Microbiology Usmle step 1 Exam questions with correct solutions

Microbiology Usmle step 1




Gram-positive organisms - answer____ have a thick peptidoglycan layer and teichoic
acid in their cell walls

Gram-negative organisms - answer____ have a thin peptidoglycan layer and a complex
outer membrane containing lipopolysaccharide (LPS).

Lipoteichoic acid - answer____ is a teichoic acid attached to a lipid abundant on Gram-
positive bacteria cell wall.

Peptostreptococci - answerClassification:
-- Anaerobic gram-positive cocci

Characteristic:
-- A normal flora of the mouth and colon.

-- This organism forms abscesses in patients with alcoholism and seizure disorder

Fusobacterium species - answerClassification:
-- Anaerobic gram-negative rods microorganism

Characteristic:
-- It forms abscesses in patients with alcoholism and seizure disorder. They are part of
the human normal flora of the mouth, colon, and female genital tract.

Staph. epidermidis - answerClassification:
-- Gram-Positive Cocci, Novobiocin sensitive.

-- Catalase positive, Coagulase-negative

Characteristic:
-- This organism infects prosthetic devices and intravenous catheters by producing
adherent biofilms. A compound of normal skin and nose flora.

,Staph. saprophyticus - answerClassification:
-- Gram-Positive Cocci, Novobiocin resistant

-- Catalase positive, Coagulase-negative

Characteristic:
-- The Second most common cause of uncomplicated UTI in young women.

Strep. pneumoniae - answerClassification:
-- A lancet-shaped encapsulated gram-positive diplococci. (Alpha hemolytic)

-- Optochin sensitive, Catalase negative.

Characteristic:
-- It causes symptoms of meningitis, otitis media (in children), pneumonia, sinusitis, and
rusty color sputum.

-- Also, secrete IgA protease

Viridans streptococci - answerClassification:
-- Gram-Positive Cocci (alpha hemolytic)

- Optochin resistant, Catalase negative.

Characteristic:
-- It is a normal flora of the oropharynx
that affects dental caries (S. Mutans) and causes subacute bacterial endocarditis at
damaged valves (S.sanguinis).

Strep. pyogenes - answerClassification:
-- Gram-Positive Cocci (beta-hemolytic)

-- Bacitracin sensitive, catalase negative

Characteristic:
-- Secretes M protein to protect itself from phagocytosis.

-- It may present with pharyngitis, cellulitis, necrotizing fasciitis, and acute
glomerulonephritis.

Strep. agalactiae (B group) - answerClassification:
-- Gram-Positive Cocci (beta-hemolytic)

--Bacitracin resistant, catalase negative.

, Characteristic:
-- It causes pneumonia, meningitis, and sepsis in babies.

Strep. bovis - answerClassification:
-- Gamma hemolytic microorganism

-- Gram-Positive Cocci that colonized the gut.

Characteristic:
-- It also causes subacute endocarditis in colon cancer patients.

Straph. aureus - answerClassification:
-- Gram-Positive Cocci

-- Catalase positive and coagulase positive organism.

Characteristic:
-- This organism is responsible for bringing toxic shock syndrome, scalded skin
syndrome (exfoliative toxin), food poisoning, and acute endocarditis (IV drug users), and
hepatic abscess.

Enterococcus faecalis, faecium (Streptococci) - answerClassification:
-- Gram-Positive Cocci

Characteristic:
-- Group D streptococci that cause endocarditis after genitourinary procedures.

Corynebacterium diphtheriae - answerClassification:
-- Gram-Positive Rod or Bacilli

-- Inactivate elongation factor (EF-2)

Characteristic:
-- Symptoms are pseudomembranous pharyngitis and severe cervical lymphadenopathy
(bull neck).

Clostridium tetani - answerClassification:
-- Gram-Positive Rod or Bacilli

Characteristic:
-- This organism secretes toxins that cleave SNARE protein and prevents the release of
inhibitory neurotransmitters in the Renshaw cell (inside the spinal cord).

-- Symptoms are spasticity and lockjaw.

Clostridium botulinum - answerClassification: