Nokea
A patient with bacterial endocarditis had Gram-positive cocci in chains isolated from blood cultures. The organism was catalase-negative. What is the most likely causative agent?
- A. Streptococcus pyogenes
- B. Staphylococcus aureus
- C. Enterococcus faecalis
- D. Micrococcus luteus
Correct Answer: A
Rationale: The correct answer is A: Streptococcus pyogenes. This is because Streptococcus pyogenes is a Gram-positive cocci in chains bacterium that is catalase-negative, making it the most likely causative agent in this scenario. Streptococcus pyogenes is a common pathogen causing bacterial endocarditis.
Summary of other choices:
B: Staphylococcus aureus is a Gram-positive cocci bacterium but is catalase-positive, so it is unlikely in this case.
C: Enterococcus faecalis is a Gram-positive cocci bacterium, but it is catalase-negative, which is not consistent with the given information.
D: Micrococcus luteus is a Gram-positive cocci bacterium but is catalase-positive, making it an unlikely causative agent.
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Spheroplasts are bacteria with defective cell walls which:
- A. Have parietal and defective peptidoglycan
- B. Have defective cytoplasmic membrane
- C. Have defective outer membrane
- D. Do not have peptidoglycan
Correct Answer: A
Rationale: The correct answer is A because spheroplasts are bacteria with defective cell walls that have a parietal layer (peptidoglycan layer) and a defective peptidoglycan layer. This is because spheroplasts have lost their rigid cell wall structure, leading to an incomplete peptidoglycan layer. Choice B is incorrect as the cytoplasmic membrane is not directly affected by the loss of cell wall integrity. Choice C is incorrect because spheroplasts do not have an outer membrane defect, but rather a defect in the peptidoglycan layer. Choice D is incorrect because spheroplasts still have peptidoglycan, even though it is defective.
The Ziehl-Neelsen stain is used to detect:
- A. Gram-positive bacteria
- B. Acid-fast bacteria
- C. Capsulated bacteria
- D. Fungal spores
Correct Answer: B
Rationale: The Ziehl-Neelsen stain is specifically designed to detect acid-fast bacteria like Mycobacterium tuberculosis. It utilizes a carbol fuchsin dye that binds to the high lipid content in the cell wall of acid-fast bacteria, making them appear red under a microscope. This staining method is crucial for the diagnosis of tuberculosis and leprosy. Gram-positive bacteria (Choice A) are not detected by this stain since they have a different cell wall structure. Capsulated bacteria (Choice C) and fungal spores (Choice D) also have distinct characteristics that are not targeted by the Ziehl-Neelsen stain.
The highly specialized structure that protects the fetus from microorganisms is the:
- A. uterus
- B. placenta
- C. vagina
- D. mammary gland
Correct Answer: B
Rationale: The correct answer is B: placenta. The placenta is a highly specialized structure that forms during pregnancy and serves as a barrier, protecting the fetus from harmful microorganisms that could potentially harm the developing baby. It facilitates the exchange of nutrients, oxygen, and waste between the mother and fetus while preventing the passage of most bacteria and viruses. The uterus (A) is the organ where the fetus develops, but it does not have the specific protective functions of the placenta. The vagina (C) is the birth canal and does not provide protection from microorganisms for the fetus. The mammary gland (D) is responsible for producing milk after birth and is not directly involved in protecting the fetus from microorganisms during pregnancy.
Which of the following organisms belongs in the group I category?
- A. Francisella tularensis
- B. Clostridium difficile
- C. Helicobacter pylori
- D. Bacillus anthracis
Correct Answer: C
Rationale: The correct answer is C: Helicobacter pylori. Group I organisms are classified based on their low infectious doses and potential for bioterrorism. H. pylori is known to cause gastric ulcers and is categorized as a Group I organism due to its ability to cause disease at low doses.
A: Francisella tularensis is a Group A bioterrorism agent, not Group I.
B: Clostridium difficile is not typically classified as a Group I organism.
D: Bacillus anthracis is a Group A bioterrorism agent, not Group I.
What is the main component of bacterial cell walls?
- A. Lipopolysaccharides
- B. Cellulose
- C. Peptidoglycan
- D. Chitin
Correct Answer: C
Rationale: The correct answer is C: Peptidoglycan. Bacterial cell walls are primarily composed of peptidoglycan, a unique molecule consisting of alternating sugar chains connected by peptide cross-links. This structure provides rigidity and shape to the cell wall, protecting the bacterial cell from osmotic pressure. Lipopolysaccharides (A) are found in the outer membrane of Gram-negative bacteria. Cellulose (B) is a component of plant cell walls, not bacterial cell walls. Chitin (D) is a polysaccharide found in the exoskeletons of arthropods and fungi, not in bacterial cell walls.