Microbiology
High Yield USMLE Step 1 Notes
Welcome to the Microbiology High-Yield Notes for Step 1.
These are the most commonly tested topics on the exam—ideal for quick review in your last
2 weeks before the exam, when you're short on time, or when you're just starting out and
want to focus on what is high yield. Created and peer-reviewed by physicians who scored
260+ on Step 1 in a Q&A style to reinforce active learning.
How to use this file
y
Read the question ➔ cover the answer ➔ self-quiz before revealing.
u
Review the NOTES ➔ they include high-yield details you don't want to
miss.
h G Need tutoring? Scan the QR code or visit The Match Guy Tutoring.
c
Watch the video explanations that go along with this file for deeper
t
understanding.
Ma
Bacteria
e
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?
Th ➔ 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
Honey-colored crusts without Flaccid bullae that rupture
Lesion Appearance
bullae easily
© 2025 TheMatchGuy LLC. All rights reserved. Page 1
Version 1.0 May 2025. No part of this document may be reproduced or distributed without written permission. For personal study only. Medical information
herein is for educational purposes and does not constitute clinical advice. Always confirm with current guidelines and your institution’s policies.
, 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.
uy
Differential Diagnosis:
Condition
h G Key Features Toxin/Cause
c
➞ Neonates with tender erythema,
t
superficial desquamation Exfoliative toxin (Staph
SSSS
➞ no mucosal involvement
a
aureus)
➞ +Nikolsky
M
➞ Drug-induced (e.g., sulfa,
anticonvulsants)
e
Toxic Epidermal Delayed type IV
➞ mucosal involvement and full-
Necrolysis (TEN) hypersensitivity
h
thickness epidermal necrosis
➞ +Nikolsky
TToxic Shock
Syndrome (TSS)
➞ High fever, hypotension, diffuse rash ➞
desquamation (esp. palms/soles),
mucosal hyperemia (e.g., strawberry
tongue
➞ hx of tampon/nasal packing
TSST-1 (superantigen
from Staph aureus)
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
© 2025 TheMatchGuy LLC. All rights reserved. Page 2
,Compare with:
Organism Virulence Factor Function
Anti-phagocytic, molecular mimicry
Strep pyogenes (GAS) M protein
➞ rheumatic fever
Binds Fc portion of IgG ➞ inhibits
Staph aureus Protein A
opsonization & 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)
uy
Note: Rheumatic heart disease
➞ Sequelae of untreated/recurrent Group A Streptococcal (GAS) pharyngitis ➞ leading
to acute rheumatic fever ➞ causing chronic rheumatic heart disease
G
➞ Mitral valve most commonly affected ➞ leads to stenosis.
➞ Histopathology shows Aschoff bodies (perivascular granulomas with Anitschkow
h
cells)
atc
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
M
dental procedure 4 weeks ago. What is the causative organism?
➔ Streptococcus mitis (viridans group streptococci) (it can cause subacute
e
endocarditis in patients with damaged valves, especially after dental work)
h
Note: Key Features of Subacute Endocarditis:
➞ Indolent symptoms (weeks of fever, fatigue)
T ➞ 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)
Previously abnormal (e.g.
Valve type Normal or abnormal
rheumatic)
Often tricuspid regurgitation
Murmur New or changing murmur
(IV drug use)
Viridans streptococci (S.
Organism Staph aureus
mitis, S. sanguinis)
© 2025 TheMatchGuy LLC. All rights reserved. Page 3
, Microbiology facts of Viridans streptococci (e.g., S. mitis, S. sanguinis):
➞ Alpha-hemolytic, optochin-resistant, bile-insoluble
➞ Prophylaxis is required before dental work in high-risk patients with Amoxicillin.
uy
h G
atc
Created with BioRender.com
M
6. A 65-year-old man is diagnosed with subacute endocarditis. Blood cultures grow Gram-
positive cocci in chains. Further evaluation reveals a mass in his colon. What is the
e
organism?
h
➔ Streptococcus gallolyticus (Endocarditis + colon cancer ➞ Think S. gallolyticus)
T Note: Part of normal gut flora; enters bloodstream via mucosal breaks (e.g., tumor)
Causes subacute endocarditis with strong link to colon cancer or polyps
If found ➞ always screen for colorectal cancer
7. A 6-year-old unvaccinated child presents with sore throat, low-grade fever, and a thick
gray coating on the pharynx. He later develops arrhythmia and signs of heart failure.What is
the causative organism?
➔ Corynebacterium diphtheriae (It produces a toxin that inhibits EF-2 ➞ leading to
pseudomembrane formation and myocarditis)
Note: Myocarditis is major cause of death ➞ due to diphtheria exotoxin ➞ Mechanism:
Toxin inactivates EF-2, halting protein synthesis ➞ leading to tissue necrosis and ⇑
cAMP ➞ causing systemic toxicity
Prevented by DTaP vaccine (toxoid vaccine)
© 2025 TheMatchGuy LLC. All rights reserved. Page 4
High Yield USMLE Step 1 Notes
Welcome to the Microbiology High-Yield Notes for Step 1.
These are the most commonly tested topics on the exam—ideal for quick review in your last
2 weeks before the exam, when you're short on time, or when you're just starting out and
want to focus on what is high yield. Created and peer-reviewed by physicians who scored
260+ on Step 1 in a Q&A style to reinforce active learning.
How to use this file
y
Read the question ➔ cover the answer ➔ self-quiz before revealing.
u
Review the NOTES ➔ they include high-yield details you don't want to
miss.
h G Need tutoring? Scan the QR code or visit The Match Guy Tutoring.
c
Watch the video explanations that go along with this file for deeper
t
understanding.
Ma
Bacteria
e
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?
Th ➔ 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
Honey-colored crusts without Flaccid bullae that rupture
Lesion Appearance
bullae easily
© 2025 TheMatchGuy LLC. All rights reserved. Page 1
Version 1.0 May 2025. No part of this document may be reproduced or distributed without written permission. For personal study only. Medical information
herein is for educational purposes and does not constitute clinical advice. Always confirm with current guidelines and your institution’s policies.
, 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.
uy
Differential Diagnosis:
Condition
h G Key Features Toxin/Cause
c
➞ Neonates with tender erythema,
t
superficial desquamation Exfoliative toxin (Staph
SSSS
➞ no mucosal involvement
a
aureus)
➞ +Nikolsky
M
➞ Drug-induced (e.g., sulfa,
anticonvulsants)
e
Toxic Epidermal Delayed type IV
➞ mucosal involvement and full-
Necrolysis (TEN) hypersensitivity
h
thickness epidermal necrosis
➞ +Nikolsky
TToxic Shock
Syndrome (TSS)
➞ High fever, hypotension, diffuse rash ➞
desquamation (esp. palms/soles),
mucosal hyperemia (e.g., strawberry
tongue
➞ hx of tampon/nasal packing
TSST-1 (superantigen
from Staph aureus)
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
© 2025 TheMatchGuy LLC. All rights reserved. Page 2
,Compare with:
Organism Virulence Factor Function
Anti-phagocytic, molecular mimicry
Strep pyogenes (GAS) M protein
➞ rheumatic fever
Binds Fc portion of IgG ➞ inhibits
Staph aureus Protein A
opsonization & 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)
uy
Note: Rheumatic heart disease
➞ Sequelae of untreated/recurrent Group A Streptococcal (GAS) pharyngitis ➞ leading
to acute rheumatic fever ➞ causing chronic rheumatic heart disease
G
➞ Mitral valve most commonly affected ➞ leads to stenosis.
➞ Histopathology shows Aschoff bodies (perivascular granulomas with Anitschkow
h
cells)
atc
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
M
dental procedure 4 weeks ago. What is the causative organism?
➔ Streptococcus mitis (viridans group streptococci) (it can cause subacute
e
endocarditis in patients with damaged valves, especially after dental work)
h
Note: Key Features of Subacute Endocarditis:
➞ Indolent symptoms (weeks of fever, fatigue)
T ➞ 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)
Previously abnormal (e.g.
Valve type Normal or abnormal
rheumatic)
Often tricuspid regurgitation
Murmur New or changing murmur
(IV drug use)
Viridans streptococci (S.
Organism Staph aureus
mitis, S. sanguinis)
© 2025 TheMatchGuy LLC. All rights reserved. Page 3
, Microbiology facts of Viridans streptococci (e.g., S. mitis, S. sanguinis):
➞ Alpha-hemolytic, optochin-resistant, bile-insoluble
➞ Prophylaxis is required before dental work in high-risk patients with Amoxicillin.
uy
h G
atc
Created with BioRender.com
M
6. A 65-year-old man is diagnosed with subacute endocarditis. Blood cultures grow Gram-
positive cocci in chains. Further evaluation reveals a mass in his colon. What is the
e
organism?
h
➔ Streptococcus gallolyticus (Endocarditis + colon cancer ➞ Think S. gallolyticus)
T Note: Part of normal gut flora; enters bloodstream via mucosal breaks (e.g., tumor)
Causes subacute endocarditis with strong link to colon cancer or polyps
If found ➞ always screen for colorectal cancer
7. A 6-year-old unvaccinated child presents with sore throat, low-grade fever, and a thick
gray coating on the pharynx. He later develops arrhythmia and signs of heart failure.What is
the causative organism?
➔ Corynebacterium diphtheriae (It produces a toxin that inhibits EF-2 ➞ leading to
pseudomembrane formation and myocarditis)
Note: Myocarditis is major cause of death ➞ due to diphtheria exotoxin ➞ Mechanism:
Toxin inactivates EF-2, halting protein synthesis ➞ leading to tissue necrosis and ⇑
cAMP ➞ causing systemic toxicity
Prevented by DTaP vaccine (toxoid vaccine)
© 2025 TheMatchGuy LLC. All rights reserved. Page 4
