Free ASCP MLS Exam Practice Questions Mock Test: Part 23 – Microbiology – Gram-Positive Cocci Identification

1. • Gram-positive coccus → Suggests Streptococcus or Enterococcus.

   • Catalase-negative → Rules out Staphylococcus (all staphylococci are catalase-positive).

   • Beta-hemolytic → Complete lysis of RBCs (clear zones on blood agar).

   • Bacitracin-resistant → Differentiates S. agalactiae (Group B Strep) from S. pyogenes (Group A Strep, which is bacitracin-sensitive).

2. Clinical Context:

   • Streptococcus agalactiae (Group B Strep) is a leading cause of neonatal sepsis/meningitis and UTIs in pregnant women.

Why Not the Others?

• a) Streptococcus pyogenes → Beta-hemolytic but bacitracin-sensitive.

• c) Streptococcus pneumoniae → Alpha-hemolytic, optochin-sensitive.

• d) Enterococcus faecalis → Gamma-hemolytic, PYR-positive.

1. Oxidase-negative – Rules out Pseudomonas and other gram-negative bacteria; staphylococci are oxidase-negative.

2. Urease-positive – S. epidermidis can produce urease (though variable, unlike S. saprophyticus, which is consistently positive).

3. Novobiocin-sensitive – S. epidermidis is sensitive, while S. saprophyticus is resistant.

Why Not the Others?

• b) Staphylococcus saprophyticus – Urease-positive but novobiocin-resistant (key difference).

• c) Enterococcus faecalis – Gram-positive coccus but catalase-negative (staphylococci are catalase-positive) and urease-negative.

• d) Streptococcus pneumoniae – Catalase-negative, alpha-hemolytic, and urease-negative.

1. Key Features of Enterococci (e.g., Enterococcus faecalis):

   • Bile esculin positive → Hydrolyzes esculin in bile-containing medium (agar turns black).

   • Growth in 6.5% NaCl → Tolerates high salt concentration (unlike most streptococci).

   • PYR test positive → Additional confirmatory test.

2. Why Not the Others?

   • a) Catalase positivity → Enterococci are catalase-negative (staphylococci are positive).

   • c) Optochin sensitivity → Used for Streptococcus pneumoniae, not enterococci.

   • d) Mannitol fermentation → Used for Staphylococcus aureus (mannitol salt agar), not enterococci.

1. Key Features of the Organism:

   • Gram-positive coccus → Suggests Staphylococcus, Streptococcus, or Enterococcus.

   • Alpha-hemolytic → Produces partial (green) hemolysis on blood agar.

   • Optochin-resistant → Rules out Streptococcus pneumoniae (which is optochin-sensitive).

2. Viridans Streptococci (e.g., S. mutans, S. mitis):

   • Alpha-hemolytic and optochin-resistant.

   • Normal flora of the oral cavity; associated with dental caries and subacute endocarditis.

Why Not the Others?

• a) Streptococcus pneumoniae → Optochin-sensitive (would be killed by optochin disk).

• c) Streptococcus pyogenes → Beta-hemolytic, not alpha-hemolytic.

• d) Enterococcus faecalis → Usually gamma-hemolytic (no hemolysis).

1. Key Features of Staphylococcus aureus:

   • Gram-positive coccus, catalase-positive, coagulase-positive.

   • Produces heat-stable enterotoxins (e.g., SEA-SEE) that cause rapid-onset food poisoning (1-6 hours after ingestion).

   • Common in contaminated dairy, meats, and salads (improper storage allows toxin production).

2. Why Not the Others?

   • b) Streptococcus pyogenes → Causes pharyngitis/cellulitis, not food poisoning.

   • c) Enterococcus faecalis → Part of normal gut flora; rarely causes foodborne illness.

   • d) Streptococcus pneumoniae → Causes pneumonia/meningitis, no enterotoxin production.

1. Key Features of Staphylococcus aureus:

   • Coagulase-positive → Differentiates S. aureus from other staphylococci (e.g., S. epidermidis, which is coagulase-negative).

   • DNase-positive → Produces deoxyribonuclease (DNase), which hydrolyzes DNA (clears agar around colonies on DNase test plates).

   • Other traits: Catalase-positive, beta-hemolytic, protein A-positive.

2. Why Not the Others?

   • a) Staphylococcus epidermidis → Coagulase-negative and DNase-negative.

   • c) Streptococcus pyogenes → Catalase-negative (staphylococci are catalase-positive).

   • d) Enterococcus faecalis → Catalase-negative, DNase-negative.

1. Toxic Shock Syndrome (TSS):

   • Staphylococcus aureus is the primary cause of staphylococcal toxic shock syndrome (TSS), mediated by toxic shock syndrome toxin-1 (TSST-1).

   • Associated with tampon use, surgical wounds, or nasal packing.

2. Streptococcal TSS:

   • Streptococcus pyogenes (Group A Strep) can also cause a similar condition (streptococcal TSS), but the question does not specify.

   • However, Staphylococcus aureus is the classic and more common cause of TSS.

Why Not the Others?

• a) Streptococcus pyogenes → Causes streptococcal TSS (less common than staphylococcal TSS).

• c) Enterococcus faecalis → Not associated with TSS.

• d) Streptococcus agalactiae → Causes neonatal sepsis, not TSS.

1. Beta-hemolytic → Rules out S. pneumoniae (alpha-hemolytic) and Enterococcus (gamma-hemolytic).

2. Catalase-negative → Confirms Streptococcus or Enterococcus (rules out Staphylococcus).

3. Bacitracin-resistant → S. agalactiae is resistant, while S. pyogenes (Group A Strep) is sensitive.

4. Gram-positive coccus → Fits all Streptococcus species.

Why Not the Others?

• a) S. pyogenes (Group A Strep, GAS):

  • Beta-hemolytic but bacitracin-sensitive (key difference from GBS).

• c) Enterococcus faecalis:

  • Gamma-hemolytic (non-hemolytic) and PYR-positive; not beta-hemolytic.

• d) S. pneumoniae:

  • Alpha-hemolytic (greenish colonies), optochin-sensitive, bile-soluble.

• Streptococcus bovis (now classified as Streptococcus gallolyticus subspecies) is strongly associated with colorectal cancer and infective endocarditis.

  • Up to 50–60% of patients with S. bovis bacteremia/endocarditis have colonic neoplasms (adenomas or carcinomas).

  • Screening colonoscopy is mandatory in these patients.

Why Not the Others?

• a) Streptococcus pyogenes – Causes pharyngitis, rheumatic fever, or cellulitis; no link to colon cancer.

• c) Staphylococcus aureus – Associated with acute endocarditis (e.g., IV drug users), but not colon cancer.

• d) Enterococcus faecalis – Causes endocarditis (often healthcare-associated) but lacks the specific colon cancer link seen with S. bovis.

1. • Gram-positive coccus, part of the viridans streptococci group.

   • Primary cause of dental caries (tooth decay) due to:

     • Acid production (ferments sugars to lactic acid).

     • Biofilm formation (creates dental plaque by adhering to enamel via glucans).

2. Why Not the Others?

   • b) Streptococcus pyogenes → Causes pharyngitis/impetigo, not oral biofilms.

   • c) Staphylococcus aureus → Catalase-positive; associated with skin infections, not caries.

   • d) Enterococcus faecalis → May cause root canal infections but not enamel caries.

1. Key Features of Enterococcus faecalis:

   • Bile esculin-positive → Hydrolyzes esculin in bile-containing medium (agar turns black).

   • PYR-positive → Produces pyrrolidonyl arylamidase (red color after adding PYR reagent).

   • Grows in 6.5% NaCl (salt-tolerant).

2. Why Not the Others?

   • a) Streptococcus bovis → Bile esculin-positive but PYR-negative.

   • c) Streptococcus pneumoniae → Bile esculin-negative and PYR-negative.

   • d) Staphylococcus aureus → Bile esculin-negative and PYR-negative (also catalase-positive).

• Coagulase-negative staphylococci (CoNS) are catalase-positive but coagulase-negative, distinguishing them from Staphylococcus aureus (coagulase-positive).

• Common CoNS species include:

  • S. epidermidis (most common, associated with biofilms on medical devices).

  • S. saprophyticus (causes UTIs in young women).

  • S. haemolyticus, S. lugdunensis (less common but clinically significant).

Why Not the Others?

• a) Group A Strep – Streptococcus pyogenes; catalase-negative, gram-positive cocci in chains.

• b) Viridans streptococci – Alpha-hemolytic, catalase-negative, part of normal oral flora.

• d) Micrococci – Catalase-positive but not staphylococci; form tetrads, are aerobic, and rarely pathogenic.

1. Bile esculin-positive → Hydrolyzes esculin, producing a black precipitate (shared with Enterococcus and other Group D streptococci).

2. Does not grow in 6.5% NaCl → Distinguishes it from Enterococcus (which grows in high salt).

3. Gram-positive coccus → Forms chains, catalase-negative.

Why Not the Others?

• a) Enterococcus faecalis – Bile esculin (+) and grows in 6.5% NaCl (salt-tolerant).

• c) Streptococcus pneumoniae – Bile esculin-negative, alpha-hemolytic, optochin-sensitive.

• d) Staphylococcus epidermidis – Catalase-positive, coagulase-negative, and does not hydrolyze bile esculin.

• Enterococcus:

  • Bile esculin-positive

  • Grows in 6.5% NaCl

  • PYR-positive

• Streptococcus bovis (Group D strep):

  • Bile esculin-positive

  • Does NOT grow in 6.5% NaCl

  • PYR-negative

Other options:

• Bile esculin hydrolysis → positive in both Enterococcus and S. bovis → cannot differentiate

• Catalase → both are catalase-negative

• PYR → Enterococcus positive, S. bovis negative (also helpful, but 6.5% NaCl growth is the classic differentiator)

• Staphylococcus saprophyticus is a common cause of UTIs in sexually active young women (second only to E. coli in this demographic).

  • It accounts for 10–20% of UTIs in women aged 17–27.

  • Adheres to uroepithelial cells and colonizes the urinary tract.

Why Not the Others?

• a) S. aureus – Rarely causes UTIs; typically associated with bloodstream infections, abscesses, or hospital-acquired infections.

• b) S. epidermidis – A coagulase-negative Staphylococcus (CoNS) that colonizes skin; causes catheter-associated UTIs but not community-acquired UTIs in healthy women.

• d) Enterococcus faecalis – More common in hospitalized patients, elderly, or those with urinary catheters (not the primary cause in young, healthy women).

• Protein A is a unique virulence factor of Staphylococcus aureus, which binds the Fc region of IgG antibodies (evading host immunity).

• The latex agglutination test uses latex beads coated with fibrinogen and IgG to detect:

  1. Protein A (on the bacterial surface).

  2. Clumping factor (bound coagulase).

• A positive test causes agglutination (clumping) of latex particles, confirming S. aureus.

Why Not the Others?

• a) Streptococcus pneumoniae – Identified by optochin sensitivity/bile solubility, not protein A.

• c) Enterococcus faecalis – No protein A; diagnosed via PYR test/bile esculin.

• d) Streptococcus agalactiae – Detected by CAMP test/hippurate hydrolysis, not protein A.

1. Urease-positive – S. saprophyticus produces urease, which hydrolyzes urea (unlike most other coagulase-negative staphylococci).

2. Catalase-positive – Confirms it is Staphylococcus (rules out Streptococcus/Enterococcus).

3. Coagulase-negative – Distinguishes it from S. aureus (coagulase-positive).

4. Novobiocin-resistant – S. saprophyticus is resistant, while S. epidermidis is sensitive.

Why Not the Others?

• a) Staphylococcus epidermidis – Urease-variable (often negative) and novobiocin-sensitive.

• c) Streptococcus bovis – Gram-positive coccus but catalase-negative and urease-negative.

• d) Enterococcus faecalis – Catalase-negative, urease-negative, and does not belong to Staphylococcus

1. Catalase-negative – Rules out Staphylococcus (all staphylococci are catalase-positive).

2. Gamma-hemolytic (non-hemolytic) – Rules out:

   • S. pneumoniae (alpha-hemolytic).

   • S. pyogenes (beta-hemolytic).

3. Gram-positive coccus – Forms pairs/chains.

Why Not the Others?

• a) Streptococcus pneumoniae – Alpha-hemolytic (greenish colonies), optochin-sensitive.

• c) Streptococcus pyogenes – Beta-hemolytic (complete lysis), bacitracin-sensitive.

• d) Staphylococcus aureus – Catalase-positive, coagulase-positive, beta-hemolytic.

• Group D Streptococci (includes Enterococcus spp. and Streptococcus bovis)

  • Can hydrolyze esculin in the presence of bile → turns medium black.

  • Enterococcus can also grow in 6.5% NaCl, while S. bovis cannot.

Hippurate hydrolysis → Positive in Streptococcus agalactiae (Group B Strep).
Optochin sensitivity → Used to identify Streptococcus pneumoniae.
Coagulase → Positive in Staphylococcus aureus, not streptococci.

1. Viridans Group Streptococci:

   • Alpha-hemolytic (greenish partial hemolysis on blood agar).

   • Normal flora of the oral cavity, GI tract, and genitourinary tract.

   • Major cause of subacute bacterial endocarditis (e.g., after dental procedures).

2. Key Pathogen:

   • Streptococcus mutans → A viridans streptococcus associated with:

     • Dental caries (due to biofilm formation).

     • Infective endocarditis (esp. in patients with pre-existing heart valve abnormalities).

Why Not the Others?

• a) Streptococcus pyogenes → Beta-hemolytic (Group A), causes pharyngitis, not endocarditis.

• c) Staphylococcus aureus → Catalase-positive, coagulase-positive; causes acute endocarditis (not subacute).

• d) Enterococcus faecalis → Part of normal gut flora; causes hospital-acquired endocarditis, but not viridans group.

• • Staphylococcus: Catalase-positive (produces bubbles when hydrogen peroxide is added due to catalase enzyme breaking down H₂O₂ into water and oxygen).

  • Streptococcus: Catalase-negative (no bubbles).

• This is the first and most critical test to distinguish between these two genera.

Why Not the Others?

• b) Oxidase: Not relevant here; used to differentiate Gram-negative bacteria (e.g., Pseudomonas vs. Enterobacteriaceae).

• c) Coagulase: Used to distinguish Staphylococcus aureus (coagulase-positive) from other staphylococci (coagulase-negative), but does not help differentiate Staphylococcus from Streptococcus.

• d) Bile solubility: Specific for identifying Streptococcus pneumoniae (bile-soluble) vs. other alpha-hemolytic streptococci (bile-insoluble).

1. Latex Agglutination Test for Protein A:

   • Protein A is a unique surface protein produced by Staphylococcus aureus.

   • The test uses latex beads coated with human IgG antibodies, which bind to Protein A, causing visible agglutination (clumping).

2. Why Staphylococcus aureus?

   • This test is specific for S. aureus because other staphylococci (e.g., S. epidermidis, S. saprophyticus) lack Protein A.

   • Rapid and reliable for differentiating S. aureus from other Gram-positive cocci.

Why Not the Others?

• a) Streptococcus pneumoniae → Identified by optochin sensitivity/bile solubility, not Protein A.

• c) Enterococcus faecalis → PYR-positive, grows in 6.5% NaCl, no Protein A.

• d) Streptococcus agalactiae → Identified by CAMP test, not Protein A.

1. Lancefield Classification:

   • Developed by Rebecca Lancefield, it groups streptococci based on antigenic differences in cell-wall carbohydrates (C-substance).

   • Examples:

     • Group A (S. pyogenes).

     • Group B (S. agalactiae).

     • Group D (Enterococci, S. bovis).

2. Why Not the Others?

   • a) Hemolysis type (alpha/beta/gamma) → Useful but separate from Lancefield grouping.

   • b) Catalase reaction → All streptococci are catalase-negative (staphylococci are positive).

   • d) Coagulase production → Used for Staphylococcus aureus (streptococci lack coagulase).

Streptococcus mutans, a member of the viridans streptococci group, is the primary bacterial pathogen implicated in dental caries (tooth decay). It metabolizes sugars to produce acid, which demineralizes tooth enamel, leading to cavities.

The other options are less relevant:

• a) Enterococcus faecalis – Associated with root canal infections, not typical caries.

• c) S. agalactiae – Group B streptococcus, linked to neonatal infections.

• d) S. aureus – Causes skin/soft tissue infections, not caries.

1. Key Features of the Organism:

   • Gram-positive coccus → Suggests Staphylococcus, Streptococcus, or Enterococcus.

   • Catalase-negative → Rules out Staphylococcus (all staphylococci are catalase-positive).

   • Gamma-hemolytic → No hemolysis on blood agar (differentiates from alpha/beta-hemolytic species).

2. Likely Organism:

   • Enterococcus faecalis →

     • Catalase-negative, gamma-hemolytic.

     • PYR-positive, bile esculin-positive, and grows in 6.5% NaCl.

     • Common in UTIs, endocarditis, and hospital-acquired infections.

Why Not the Others?

• a) Streptococcus pneumoniae → Alpha-hemolytic (green partial hemolysis).

• c) Streptococcus pyogenes → Beta-hemolytic (complete hemolysis).

• d) Staphylococcus aureus → Catalase-positive (excluded by the question’s criteria).

1. Bile Esculin Test:

   • Enterococci (e.g., E. faecalis) and Group D streptococci (e.g., S. bovis) hydrolyze esculin in bile-containing medium, turning the agar black.

   • Bile tolerance and esculin hydrolysis are key traits for identification.

2. Why Not the Others?

   • a) S. pyogenes → Beta-hemolytic, bile esculin-negative.

   • b) S. pneumoniae → Alpha-hemolytic, bile-soluble, but bile esculin-negative.

   • d) S. aureus → Catalase-positive, coagulase-positive, bile esculin-negative.

Mannitol Salt Agar (MSA) is both selective (7.5% NaCl inhibits most bacteria except Staphylococci) and differential (mannitol fermentation).

• S. aureus ferments mannitol, producing acid, which turns the phenol red indicator yellow (yellow colonies/halo).

• Other Staphylococci (e.g., S. epidermidis) grow but do not ferment mannitol, leaving the agar red/pink.

Why Not the Others?

• a) No growth – Incorrect; S. aureus grows well on MSA (salt-tolerant).

• b) Red colonies – Seen in non-mannitol-fermenters (e.g., S. epidermidis).

• d) Black precipitate – Occurs in tests like H₂S production (e.g., Salmonella on TSI agar), not MSA.

Streptococcus pneumoniae is:

• Encapsulated (has a polysaccharide capsule, a major virulence factor).

• Alpha-hemolytic (produces partial hemolysis, causing greenish discoloration on blood agar).

• Lancet-shaped (diplococci with pointed ends under microscopy).

Why not the others?

• a) S. pyogenes – Beta-hemolytic (complete hemolysis), Group A Streptococcus, not lancet-shaped.

• b) S. agalactiae – Beta-hemolytic, Group B Streptococcus, not lancet-shaped.

• d) E. faecalis – Gamma-hemolytic (non-hemolytic), not a Streptococcus (though formerly classified as Group D Streptococcus), and not lancet-shaped.

1. • Coagulase-negative (differentiates from S. aureus, which is coagulase-positive).

   • Novobiocin-sensitive (zone of inhibition around novobiocin disk).

   • Part of normal skin flora; causes biofilm-associated infections (e.g., catheter-related).

2. Why Not the Others?

   • a) S. aureus → Coagulase-positive (excluded by the question).

   • b) S. saprophyticus → Coagulase-negative but novobiocin-resistant (UTIs in young women).

   • d) Enterococcus faecalis → Not a staphylococcus (Gram-positive coccus, but catalase-negative).

1. PYR Test (Pyrrolidonyl Arylamidase Test):

   • Detects the enzyme pyrrolidonyl arylamidase, which hydrolyzes the substrate L-pyrrolidonyl-β-naphthylamide (PYR).

   • A positive result (red color after adding PYR reagent) helps identify certain Gram-positive cocci.

2. PYR-Positive Organisms:

   • Streptococcus pyogenes (Group A Strep) → PYR-positive (key diagnostic feature).

   • Enterococcus spp. (e.g., E. faecalis) → Also PYR-positive.

Why Not the Others?

• b) Staphylococcus epidermidis → PYR-negative.

• c) Streptococcus pneumoniae → PYR-negative.

• d) Micrococcus luteus → PYR-negative

1. Key Characteristics of Enterococcus Species:

   • Vancomycin Resistance: Some strains (e.g., Enterococcus faecium) are vancomycin-resistant (VRE), a major concern in hospitals.

   • Salt Tolerance: Grows in 6.5% NaCl (unlike most streptococci).

   • Bile Esculin Hydrolysis: Positive (turns agar black).

   • PYR Test: Positive (red color).

2. Why Not the Others?

   • a) Sensitive to penicillin → Enterococci are intrinsically less susceptible to penicillin (require combo therapy like ampicillin + aminoglycoside).

   • b) Does not grow in 6.5% NaCl → Incorrect; Enterococci grow in 6.5% NaCl (a defining trait).

   • c) Hydrolyzes hippurate → Streptococcus agalactiae (Group B) is hippurate-positive, not Enterococcus.

1. Key Features of Streptococcus pneumoniae:

   • Gram-positive diplococci (lancet-shaped pairs).

   • Alpha-hemolytic (green partial hemolysis on blood agar).

   • Bile-soluble (lyses in bile salts, unlike viridans streptococci).

   • Optochin-sensitive (definitive test).

2. Clinical Relevance:

   • Leading cause of community-acquired pneumonia, meningitis, and otitis media.

3. Why Not the Others?

   • a) S. pyogenes → Beta-hemolytic (complete hemolysis), arranged in chains.

   • b) S. agalactiae → Beta-hemolytic (Group B Strep), no bile solubility.

   • d) Enterococcus faecalis → Gamma-hemolytic (no hemolysis), bile esculin-positive.

1. Key Difference Between Staphylococcus and Micrococcus:

   • Both are Gram-positive cocci and catalase-positive, so the catalase test (b) cannot differentiate them.

   • Oxidase test:

     • Micrococcus → Oxidase-positive (turns purple with oxidase reagent).

     • Staphylococcus → Oxidase-negative (no color change).

2. Why Not the Others?

   • b) Catalase → Both are positive, so it doesn’t help differentiate.

   • c) Coagulase → Only identifies Staphylococcus aureus (coagulase-positive) vs. other staphylococci, not Micrococcus.

   • d) Bile solubility → Used for Streptococcus pneumoniae, irrelevant here.

1. Key Features of the Organism:

   • Gram-positive coccus → Suggests Staphylococcus or Micrococcus.

   • Catalase-positive → Confirms Staphylococcus or Micrococcus (both are catalase-positive).

   • Novobiocin-sensitive → Staphylococcus epidermidis is sensitive, while S. saprophyticus is resistant.

2. Differentiation:

   • Staphylococcus epidermidis →

     • Coagulase-negative (unlike S. aureus).

     • Novobiocin-sensitive (key test to differentiate from S. saprophyticus).

     • Part of normal skin flora; causes biofilm-associated infections (e.g., catheter-related).

Why Not the Others?

• b) Staphylococcus saprophyticus → Novobiocin-resistant (UTIs in young women).

• c) Staphylococcus aureus → Coagulase-positive (not relevant here) and novobiocin-sensitive, but S. epidermidis is the more common choice for this question.

• d) Micrococcus luteus → Catalase-positive but usually novobiocin-resistant.

1. Key Features of the Organism:

   • Gram-positive coccus → Suggests Staphylococcus, Streptococcus, or Enterococcus.

   • Grows in 6.5% NaCl (salt tolerance) → Rules out most Streptococcus spp. (except some Group D strep, but they usually don’t grow well in high salt).

   • Hydrolyzes bile esculin (bile esculin positive, turns agar black) → Confirms Group D streptococci/enterococci.

2. Differentiation:

   • Enterococcus faecalis → Grows in 6.5% NaCl + bile esculin positive (classic for enterococci).

   • Streptococcus bovis (Group D Strep) → Bile esculin positive but does not grow in 6.5% NaCl.

   • Streptococcus pneumoniae → Bile esculin negative, no growth in 6.5% NaCl.

   • Staphylococcus saprophyticus → Grows in NaCl but bile esculin negative.

1. Optrochin (Ethylhydrocupreine) Sensitivity Test:

   • Used to differentiate Streptococcus pneumoniae (optochin-sensitive) from other alpha-hemolytic streptococci (e.g., Viridans streptococci, which are optochin-resistant).

   • A zone of inhibition ≥14 mm around an optochin disk confirms S. pneumoniae.

2. Key Features of S. pneumoniae:

   • Gram-positive coccus (lancet-shaped diplococci).

   • Alpha-hemolytic (greenish partial hemolysis on blood agar).

   • Bile-soluble (another confirmatory test).

Why Not the Others?

• a) Streptococcus pyogenes → Beta-hemolytic, optochin-resistant.

• b) Streptococcus agalactiae → Beta-hemolytic, optochin-resistant.

• d) Enterococcus faecalis → Gamma-hemolytic, optochin-resistant.

• Thick peptidoglycan layer → retains crystal violet stain → appears purple on Gram stain.

• Teichoic acids present in cell wall.

• No outer membrane (unlike Gram-negative).

• No lipopolysaccharide (LPS) (endotoxin only in Gram-negative).

Thin peptidoglycan layer → Gram-negative
 Outer membrane → Gram-negative
 LPS in cell wall → Gram-negative

1. Key Features of the Organism:

   • Gram-positive coccus → Suggests Staphylococcus, Streptococcus, or Enterococcus.

   • Catalase-negative → Rules out Staphylococcus (all staphylococci are catalase-positive).

   • Alpha-hemolytic → Produces partial (green) hemolysis on blood agar.

2. Likely Organism:

   • Streptococcus pneumoniae → Catalase-negative, alpha-hemolytic (and optochin-sensitive, bile-soluble).

   • Other alpha-hemolytic streptococci (e.g., Viridans streptococci) are also catalase-negative but differ in clinical context (e.g., dental caries, endocarditis).

Why Not the Others?

• a) Streptococcus pyogenes → Beta-hemolytic (complete hemolysis).

• c) Staphylococcus epidermidis → Catalase-positive, non-hemolytic or gamma-hemolytic.

• d) Enterococcus faecalis → Catalase-negative, but usually gamma-hemolytic (no hemolysis).

1. Common in young women – Staphylococcus saprophyticus causes 10–20% of UTIs in sexually active women aged 17–27 (second only to E. coli).

2. Novobiocin-resistant – Unlike other coagulase-negative staphylococci (CoNS), it resists novobiocin, a key lab identifier.

3. Adherence to uroepithelium – Binds directly to urinary tract cells, enabling colonization.

Why Not the Others?

• a) S. aureus – Rarely causes UTIs; associated with skin infections, bacteremia, or hospital-acquired infections.

• c) S. epidermidis – A CoNS that colonizes catheters but rarely causes community-acquired UTIs.

• d) S. haemolyticus – Hospital-associated CoNS; linked to bloodstream infections, not typical UTIs.

1. Optochin-sensitive – S. pneumoniae shows a zone of inhibition around an optochin (P disk) on blood agar, while other alpha-hemolytic strep (e.g., viridans group) are resistant.

2. Alpha-hemolytic – Produces greenish discoloration (partial hemolysis) on blood agar.

3. Gram-positive coccus – Appears as lancet-shaped diplococci under microscopy.

Why Not the Others?

• a) Streptococcus pyogenes – Beta-hemolytic (complete hemolysis), bacitracin-sensitive, and optochin-resistant.

• b) Streptococcus agalactiae – Beta-hemolytic (Group B Strep), CAMP test-positive, and optochin-resistant.

• d) Enterococcus faecium – Gamma-hemolytic (non-hemolytic), PYR-positive, grows in 6.5% NaCl, and optochin-resistant.

1. Key Features of Staphylococcus saprophyticus:

   • Gram-positive coccus, coagulase-negative staphylococcus (CoNS).

   • Second most common cause of UTIs in young, sexually active women (after E. coli).

   • Adheres to uroepithelial cells via surface adhesins.

2. Why Not the Others?

   • a) Staphylococcus aureus → Causes skin infections, abscesses, and systemic infections (not typical for uncomplicated UTIs).

   • c) Streptococcus pyogenes → Causes pharyngitis, cellulitis (not UTIs).

   • d) Enterococcus faecium → More common in hospital-acquired UTIs (e.g., catheter-associated).

• Bile solubility is a key diagnostic feature of Streptococcus pneumoniae.

  • When exposed to bile salts (e.g., sodium deoxycholate), S. pneumoniae undergoes lysis (dissolves), while other alpha-hemolytic streptococci (e.g., viridans group) remain intact.

Why Not the Others?

• a) Latex agglutination of coagulase – Used for Staphylococcus aureus (coagulase-positive), not S. pneumoniae.

• c) Novobiocin resistance – Used to differentiate Staphylococcus saprophyticus (resistant) from other coagulase-negative staphylococci.

• d) Catalase positivity – S. pneumoniae is catalase-negative (like all streptococci); catalase is used to distinguish Staphylococcus (positive) from Streptococcus (negative).

1. Beta-hemolytic → Causes complete lysis of red blood cells (clear zone on blood agar).

   • Streptococcus pyogenes (Group A) and Streptococcus agalactiae (Group B) are beta-hemolytic.

   • Streptococcus pneumoniae is alpha-hemolytic, and Enterococcus faecium is usually non-hemolytic or gamma-hemolytic.

2. Bacitracin-sensitive → Helps differentiate S. pyogenes (Group A, sensitive) from S. agalactiae (Group B, resistant).

Why not the others?

• a) Streptococcus agalactiae → Beta-hemolytic but bacitracin-resistant.

• b) Streptococcus pneumoniae → Alpha-hemolytic, not beta-hemolytic.

• d) Enterococcus faecium → Not beta-hemolytic, and bacitracin-resistant.

• Group A Streptococcus (GAS), specifically Streptococcus pyogenes, is the primary cause of:

  • Pharyngitis (strep throat).

  • Impetigo (a superficial skin infection characterized by honey-colored crusts).

Why Not the Others?

• a) Viridans streptococci – Part of normal oral flora; associated with dental caries and endocarditis, not impetigo/pharyngitis.

• b) Enterococcus – Causes UTIs and endocarditis; no role in impetigo/pharyngitis.

• d) S. aureus – Can cause impetigo (especially bullous impetigo) but not pharyngitis (strep throat is classic for S. pyogenes).

• Coagulase test is a key test to differentiate Staphylococcus aureus (coagulase-positive) from other staphylococci (coagulase-negative).

  • S. aureus produces coagulase, an enzyme that clots plasma (a major virulence factor).

  • Other staphylococci like S. epidermidis and S. saprophyticus are coagulase-negative.

• Streptococcus pyogenes is a Gram-positive coccus but is not staphylococcus (it is catalase-negative and grows in chains).

Why Not the Others?

• a) Staphylococcus epidermidis → Coagulase-negative (part of normal skin flora).

• b) Staphylococcus saprophyticus → Coagulase-negative (causes UTIs in young women).

• d) Streptococcus pyogenes → Gram-positive coccus, but not staphylococcus (no coagulase).

1. Optochin Test (Ethylhydrocupreine):

   • S. pneumoniae → Sensitive (zone of inhibition ≥14 mm around optochin disk).

   • Viridans streptococci (e.g., S. mutans, S. mitis) → Resistant (no zone or small zone).

2. Supportive Tests for S. pneumoniae:

   • Bile solubility (S. pneumoniae dissolves in bile, viridans strep do not).

   • Alpha-hemolysis (green partial hemolysis on blood agar).

3. Why Not the Others?

   • a) Bile esculin → Used for Group D streptococci/enterococci, not pneumococci.

   • b) Hippurate hydrolysis → Used for Streptococcus agalactiae (Group B Strep).

   • d) Coagulase → Used for Staphylococcus aureus.

1. Catalase Test Procedure:

   • A drop of 3% hydrogen peroxide (H₂O₂) is added to bacterial colonies.

   • Catalase-positive organisms (e.g., Staphylococcus) produce bubbles (O₂ gas) due to breakdown of H₂O₂ into water and oxygen.

   • Catalase-negative organisms (e.g., Streptococcus) show no reaction.

2. Why Not the Others?

   • a) Iodine → Used in Gram staining or starch hydrolysis tests.

   • c) Crystal violet → Used in Gram staining.

   • d) Bile salt → Used in bile solubility or bile esculin tests.

1. DNase-positive – S. aureus produces DNase (deoxyribonuclease), an enzyme that hydrolyzes DNA. This is a key lab test to identify it.

2. Causes food poisoning – S. aureus produces heat-stable enterotoxins (e.g., SEA-SEE) that cause rapid-onset vomiting/diarrhea (1–6 hours after eating contaminated food like dairy, meats, or salads).

Why Not the Others?

• a) Staphylococcus epidermidis – DNase-negative; part of normal skin flora, not a foodborne pathogen.

• c) Streptococcus pyogenes – DNase-positive (produces Streptodornase), but causes pharyngitis/cellulitis, not food poisoning.

• d) Enterococcus faecalis – DNase-negative; associated with UTIs/endocarditis, not foodborne illness.

• The CAMP test is used to identify Group B Streptococcus (Streptococcus agalactiae).

• S. agalactiae produces a CAMP factor (protein) that synergizes with the beta-lysin (sphingomyelinase C) secreted by Staphylococcus aureus.

• This interaction creates an arrowhead-shaped zone of enhanced hemolysis on sheep blood agar.

Why Not the Others?

• b) Streptolysin O – Produced by S. pyogenes (Group A Strep), but this toxin is oxygen-labile and detected via ASO antibody testing, not the CAMP test.

• c) Bile solubility – Used to identify S. pneumoniae (dissolves in bile salts), unrelated to hemolysis patterns.

• d) Optochin sensitivity – Differentiates S. pneumoniae (sensitive) from other alpha-hemolytic streptococci.

1. Post-Streptococcal Glomerulonephritis (PSGN):

   • A type III hypersensitivity reaction triggered by immune complexes deposited in the kidneys.

   • Occurs 1-3 weeks after infection with nephritogenic strains of Streptococcus pyogenes (Group A Strep).

2. Key Features of S. pyogenes:

   • Causes pharyngitis or skin infections (e.g., impetigo) that precede PSGN.

   • M-protein serotypes (e.g., types 1, 4, 12) are linked to PSGN.

Why Not the Others?

• b) Streptococcus agalactiae (Group B) → Causes neonatal sepsis/meningitis, not PSGN.

• c) Streptococcus pneumoniae → Causes pneumonia/meningitis, no association with PSGN.

• d) Enterococcus faecalis → Causes UTIs/endocarditis, unrelated to PSGN.

1. Key Features of the Organism:

   • Gram-positive coccus → Suggests Staphylococcus, Streptococcus, or Enterococcus.

   • Bile esculin-positive → Hydrolyzes esculin in bile-containing medium (agar turns black).

     • Seen in Group D streptococci (e.g., S. bovis) and Enterococci.

   • Does not grow in 6.5% NaCl → Rules out Enterococcus (which is salt-tolerant).

2. Differentiation:

   • Streptococcus bovis → Bile esculin-positive, salt-intolerant (no growth in 6.5% NaCl).

     • Associated with colorectal cancer and endocarditis.

   • Enterococcus faecalis → Bile esculin-positive and grows in 6.5% NaCl.

Why Not the Others?

• a) Enterococcus faecalis → Grows in 6.5% NaCl.

• c) Streptococcus pyogenes → Beta-hemolytic, bile esculin-negative.

• d) Staphylococcus aureus → Catalase-positive, coagulase-positive, bile esculin-negative.

• Teichoic acids are unique to gram-positive bacteria, embedded in their thick peptidoglycan layer.

  • Function: Cell wall stability, cation binding, and pathogenicity (e.g., S. aureus lipoteichoic acid triggers inflammation).

Why Not the Others?

• b) Outer membrane containing LPS – Exclusive to gram-negative bacteria (e.g., E. coli).

• c) Thin peptidoglycan layer – Gram-negatives have thin peptidoglycan (vs. thick in gram-positives).

• d) Production of exotoxin – Not specific; both gram-positive (S. aureus, C. tetani) and gram-negative (V. cholerae, E. coli) bacteria produce exotoxins.

• Lancefield classification groups streptococci (and some related bacteria) based on antigenic differences in their cell-wall carbohydrates (C-substance).

• Serologic testing uses specific antibodies to detect these carbohydrates (e.g., Group A, B, D, etc.).

Why Not the Others?

• a) PYR test – Identifies S. pyogenes (Group A) and Enterococcus (Group D), but doesn’t classify Lancefield groups broadly.

• b) Hippurate hydrolysis – Used to confirm S. agalactiae (Group B), not a general Lancefield tool.

• d) Catalase test – Only distinguishes Staphylococcus (+) from Streptococcus (−); irrelevant to Lancefield grouping.

1. Catalase Test:

   • Staphylococcus → Catalase-positive (bubbles form with hydrogen peroxide).

   • Streptococcus → Catalase-negative (no bubbles).

   • This is the primary test to distinguish the two genera.

2. Why Not the Others?

   • a) Coagulase → Only differentiates S. aureus (positive) from other staphylococci, not streptococci.

   • c) Optochin sensitivity → Used to identify S. pneumoniae (a streptococcus), not staphylococci.

   • d) Bile solubility → Used for S. pneumoniae, irrelevant for Staphylococcus.

1. Group A Streptococcus (S. pyogenes) Infections:

   • Scarlet fever → Caused by erythrogenic toxin-producing strains, leading to a sandpaper-like rash and strawberry tongue.

   • Other key diseases:

     • Pharyngitis (strep throat).

     • Impetigo (skin infection).

     • Rheumatic fever (post-streptococcal complication).

     • Necrotizing fasciitis (“flesh-eating” infection).

2. Why Not the Others?

   • a) Endocarditis → More associated with Viridans streptococci or S. aureus.

   • c) UTIs → Caused by E. coli, S. saprophyticus, or enterococci (not Group A Strep).

   • d) Dental caries → Caused by S. mutans (viridans group).

1. Key Features of the Organism:

   • Gram-positive coccus → Suggests Staphylococcus or Streptococcus.

   • Catalase-negative → Rules out Staphylococcus (all staphylococci are catalase-positive).

   • Beta-hemolytic → Causes complete lysis of RBCs (clear zones on blood agar).

2. Likely Organism:

   • Streptococcus pyogenes (Group A Strep) → Catalase-negative, beta-hemolytic.

     • Associated with pharyngitis, impetigo, rheumatic fever.

   • Other beta-hemolytic streptococci (e.g., S. agalactiae – Group B) are also catalase-negative but differ in clinical context.

Why Not the Others?

• a) Staphylococcus aureus → Catalase-positive (would not fit).

• c) Micrococcus luteus → Catalase-positive, usually non-hemolytic.

• d) Staphylococcus epidermidis → Catalase-positive, non-hemolytic or gamma-hemolytic.

• Bile solubility test is used to differentiate Streptococcus pneumoniae (bile-soluble) from other alpha-hemolytic streptococci (e.g., Viridans streptococci, which are bile-insoluble).

  • When sodium deoxycholate (bile salt) is added to S. pneumoniae, the bacterial cells lyse, clearing the turbidity in the suspension.

  • Other alpha-hemolytic streptococci remain unaffected.

Why Not the Others?

• a) Catalase → Used to differentiate staphylococci (positive) from streptococci (negative), but not within streptococci.

• c) Coagulase → Used for identifying Staphylococcus aureus (coagulase-positive).

• d) Urease → Used for organisms like Proteus and Helicobacter, not streptococci.

1. CAMP Test Purpose:

   • The CAMP test is used to identify Streptococcus agalactiae (Group B Streptococcus, GBS).

   • It detects the production of CAMP factor, a protein that enhances the hemolysis caused by Staphylococcus aureus beta-toxin.

2. How It Works:

   • S. agalactiae is streaked perpendicular to S. aureus on a blood agar plate.

   • A characteristic arrowhead-shaped zone of enhanced hemolysis appears at the junction, confirming S. agalactiae.

Why Not the Others?

• a) Staphylococcus aureus → Used in the test but is not identified by it.

• c) Streptococcus pneumoniae → Alpha-hemolytic, does not produce CAMP factor.

• d) Enterococcus faecalis → Gamma-hemolytic, does not interact with CAMP factor.

• Coagulase test is used to differentiate Staphylococcus aureus (coagulase-positive) from other staphylococci (usually coagulase-negative).

• S. aureus produces coagulase, an enzyme that clots plasma, while most other staphylococci do not.

• Catalase (c) helps differentiate staphylococci (catalase-positive) from streptococci (catalase-negative), but not within staphylococci.

• Oxidase (a) and Optochin susceptibility (d) are not relevant for identifying S. aureus.

1. Coagulase Test:

   • Coagulase-positive → Staphylococcus aureus (pathogenic, produces coagulase enzyme to clot plasma).

   • Coagulase-negative staphylococci (CoNS) → S. epidermidis, S. saprophyticus, S. lugdunensis (usually non-pathogenic or opportunistic).

2. Exceptions:

   • S. lugdunensis is a rare CoNS that may act aggressively (e.g., causing endocarditis) but is still coagulase-negative.

1. Staphylococci are Gram-positive cocci that divide irregularly, forming grape-like clusters (visible on microscopy).

   • This distinguishes them from:

     • Streptococci (chains or pairs).

     • Micrococci (tetrads or packets).

2. Why Not the Others?

   • a) Chains → Streptococcus (e.g., S. pyogenes).

   • b) Pairs → Streptococcus pneumoniae (lancet-shaped diplococci).

   • d) Tetrads → Micrococcus or Gaffkya.

1. Group B Streptococcus (GBS) → Streptococcus agalactiae is a major cause of neonatal sepsis, meningitis, and pneumonia.

   • Transmitted during childbirth from colonized mothers.

   • Identified by:

     • Beta-hemolysis on blood agar.

     • Positive CAMP test (enhances hemolysis near S. aureus).

     • Hippurate hydrolysis (biochemical test).

2. Why Not the Others?

   • a) Streptococcus pyogenes → Group A Strep (causes pharyngitis, rheumatic fever).

   • c) Streptococcus pneumoniae → Alpha-hemolytic; causes pneumonia/meningitis (not neonatal sepsis).

   • d) Enterococcus faecalis → Part of normal gut flora; causes UTIs, endocarditis (not neonatal sepsis).

1. Caused by S. pyogenes – Specifically, strains that produce erythrogenic exotoxins (e.g., SpeA, SpeB, SpeC).

2. Clinical Triad:

   • Strep pharyngitis/tonsillitis (sore throat, fever).

   • Strawberry tongue (white coating → red, bumpy).

   • Sandpaper-like rash (blanching, starts on trunk/spreads outward).

Why Not the Others?

• b) S. agalactiae (Group B Strep) – Causes neonatal sepsis/meningitis, not scarlet fever.

• c) S. pneumoniae – Causes pneumonia/meningitis; alpha-hemolytic, optochin-sensitive.

• d) Enterococcus faecalis – Associated with UTIs/endocarditis; gamma-hemolytic.

• Coagulase test is used to differentiate Staphylococcus aureus (coagulase-positive) from other staphylococci (coagulase-negative).

  • Not used for streptococci, as streptococci are coagulase-negative by definition.

Tests Used to Identify Streptococci:

1. a) Optochin (P disk) – Differentiates:

   • S. pneumoniae (sensitive, zone of inhibition) vs. other alpha-hemolytic streptococci (resistant).

2. c) Hippurate hydrolysis – Identifies:

   • Group B Streptococcus (S. agalactiae), which hydrolyzes hippurate (positive test).

3. d) Bile esculin – Identifies:

   • Enterococcus and Group D Streptococci (blackening due to esculin hydrolysis).

Why Not the Others?

• Optochin, hippurate hydrolysis, and bile esculin are standard tests for streptococcal speciation.

• Coagulase is exclusive to Staphylococcus identification

• The catalase test distinguishes Staphylococcus (catalase-positive) from Streptococcus (catalase-negative).

  • Staphylococcus produces catalase, which breaks down hydrogen peroxide into water and oxygen (bubbles form).

  • Streptococcus does not produce catalase (no bubbles).

Why Not the Others?

• a) Staphylococcus vs Enterococcus – While Staphylococcus is catalase-positive and Enterococcus is catalase-negative, this is not the primary purpose of the test (Enterococcus is a Gram-positive coccus but not a major genus compared to Streptococcus in this context).

• c) Streptococcus vs Enterococcus – Both are catalase-negative, so the test cannot distinguish them.

• d) All of the above – Incorrect because the test is mainly used for Staphylococcus vs Streptococcus.

1. Reverse CAMP Test Purpose:

   • Used to identify Clostridium perfringens (a Gram-positive, anaerobic, spore-forming rod).

   • Detects phospholipase C (lecithinase) activity, which enhances hemolysis caused by Streptococcus agalactiae (Group B Strep).

2. How It Works:

   • C. perfringens is streaked perpendicular to S. agalactiae on blood agar.

   • A “square-shaped” zone of enhanced hemolysis appears at the junction, confirming C. perfringens.

Why Not the Others?

• a) Staphylococcus aureus → Identified by coagulase/protein A tests, not reverse CAMP.

• c) Streptococcus agalactiae → Identified by standard CAMP test (not reverse).

• d) Enterococcus faecalis → PYR-positive, bile esculin-positive; no role in reverse CAMP.

1. PYR-positive → Hydrolyzes L-pyrrolidonyl-β-naphthylamide (turns red), seen in:

   • Enterococcus (e.g., E. faecalis, E. faecium).

   • Group A Strep (S. pyogenes), but S. pyogenes does not grow in 6.5% NaCl.

2. Grows in 6.5% NaCl → Salt tolerance is a hallmark of Enterococcus (not most streptococci).

Why Not the Others?

• a) Streptococcus bovis (Group D Strep):

  • PYR-negative (distinguishes it from Enterococcus).

  • Some strains grow in 6.5% NaCl, but not PYR-positive.

• c) Streptococcus pneumoniae:

  • PYR-negative, fails to grow in 6.5% NaCl, optochin-sensitive.

• d) Staphylococcus epidermidis:

  • Catalase-positive (not relevant here), PYR-negative.

1. Catalase-positive → Confirms it is Staphylococcus (rules out Streptococcus).

2. Coagulase-negative → Rules out S. aureus (which is coagulase-positive).

3. Novobiocin-resistant → Distinguishes S. saprophyticus from other coagulase-negative staphylococci (CoNS), which are typically sensitive.

Why Not the Others?

• a) S. aureus – Coagulase-positive (excluded by the question).

• b) S. epidermidis – Novobiocin-sensitive (most common CoNS but does not fit resistance pattern).

• d) S. haemolyticus – Novobiocin-sensitive (less common CoNS, typically hospital-associated).

1. Gram-positive cocci in clusters → Suggests Staphylococcus (not Streptococcus or Enterococcus, which are usually in chains or pairs).

2. Catalase-positive → Confirms Staphylococcus (since Streptococcus and Enterococcus are catalase-negative).

3. Coagulase-negative → Rules out Staphylococcus aureus (which is coagulase-positive).

The most common coagulase-negative staphylococcus (CoNS) is Staphylococcus epidermidis, a normal skin commensal and a frequent cause of hospital-acquired infections (e.g., catheter-related infections).

Why not the others?

• a) Staphylococcus aureus → Coagulase-positive.

• c) Streptococcus pyogenes → Catalase-negative, gram-positive cocci in chains.

• d) Enterococcus faecalis → Catalase-negative, gram-positive cocci in pairs/chains.

• S. epidermidis

  • Normal skin flora

  • Coagulase-negative staphylococcus (CoNS)

  • Major cause of prosthetic device and catheter-related infections

  • Forms biofilms via polysaccharide slime layer → protects against antibiotics & host defenses

S. aureus → Causes skin abscesses, pneumonia, TSS, food poisoning
S. saprophyticus → Causes UTIs in young women
S. pneumoniae → Causes pneumonia, meningitis, otitis media, sinusitis (not a staph)

Viridans streptococci – Key features:

• α-hemolytic (like S. pneumoniae, but different tests separate them)

• No Lancefield antigens (unlike Groups A, B, D, etc.)

• Optochin resistant (vs. S. pneumoniae, which is sensitive)

• Bile insoluble

• Normal oral flora → cause dental caries (S. mutans), subacute endocarditis (esp. after dental procedures)

• Catalase positivity → staphylococci, not streptococciLancefield Group B antigen → S. agalactiae

Resistance to optochin → true for viridans, but not the defining feature; the combination of α-hemolysis + no Lancefield grouping best describes them

So the hallmark is α-hemolysis with no Lancefield classification.

Do you want me to put together a comparison table: S. pneumoniae vs. Viridans streptococci for quick recall?

• Novobiocin susceptibility is used to differentiate Staphylococcus species (e.g., S. saprophyticus is resistant, S. epidermidis is sensitive). It has no role in distinguishing S. pneumoniae from Viridans streptococci.

Tests That Are Useful for Differentiation:

1. a) Optochin sensitivity – S. pneumoniae is sensitive (zone of inhibition around P disk), while Viridans strep are resistant.

2. b) Bile solubility – S. pneumoniae lyses in bile salts, whereas Viridans strep do not.

3. d) Alpha-hemolysis – Both S. pneumoniae and Viridans strep are alpha-hemolytic (greenish colonies), so this alone cannot differentiate them. However, it rules out beta- or gamma-hemolytic species.

1. Key Features of the Organism:

   • Gram-positive coccus → Suggests Staphylococcus or Micrococcus.

   • Catalase-positive → Confirms Staphylococcus or Micrococcus (both are catalase-positive).

   • Novobiocin-resistant → Staphylococcus saprophyticus is resistant, while S. epidermidis is sensitive.

2. Differentiation:

   • Staphylococcus saprophyticus →

     • Catalase-positive, coagulase-negative.

     • Novobiocin-resistant (key test to differentiate from S. epidermidis).

     • Causes UTIs in young women.

   • Micrococcus luteus → Catalase-positive but novobiocin-sensitive.

Why Not the Others?

• a) Staphylococcus aureus → Coagulase-positive (not relevant here) and novobiocin-sensitive.

• c) Staphylococcus epidermidis → Novobiocin-sensitive.

• d) Micrococcus luteus → Novobiocin-sensitive.

• Growth in 6.5% NaCl is a defining feature of Enterococcus (e.g., E. faecalis, E. faecium).

  • Enterococci are salt-tolerant and can grow in high NaCl concentrations, unlike most other streptococci.

  • This test helps differentiate Enterococcus (which grows) from Streptococcus species (which generally do not).

Why Not the Others?

• a) Viridans streptococci – Cannot grow in 6.5% NaCl (salt-sensitive).

• c) S. pneumoniae – Fails to grow in 6.5% NaCl.

• d) S. agalactiae (Group B Strep) – Does not tolerate high salt concentrations.

1. Coagulase Test:

   • Staphylococcus aureus is coagulase-positive (produces an enzyme that clots plasma).

   • Other staphylococci (e.g., S. epidermidis, S. saprophyticus) are coagulase-negative.

2. Why Not the Others?

   • a) EMB agar → Used for Gram-negative enteric bacteria (e.g., E. coli), not staphylococci.

   • c) Optochin → Used to identify Streptococcus pneumoniae (a streptococcus).

   • d) Hippurate hydrolysis → Used for Streptococcus agalactiae (Group B Strep).

1. Novobiocin Resistance Test:

   • Used to differentiate coagulase-negative staphylococci (CoNS).

   • S. saprophyticus → Resistant to novobiocin (no zone of inhibition).

   • S. epidermidis and S. aureus → Sensitive (zone of inhibition present).

2. Clinical Relevance:

   • S. saprophyticus causes UTIs in young women, and its novobiocin resistance helps distinguish it from other CoNS.

3. Why Not the Others?

   • a) S. epidermidis → Sensitive to novobiocin.

   • c) S. aureus → Sensitive (though coagulase-positive, so rarely tested this way).

   • d) S. pneumoniae → A streptococcus; novobiocin resistance is irrelevant (optochin sensitivity is used instead).

• Coagulase-positive staphylococci are pathogenic and include:

  • Staphylococcus aureus (most clinically significant).

  • Rarely, S. intermedius/pseudintermedius (animal pathogens).

• Coagulase-negative staphylococci (CoNS) are typically commensals or opportunistic pathogens, including:

  • S. epidermidis (biofilms on medical devices).

  • S. saprophyticus (UTIs in young women).

  • S. lugdunensis (aggressive CoNS, can mimic S. aureus virulence).

Why Not the Others?

• a) S. epidermidis – CoNS (coagulase-negative), part of normal skin flora.

• b) S. saprophyticus – CoNS, novobiocin-resistant, causes UTIs.

• d) S. lugdunensis – CoNS (but can cause endocarditis and abscesses like S. aureus).

• Catalase-negative: Streptococcus does not produce the enzyme catalase (no bubbles when exposed to H₂O₂).

• Forms chains: Under microscopy, Streptococcus appears as chains of cocci (due to division in a single plane).

Why Not the Others?

• a) Staphylococcus – Catalase-positive and forms clusters (grape-like).

• c) Micrococcus – Catalase-positive and forms tetrads/packets.

• d) Bacillus – Catalase-positive and forms rods (not cocci).

Clinical Relevance:

Streptococcus includes pathogens like:

• S. pyogenes (pharyngitis, rheumatic fever).

• S. pneumoniae (pneumonia, meningitis).

• S. agalactiae (neonatal sepsis).

• S. pyogenes (Group A Streptococcus, GAS) does not produce endotoxin (a component of the outer membrane of Gram-negative bacteria).

• Its virulence factors are exotoxins and surface proteins, including:

  1. a) M protein – Anti-phagocytic, major virulence factor (antibodies to M protein confer type-specific immunity).

  2. b) Streptokinase – Dissolves clots (activates plasminogen to plasmin).

  3. d) Streptolysin – Cytolytic exotoxins:

     • Streptolysin O (oxygen-labile, immunogenic → ASO antibodies used in rheumatic fever diagnosis).

     • Streptolysin S (oxygen-stable, hemolysis on blood agar).

Why Not the Others?

• M protein, streptokinase, and streptolysin are classic GAS virulence factors.

• Endotoxin (LPS) is exclusive to Gram-negative bacteria (e.g., E. coli, Salmonella).

1. CAMP Test for Streptococcus agalactiae (Group B Strep):

   • CAMP factor produced by S. agalactiae enhances hemolysis by S. aureus beta-toxin, creating an arrowhead-shaped zone of hemolysis on blood agar.

   • Hippurate hydrolysis is another confirmatory test.

2. Why Not the Others?

   • a) Optochin sensitivity → Used for S. pneumoniae (Group B Strep is resistant).

   • c) Bile solubility → Used for S. pneumoniae (Group B Strep is insoluble).

   • d) Coagulase positivity → Used for Staphylococcus aureus (streptococci are coagulase-negative).