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A wound culture grew Gram-negative rods that were oxidase-positive and lactose non-fermenters. What is the most likely microorganism?
- A. Pseudomonas aeruginosa
- B. Escherichia coli
- C. Salmonella typhi
- D. Shigella dysenteriae
Correct Answer: A
Rationale: The correct answer is A: Pseudomonas aeruginosa. P. aeruginosa is a Gram-negative rod that is oxidase-positive and lactose non-fermenting. The positive oxidase test indicates the presence of cytochrome c oxidase, a characteristic of P. aerugin. E. coli is lactose fermenting. Salmonella and Shigella are also Gram-negative rods, but they are lactose fermenters, unlike the lactose non-fermenting P. aeruginosa.
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Which of the following bacteria is known for causing the disease tetanus?
- A. Clostridium tetani
- B. Clostridium perfringens
- C. Streptococcus pyogenes
- D. Escherichia coli
Correct Answer: A
Rationale: The correct answer is A: Clostridium tetani. This bacterium is known for causing tetanus due to its ability to produce tetanospasmin, a neurotoxin that leads to the characteristic muscle stiffness and spasms seen in tetanus. Clostridium perfringens (Choice B) is associated with gas gangrene, Streptococcus pyogenes (Choice C) causes streptococcal infections, and Escherichia coli (Choice D) is known for causing various gastrointestinal infections. Therefore, Choices B, C, and D are incorrect as they are not the causative agents of tetanus.
A patient's sputum was stained using Ziehl-Neelsen method, which revealed red-colored bacilli on a blue background. What structural component of the cell is responsible for this staining property?
- A. Mycolic acid
- B. Capsule
- C. Flagella
- D. Endospores
Correct Answer: A
Rationale: The correct answer is A: Mycolic acid. Ziehl-Neelsen method is used to stain acid-fast bacteria, such as Mycobacterium species. Mycolic acid in the cell wall of these bacteria resists decolorization, leading to red-colored bacilli on a blue background. Capsule, flagella, and endospores do not play a role in this staining property. Capsules are usually stained using special techniques, flagella are not typically visible with this stain, and endospores are stained using the Schaeffer-Fulton method.
What reaction is catalyzed by a beta-lactamase enzyme
- A. the final cross-linking reaction to form the bacterial cell wall
- B. the biosynthesis of the penicillin structure from the amino acid valine and cysteine
- C. the hydrolysis of the four-membered ring present in penicillin
- D. the hydrolysis of the acyl side chain from penicillin structures
Correct Answer: C
Rationale: The correct answer is C: the hydrolysis of the four-membered ring present in penicillin. Beta-lactamase enzyme catalyzes the hydrolysis of the four-membered beta-lactam ring present in penicillin, rendering the antibiotic ineffective against bacterial cell walls. This enzyme breaks the bond in the beta-lactam ring, thus inactivating the antibiotic.
Incorrect choices:
A: the final cross-linking reaction to form the bacterial cell wall - This is incorrect as beta-lactamase does not participate in the formation of bacterial cell walls.
B: the biosynthesis of the penicillin structure from the amino acid valine and cysteine - This is incorrect as beta-lactamase is not involved in the biosynthesis of penicillin.
D: the hydrolysis of the acyl side chain from penicillin structures - This is incorrect as beta-lactamase specifically targets the beta-lactam ring, not the
If skin-allergic samples for tuberculosis are negative:
- A. Antibiotics are prescribed
- B. Tuberculin is introduced
- C. A vaccine is administered
- D. None of the above
Correct Answer: B
Rationale: Step-by-step rationale for why choice B (Tuberculin is introduced) is correct:
1. Tuberculin test helps identify individuals with latent TB infection.
2. Negative skin-allergic samples suggest absence of active TB.
3. Introducing tuberculin can help confirm latent TB infection.
4. Antibiotics are not prescribed for negative skin-allergic samples.
5. Vaccines are not administered for TB diagnosis.
Summary:
- Choice A is incorrect as antibiotics are not prescribed for negative skin-allergic samples.
- Choice C is incorrect as vaccines are not used for TB diagnosis.
- Choice D is incorrect as introducing tuberculin is the appropriate step in this scenario.
Microscopy of sputum from a tuberculosis patient revealed acid-fast rods stained red using the Ziehl-Neelsen method. What structural feature allows this staining?
- A. Mycolic acid in the cell wall
- B. Capsule surrounding the cell
- C. Flagella on the cell surface
- D. Thick peptidoglycan layer
Correct Answer: A
Rationale: The correct answer is A: Mycolic acid in the cell wall. Mycolic acid is a unique component of the cell wall of Mycobacterium species, including the causative agent of tuberculosis. The Ziehl-Neelsen method uses acid-fast staining, where mycolic acid confers resistance to decolorization by acid-alcohol, causing the bacteria to retain the red stain. Choices B, C, and D are incorrect as capsules, flagella, and peptidoglycan layers do not directly contribute to acid-fast staining characteristic of Mycobacterium species.