Bacteria
1. A 5 year old child presents with itchy, honey-colored crusts
around the mouth and nose, along with bullous lesions. What
is the causative organism?
➔ Staph aureus (symptoms suggestive of Non-bullous
Impetigo → caused by Staph aureus)
Note: Comparison of Non-bullous vs. Bullous Impetigo
Feature Non-bullous Impetigo Bullous Impetigo
Strep pyogenes (GAS) or Staph
Cause Staph aureus
aureus
Toxin Involved not toxin-mediated Exfoliative toxin A/B
Lesion Honey-colored crusts without Flaccid bullae that rupture
Appearance bullae easily
Vs. Herpes simplex (HSV) which presents with:
→ Painful grouped vesicles on erythematous base
→ +Tzanck smear.
Vs. Contact dermatitis which presents with:
→ Linear/geometric pruritic plaques with no
crusts/bullae
→ History of allergen exposure.
2. A 2-day-old neonate develops generalized erythema, tender
skin peeling, and Nikolsky sign. No mucosal involvement. What is
the key toxin responsible?
➔ Exfoliative toxin (It is produced by Staphylococcus
,aureus → cleaves desmoglein-1 → leading to
Staphylococcal Scalded Skin Syndrome)
Note: Staphylococcal Scalded Skin Syndrome (SSSS):
→ Has no mucosal involvement as desmoglein-3
which is found in mucosa → is not affected
→ Has + Nikolsky sign which means that gentle
pressure causes skin to slough off.
Differential Diagnosis:
Condition Key Features Toxin/Cause
→ Neonates with tender erythema,
superficial desquamation Exfoliative toxin
SSSS
→ no mucosal involvement (Staph aureus)
→ +Nikolsky
→ Drug-induced (e.g., sulfa,
Toxic
anticonvulsants)
Epidermal Delayed type IV
→ mucosal involvement and full-thickness
Necrolysis hypersensitivity
epidermal necrosis
(TEN)
→ +Nikolsky
→ High fever, hypotension, diffuse rash →
Toxic Shock TSST-1
desquamation (esp. palms/soles), mucosal
Syndrome (superantigen from
hyperemia (e.g., strawberry tongue)
(TSS) Staph aureus)
→ hx of tampon/nasal packing
3. A 5 year old child presents with acute pharyngitis, high fever,
and swollen tonsils with exudate. The rapid antigen test is
positive. What is the key virulence factor of the causative
organism?
,➔ M protein (symptoms suggestive of Streptococcal
pharyngitis which is → caused by Streptococcus pyogenes
(Group A Strep, GAS))
Note: Key virulence factor for GAS is M protein:
→ It is anti-phagocytic
→ Causes Molecular mimicry as it Shares structural
similarity with cardiac myosin → leading to
autoimmune cross-reactivity → causing Rheumatic
fever
Compare with:
Virulence
Organism Function
Factor
Strep pyogenes Anti-phagocytic, molecular mimicry →
M protein
(GAS) rheumatic fever
Binds Fc portion of IgG → inhibits opsonization
Staph aureus Protein A
& phagocytosis
4. 46 year old woman presents with progressive orthopnea (now
requiring 2 pillows), rusty-colored sputum, and a mid-diastolic
rumbling murmur. Echo reveals mitral stenosis with left atrial
dilation. What is the underlying cause?
➔ Recurrent group A streptococcal infections (Symptoms
suggestive of Rheumatic Mitral Stenosis)
Note: Rheumatic heart disease
→ Sequelae of untreated/recurrent Group A
Streptococcal (GAS) pharyngitis → leading to acute
rheumatic fever → causing chronic rheumatic heart
disease
, → Mitral valve most commonly affected → leads to
stenosis.
→ Histopathology shows Aschoff bodies (perivascular
granulomas with Anitschkow cells)
5. A 28-year-old woman with a history of rheumatic fever and
mitral regurgitation presents with 2 weeks of fever, fatigue, and a
new diastolic murmur at the apex. She underwent a dental
procedure 4 weeks ago. What is the causative organism?
➔ Streptococcus mitis (viridans group
streptococci)
(it can cause subacute endocarditis in patients with
damaged valves, especially after dental work)
Note: Key Features of Subacute Endocarditis:
→ Indolent symptoms (weeks of fever, fatigue)
→ Predisposing factors include Rheumatic heart
disease, dental procedures
Differential Diagnosis: Acute vs Subacute Endocarditis
Feature Subacute (Viridans strep) Acute (Staph aureus)
Onset Weeks Rapid (days)
Valve Previously abnormal (e.g.
Normal or abnormal
type rheumatic)
Often tricuspid regurgitation (IV
Murmur New or changing murmur
drug use)
Organis Viridans streptococci (S. mitis, S.
Staph aureus
m sanguinis)
1. A 5 year old child presents with itchy, honey-colored crusts
around the mouth and nose, along with bullous lesions. What
is the causative organism?
➔ Staph aureus (symptoms suggestive of Non-bullous
Impetigo → caused by Staph aureus)
Note: Comparison of Non-bullous vs. Bullous Impetigo
Feature Non-bullous Impetigo Bullous Impetigo
Strep pyogenes (GAS) or Staph
Cause Staph aureus
aureus
Toxin Involved not toxin-mediated Exfoliative toxin A/B
Lesion Honey-colored crusts without Flaccid bullae that rupture
Appearance bullae easily
Vs. Herpes simplex (HSV) which presents with:
→ Painful grouped vesicles on erythematous base
→ +Tzanck smear.
Vs. Contact dermatitis which presents with:
→ Linear/geometric pruritic plaques with no
crusts/bullae
→ History of allergen exposure.
2. A 2-day-old neonate develops generalized erythema, tender
skin peeling, and Nikolsky sign. No mucosal involvement. What is
the key toxin responsible?
➔ Exfoliative toxin (It is produced by Staphylococcus
,aureus → cleaves desmoglein-1 → leading to
Staphylococcal Scalded Skin Syndrome)
Note: Staphylococcal Scalded Skin Syndrome (SSSS):
→ Has no mucosal involvement as desmoglein-3
which is found in mucosa → is not affected
→ Has + Nikolsky sign which means that gentle
pressure causes skin to slough off.
Differential Diagnosis:
Condition Key Features Toxin/Cause
→ Neonates with tender erythema,
superficial desquamation Exfoliative toxin
SSSS
→ no mucosal involvement (Staph aureus)
→ +Nikolsky
→ Drug-induced (e.g., sulfa,
Toxic
anticonvulsants)
Epidermal Delayed type IV
→ mucosal involvement and full-thickness
Necrolysis hypersensitivity
epidermal necrosis
(TEN)
→ +Nikolsky
→ High fever, hypotension, diffuse rash →
Toxic Shock TSST-1
desquamation (esp. palms/soles), mucosal
Syndrome (superantigen from
hyperemia (e.g., strawberry tongue)
(TSS) Staph aureus)
→ hx of tampon/nasal packing
3. A 5 year old child presents with acute pharyngitis, high fever,
and swollen tonsils with exudate. The rapid antigen test is
positive. What is the key virulence factor of the causative
organism?
,➔ M protein (symptoms suggestive of Streptococcal
pharyngitis which is → caused by Streptococcus pyogenes
(Group A Strep, GAS))
Note: Key virulence factor for GAS is M protein:
→ It is anti-phagocytic
→ Causes Molecular mimicry as it Shares structural
similarity with cardiac myosin → leading to
autoimmune cross-reactivity → causing Rheumatic
fever
Compare with:
Virulence
Organism Function
Factor
Strep pyogenes Anti-phagocytic, molecular mimicry →
M protein
(GAS) rheumatic fever
Binds Fc portion of IgG → inhibits opsonization
Staph aureus Protein A
& phagocytosis
4. 46 year old woman presents with progressive orthopnea (now
requiring 2 pillows), rusty-colored sputum, and a mid-diastolic
rumbling murmur. Echo reveals mitral stenosis with left atrial
dilation. What is the underlying cause?
➔ Recurrent group A streptococcal infections (Symptoms
suggestive of Rheumatic Mitral Stenosis)
Note: Rheumatic heart disease
→ Sequelae of untreated/recurrent Group A
Streptococcal (GAS) pharyngitis → leading to acute
rheumatic fever → causing chronic rheumatic heart
disease
, → Mitral valve most commonly affected → leads to
stenosis.
→ Histopathology shows Aschoff bodies (perivascular
granulomas with Anitschkow cells)
5. A 28-year-old woman with a history of rheumatic fever and
mitral regurgitation presents with 2 weeks of fever, fatigue, and a
new diastolic murmur at the apex. She underwent a dental
procedure 4 weeks ago. What is the causative organism?
➔ Streptococcus mitis (viridans group
streptococci)
(it can cause subacute endocarditis in patients with
damaged valves, especially after dental work)
Note: Key Features of Subacute Endocarditis:
→ Indolent symptoms (weeks of fever, fatigue)
→ Predisposing factors include Rheumatic heart
disease, dental procedures
Differential Diagnosis: Acute vs Subacute Endocarditis
Feature Subacute (Viridans strep) Acute (Staph aureus)
Onset Weeks Rapid (days)
Valve Previously abnormal (e.g.
Normal or abnormal
type rheumatic)
Often tricuspid regurgitation (IV
Murmur New or changing murmur
drug use)
Organis Viridans streptococci (S. mitis, S.
Staph aureus
m sanguinis)
