Microbiology · USMLE Step 1

USMLE Microbiology: Bugs and Diseases I

Bacteria and viruses (gram-positive/negative, key virus families) matched to the disease they cause and one distinguishing lab feature.

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What gram-positive coccus is catalase-positive and coagulase-positive, causing skin infections and toxic shock syndrome?
Staphylococcus aureus
Which gram-positive coccus is beta-hemolytic, catalase-negative, and causes pharyngitis and scarlet fever?
Streptococcus pyogenes (Group A Streptococcus)
Streptococcus pneumoniae is gram-positive, alpha-hemolytic, and optochin-sensitive. What two serious infections does it cause?
Pneumonia and meningitis
Enterococcus species grow in 6.5% NaCl and are PYR-positive gram-positive cocci. Name one infection they cause.
Urinary tract infection (UTI) or endocarditis
Bacillus anthracis is a gram-positive, spore-forming rod. What is the disease it causes?
Anthrax (cutaneous, pulmonary, or gastrointestinal form)
Listeria monocytogenes causes meningitis in neonates and immunocompromised patients. What is its characteristic motility at cold temperatures?
Tumbling motility; grows during cold enrichment at 4°C
Clostridium tetani is an anaerobic gram-positive rod. What neurotoxin does it produce?
Tetanospasmin, causing muscle rigidity and lockjaw
Clostridium botulinum produces a neurotoxin causing flaccid paralysis. What is this condition called?
Botulism
Which anaerobic gram-positive rod produces toxins A and B, causing pseudomembranous colitis?
Clostridium difficile
Neisseria meningitidis is a gram-negative diplococcus. Is the oxidase test positive or negative, and what is the primary disease?
Oxidase-positive; causes meningitis and meningococcal septicemia
Neisseria gonorrhoeae is oxidase-positive and causes what sexually transmitted infection?
Gonorrhea (urethritis, cervicitis, or pelvic inflammatory disease)
E. coli is a gram-negative rod that ferments lactose and tests oxidase-negative. Name two infections it causes.
Urinary tract infection and gastroenteritis
Klebsiella pneumoniae forms mucoid colonies on agar and ferments lactose. What is its primary clinical presentation?
Pneumonia; also urinary tract infection and sepsis, especially in hospitalized patients
Proteus mirabilis is urease-positive and produces swarming motility on agar plates. What infection does it cause?
Urinary tract infection; often associated with kidney stone formation due to urease
Pseudomonas aeruginosa is oxidase-positive and produces a distinctive green pigment. Name one type of infection it causes.
Wound infection, respiratory infection in cystic fibrosis, or nosocomial sepsis

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