microbiology test bank

Correct Answer: A

Rationale: The correct answer is A: Streptococcus pneumoniae. The given characteristics match this bacterium: Gram-positive diplococci, catalase-negative, and alpha-hemolytic. S. pneumoniae is a common cause of community-acquired pneumonia. Other choices are incorrect because Staphylococcus aureus is catalase-positive, Enterococcus faecalis is not alpha-hemolytic, and Klebsiella pneumoniae is not Gram-positive diplococci.

Correct Answer: B

Rationale: The correct answer is B because the structure of the cytoplasmic membrane in gram-positive and gram-negative bacteria is fundamentally different. Gram-positive bacteria have a single-layered cytoplasmic membrane, while gram-negative bacteria have a double-layered cytoplasmic membrane with an additional outer membrane. This key difference is crucial for various functions such as antibiotic resistance and nutrient transport. Rationale for the incorrect choices: A: Gram-positive bacteria have a single-layered cytoplasmic membrane, not a double-layered one. C: Prokaryotes do have a cytoplasmic membrane, which is essential for maintaining cell integrity and regulating the passage of molecules. D: Gram-negative bacteria have a double-layered cytoplasmic membrane, not a three-layered one.

Correct Answer: C

Rationale: The correct answer is C because diphtheria exotoxin is absorbed into the underlying healthy tissues and spreads throughout the body. This is due to the toxin's ability to enter the bloodstream and target various organs, causing systemic effects. Choice A is incorrect as the toxin does not stay localized at the site of infection. Choice B is incorrect as diphtheria exotoxin does not directly affect neuro-muscular transmission. Choice D is also incorrect as the toxin does spread throughout the body.

Correct Answer: B

Rationale: The correct answer is B: Motility. The bacterial flagellum is a whip-like appendage that helps bacteria move by rotating like a propeller. This motility allows bacteria to navigate their environment, find nutrients, and avoid harmful conditions. Choices A, C, and D are incorrect because the bacterial flagellum is not involved in protein synthesis, DNA replication, or spore formation. Protein synthesis occurs in ribosomes, DNA replication takes place in the nucleus or nucleoid, and spore formation is a specialized process involving the production of spores for survival in harsh conditions.

Correct Answer: A

Rationale: The correct answer is A: Candida fungi. Candida fungi are known to cause oral thrush, characterized by whitish spots on the oral mucosa. The presence of gram-positive oval budding cells in the smear preparations is indicative of Candida species. Staphylococci (choice B) are gram-positive cocci, not oval budding cells. Sarcinae (choice C) are gram-positive bacteria that form cuboidal packets, not oval budding cells. Actinomycetes (choice D) are filamentous gram-positive bacteria, not oval budding cells like Candida fungi.