Systemic Bacteriology

Topics in this chapter

• Gram-Positive Bacilli
• Spirochetal Infections
• Neisseria Infections
• Enteric and Gastrointestinal Pathogens
• Bacterial Pneumonia Pathogens
• Bacterial Identification Methods

Practice questions

1. Question 1Q-8EC16E

   A 35-year-old immigrant worker presents with a non-healing, painless ulcer on his hand that has a black eschar, surrounded by massive non-pitting edema. He reports working with imported goat wool. Gram stain of the wound exudate reveals large, encapsulated, gram-positive bacilli in chains resembling 'boxcars'. Which of the following is the primary virulence factor responsible for the systemic manifestations and edema in this condition?

   1. A.Exotoxin A (inhibits protein synthesis via EF-2 elongation factor)
   2. B.Tripartite toxin consisting of protective antigen, lethal factor, and edema factor
   3. C.Heat-labile enterotoxin increasing intracellular cAMP
   4. D.Cord factor (trehalose 6,6'-dimycolate) inhibiting phagolysosome fusion
   See the answer and solution
2. Question 2Q-409656

   A 24-year-old medical student develops a sore throat, low-grade fever, and malaise. Examination of the posterior pharynx reveals a dirty-gray, firmly adherent pseudomembrane that bleeds upon scraping. Cervical lymphadenopathy gives a 'bull-neck' appearance. Which laboratory finding or test is most specific for confirming the mechanism of pathogenesis of this organism?

   1. A.Elek gel diffusion test for in vitro toxin production
   2. B.Nagler reaction detecting lecithinase activity
   3. C.Quellung reaction showing capsular swelling
   4. D.CAMP test showing an arrowhead zone of hemolysis
   See the answer and solution
3. Question 3Q-E264DC

   A 68-year-old man with acute myeloid leukemia undergoing chemotherapy presents with high-grade fever and chills. Blood cultures grow gram-positive bacilli that form colonies with narrow zones of beta-hemolysis on blood agar. The organism exhibits tumbling motility at room temperature (25°C) and forms an umbrella-like pattern of motility in semisolid agar at room temperature. What is the most likely etiologic agent?

   1. A.Erysipelothrix rhusiopathiae
   2. B.Listeria monocytogenes
   3. C.Corynebacterium jeikeium
   4. D.Bacillus cereus
   See the answer and solution
4. Question 4Q-E2D9A6

   A 55-year-old chronic alcoholic develops a severe, necrotizing pneumonia characterized by 'currant jelly' sputum. Gram stain of the sputum shows gram-negative bacilli with prominent clear spaces surrounding them, indicative of a large capsule. The organism grows as mucoid, highly viscous colonies on MacConkey agar and is non-motile, lactose-fermenting, and urease-positive. Which organism is responsible?

   1. A.Pseudomonas aeruginosa
   2. B.Klebsiella pneumoniae
   3. C.Legionella pneumophila
   4. D.Streptococcus pneumoniae
   See the answer and solution
5. Question 5Q-1AF8F8

   A 22-year-old male presents with acute urethritis characterized by a purulent urethral discharge. Gram stain of the exudate reveals intracellular gram-negative diplococci within neutrophils. The organism requires specialized media containing hemin (Factor X) and NAD (Factor V) to grow, or can be cultured on Chocolate agar enriched with these factors. Wait, re-evaluating the organism causing purulent urethritis with gram-negative intracellular diplococci: what is the classic diagnostic growth requirement for Neisseria gonorrhoeae vs Haemophilus influenzae? Which growth medium is selectively used to isolate Neisseria gonorrhoeae from clinical specimens?

   1. A.Thayer-Martin medium
   2. B.Lowenstein-Jensen medium
   3. C.TCBS medium
   4. D.Buffered Charcoal Yeast Extract (BCYE) agar
   See the answer and solution