Microbiology · USMLE Step 1

USMLE Microbiology: Bacteria Classification

Bacterial classification by gram stain, shape, and growth requirements, with the key lab test used to identify each organism.

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What is the Gram stain and cellular arrangement of Staphylococcus?
Gram-positive cocci in clusters
What is the Gram stain and cellular arrangement of Streptococcus?
Gram-positive cocci in chains
Which lab test distinguishes Staphylococcus from Streptococcus, based on breakdown of hydrogen peroxide?
Catalase test (Staphylococcus is catalase-positive)
Is Streptococcus catalase-positive or catalase-negative?
Catalase-negative
Which coagulase test result identifies Staphylococcus aureus among staphylococci?
Coagulase-positive
Which coagulase-negative staphylococcus is novobiocin-sensitive?
Staphylococcus epidermidis
Which coagulase-negative staphylococcus is novobiocin-resistant?
Staphylococcus saprophyticus
Which streptococcus species is optochin-sensitive?
Streptococcus pneumoniae
Which alpha-hemolytic streptococcal group is optochin-resistant?
Viridans streptococci
Which streptococcus is bacitracin-sensitive?
Group A Streptococcus (Streptococcus pyogenes)
Which streptococcus is bacitracin-resistant?
Group B Streptococcus (Streptococcus agalactiae)
Which streptococcus gives a positive CAMP test?
Group B Streptococcus (Streptococcus agalactiae)
What does alpha hemolysis look like on blood agar?
Partial hemolysis with a greenish discoloration
What does beta hemolysis look like on blood agar?
Complete clearing around the colony
What does gamma hemolysis look like on blood agar?
No hemolysis

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