile esculin positive, but only Enterococcus grows in 6.5% NaCl. That's the final differentiator.. Important cause of UTIs, biliary infections, and VREVancomycin-Resistant Enterococcus. A major nosocomial pathogen. Treat with linezolid or daptomycin. VRE modified the D-Ala-D-Ala peptidoglycan target to D-Ala-D-Lac, so vancomycin can't bind..

An alpha-hemolytic organism is isolated. You run an optochin sensitivity test. It's RESISTANT. What is the organism?

Sort Them · Drag Each Organism Home

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Drag each organism into the correct category. No peeking back up.

Round 1: Catalase · Staph or Strep?

Staph (Catalase +)

Strep (Catalase −)

The Decision Tree · From Gram Stain to Organism

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This is the algorithm. You see gram-positive cocci. Walk through each branch. At each fork, you'll be quizzed before the path reveals.

Gram stain shows gram-positive cocci. You see them arranged in clusters. What test do you run first?

Catalase first. Always.

Clusters suggest Staph, but you confirm with catalase. Coagulase comes AFTER you've established it's Staph. Optochin is for differentiating alpha-hemolytic Strep · way too early.

Catalase is POSITIVE (bubbles). It's Staph. What test do you run next?

Coagulase. This is how you find S. aureus.

Bacitracin is for differentiating beta-hemolytic Strep (Group A vs B). Bile esculin is for Enterococcus. Coagulase splits the Staph family: positive = S. aureus, negative = CoNS.

Coagulase is NEGATIVE. It's CoNS. The patient is a 21-year-old woman with a UTI. What test differentiates S. epidermidis from S. saprophyticus?

Novobiocin. S. saprophyticus is resistant, S. epidermidis is sensitive.

In a young woman with a UTI + coagulase-negative staph, you're already thinking saprophyticus. But the lab confirmation is novobiocin resistance. Optochin differentiates pneumococcus from viridans (alpha strep, not staph). Bile solubility also goes with pneumococcus.

Now a different scenario: gram-positive cocci in CHAINS, catalase NEGATIVE. You plate it on blood agar and see complete clearing around the colonies (clear zone). What hemolysis pattern is this?

Beta. Complete clearing = complete lysis of red blood cells.

Beta = bloody destruction (clear zone). Alpha = almost (green zone, partial). Gamma = gone · no hemolysis at all.

Beta-hemolytic strep. What test differentiates Group A from Group B?

Bacitracin. Group A (S. pyogenes) is sensitive. Group B (S. agalactiae) is resistant.

Optochin = alpha-hemolytic strep (pneumo vs viridans). Novobiocin = coagulase-negative staph (epidermidis vs saprophyticus). Each test belongs to a specific branch of the tree. Don't cross the streams.

New scenario: alpha-hemolytic strep (green zone). What test differentiates S. pneumoniae from viridans group?

Optochin. Pneumococcus is sensitive. Viridans is resistant.

You can also use bile solubility (pneumococcus dissolves in bile, viridans doesn't). But optochin is the classic board answer. Remember: OPTochin = Pneumo is Sensitive.

The Full Algorithm (reference):

Gram-positive cocci

↓ Catalase test

Catalase +
STAPH (clusters)

Catalase −
STREP (chains)

STAPH branch
Coagulase test

↓

Coag +
S. aureus

Coag − (CoNS)
Novobiocin test

CoNS + Novobiocin

↓

Sensitive
S. epidermidis

Resistant
S. saprophyticus

STREP branch
Hemolysis pattern

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Beta
Bacitracin

Alpha
Optochin

Gamma
Bile esculin + NaCl

Bacitracin S
GAS (pyogenes)

Bacitracin R
GBS (agalactiae)

Optochin S
S. pneumoniae

Optochin R
Viridans

Bile esc +, NaCl +
Enterococcus

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Decision Tree: Gram-Positive Cocci: Staph or Strep?

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Pick the arrangement on Gram stain. Follow the branch to organism and treatment.

Clusters or chains on Gram stain?

The Cheat Sheet · All 8 Organisms

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Everything on one table. Scroll sideways on mobile.

Organism	Catalase	Key Test	Hallmark	Top Disease
S. aureus	+	Coagulase +	Golden pigment, protein A	Skin, endocarditis, osteo, MRSA
S. epidermidis	+	Novobiocin S	Biofilm on prosthetics	Prosthetic valve/joint infections
S. saprophyticus	+	Novobiocin R	UTI bug in young women	UTI (#2 after E. coli)
S. pyogenes (GAS)	−	Bacitracin S	Beta-hemolysis	Pharyngitis, rheumatic fever, NF
S. agalactiae (GBS)	−	Bacitracin R	Beta-hemolysis	Neonatal meningitis/sepsis
S. pneumoniae	−	Optochin S	Alpha, lancet diplococci	Meningitis, pneumonia (#1 both)
Viridans group	−	Optochin R	Alpha, dental flora	Subacute endocarditis (damaged valves)
Enterococcus	−	Bile esculin +, 6.5% NaCl	Gamma (no hemolysis)	UTIs, biliary, VRE

💡 Test-to-branch cheat: Coagulase = Staph. Bacitracin = Beta Strep. Optochin = Alpha Strep. Novobiocin = CoNS. Bile esculin + NaCl = Enterococcus. Each test lives on ONE branch. Don't cross them.

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The Rogue's Gallery · Tap to Flip

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Tap any card to reveal the dossier. Tap again to flip back.

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S. aureus

The Golden Menace

S. aureus

ID: Clusters, catalase +, coagulase +
Toxins: Protein A, TSST-1, PVL (Panton-Valentine leukocidin)
Diseases: Skin abscesses, endocarditis, osteomyelitis, food poisoning, SSSS, TSS
MRSA: Altered PBP2a; treat with vancomycin or linezolid
Hook: IVDU + acute endocarditis + tricuspid valve

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S. epidermidis

The Biofilm Builder

S. epidermidis

ID: Clusters, catalase +, coagulase -, novobiocin SENSITIVE
Virulence: Slime-layer biofilm on prosthetic material
Diseases: Prosthetic valve endocarditis, catheter BSI, prosthetic joint infection
Key: Low virulence; dangerous only on implanted foreign bodies
Hook: Coagulase-negative staph on a prosthetic valve = epidermidis

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S. pyogenes

Group A · The Flesh Eater

S. pyogenes (GAS)

ID: Chains, catalase -, beta-hemolytic, bacitracin SENSITIVE
Toxins: Streptolysin O/S, erythrogenic toxin, M protein
Diseases: Pharyngitis, scarlet fever, impetigo, necrotizing fasciitis, rheumatic fever, PSGN
Key: ASO titer confirms post-strep complications
Hook: Rheumatic fever only after PHARYNGITIS, not skin infection

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S. agalactiae

Group B · The Neonatal Threat

S. agalactiae (GBS)

ID: Chains, catalase -, beta-hemolytic, bacitracin RESISTANT
Tests: CAMP test positive; hippurate hydrolysis positive
Diseases: Neonatal meningitis, sepsis, pneumonia; UTIs in pregnant women
Prevention: Screen at 35-37 weeks; intrapartum ampicillin if positive
Hook: Neonate + beta-hemolytic strep + bacitracin resistant = GBS

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S. pneumoniae

King of Upper Resp

S. pneumoniae

ID: Lancet-shaped diplococci, alpha-hemolytic, optochin SENSITIVE, bile soluble
Special: Quellung reaction +, polysaccharide capsule = major virulence factor
Diseases: #1 bacterial pneumonia, #1 adult bacterial meningitis, otitis media
Risk groups: Sickle cell, asplenia, alcoholics, elderly
Hook: Post-splenectomy + encapsulated organism = pneumococcus first

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Enterococcus

The VRE Threat

Enterococcus

ID: Pairs/short chains, catalase -, gamma-hemolytic, grows in 6.5% NaCl, bile esculin +
Diseases: UTIs, biliary infections, nosocomial endocarditis
Resistance: VRE: D-Ala-D-Lac substitution blocks vancomycin; treat with linezolid or daptomycin
Intrinsic: Resistant to cephalosporins and clindamycin
Hook: Elderly man after prostate surgery + gram-positive UTI = Enterococcus

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Clinical Images · What They Actually Look Like

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Tap any image to expand. Images via Wikimedia Commons.

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Quiz · 4 Lab Results, 4 Organisms

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5 patients just showed up with infections. The micro lab sent results. Match the bug to the patient before rounds start.