A patient with pneumonia had sputum stained with Gram's method, revealing purple-stained diplococci with pointed ends. What is the likely causative agent?

Correct Answer: A

Rationale: The correct answer is A: Streptococcus pneumoniae. This bacterium appears as purple-stained diplococci with pointed ends on Gram staining. S. pneumoniae is a common cause of pneumonia. It is encapsulated and has a lancet-shaped appearance, which is characteristic of the pointed ends seen in the staining. Staphylococcus aureus (B) typically appears as clusters of gram-positive cocci, not diplococci. Klebsiella pneumoniae (C) is a gram-negative rod, not a diplococci. Neisseria meningitidis (D) is a gram-negative diplococci but typically lacks the pointed ends seen in the staining of S. pneumoniae.

Correct Answer: C

Rationale: The bacterial capsule is composed of polysaccharides. This outer layer helps bacteria evade the host immune system, protect against desiccation, and aid in attachment to host cells. Polysaccharides are long chains of sugar molecules, providing a protective barrier for the bacteria. Lipopolysaccharides (choice A) are found in the outer membrane of Gram-negative bacteria and are not a major component of the capsule. Teichoic acid (choice B) is a component of the cell wall in Gram-positive bacteria, not the capsule. Peptidoglycan (choice D) is another component of the cell wall but is not part of the capsule.

Correct Answer: D

Rationale: The correct answer is D because all three bacteria - Salmonella enterica, Clostridium botulinum, and Escherichia coli - are commonly associated with foodborne illnesses. Salmonella causes gastroenteritis, Clostridium botulinum causes botulism, and E. coli can lead to severe food poisoning. Each bacterium has different mechanisms of causing illness, but all can be transmitted through contaminated food. Choices A, B, and C alone are incorrect because they do not encompass the full spectrum of bacteria commonly linked to foodborne illnesses.

Correct Answer: A

Rationale: The correct answer is A: Cell wall. In the Gram staining method, the primary differentiation of bacteria is based on the composition of their cell walls. Gram-positive bacteria have a thick layer of peptidoglycan in their cell walls, which retains the crystal violet dye and appears purple under the microscope. On the other hand, Gram-negative bacteria have a thinner layer of peptidoglycan and an outer membrane, which allows the crystal violet dye to be washed out during the decolorization step and take up the counterstain safranin, appearing pink. The different responses to stains are primarily due to the structural differences in the cell walls of these two types of bacteria. Summary: - Choice A: Cell wall is correct because it determines the differential staining in Gram staining. - Choice B: Cytoplasm is incorrect as it does not directly influence the staining properties of bacteria. - Choice C: Cytoplasmic membrane is incorrect as it is not the main structural

Correct Answer: A

Rationale: The correct answer is A: Bacillus anthracis. The presence of Gram-positive rods with central spores, catalase-positive, and aerobic growth characteristics are indicative of Bacillus anthracis. Bacillus anthracis is known for its central spores and ability to grow aerobically. Summary of why other choices are incorrect: B: Clostridium tetani is a Gram-positive rod with terminal spores and is anaerobic. C: Streptococcus pyogenes is a Gram-positive cocci and catalase-negative. D: Klebsiella pneumoniae is a Gram-negative rod and catalase-positive, but is not associated with central spores.

Correct Answer: A

Rationale: Step 1: Vibrio cholerae is a bacterium known to cause cholera, a foodborne illness transmitted through contaminated water. Step 2: Cholera outbreaks are often linked to unsanitary water sources, making Vibrio cholerae the correct choice. Step 3: Clostridium difficile causes gastrointestinal infections, not typically associated with contaminated water. Step 4: Mycobacterium tuberculosis causes tuberculosis, a respiratory disease, not related to foodborne illnesses. Step 5: Neisseria gonorrhoeae is a sexually transmitted bacterium, not linked to foodborne illnesses from contaminated water.

Correct Answer: B

Rationale: The correct answer is B: slime mold. Plasmodium is a type of slime mold, specifically a parasitic protist that causes malaria in humans. Slime molds are a group of organisms that share characteristics of both fungi and protozoa. Plasmodium does not belong to the other categories listed: A) protozoan refers to a broad category of single-celled eukaryotic organisms, C) spirochetes are a type of bacteria, and D) helminths are parasitic worms. Therefore, the correct classification for Plasmodium is slime mold.

Correct Answer: C

Rationale: The correct answer is C: To perform serodiagnostics of tuberculosis. Rationale: The negative reaction to the Mantoux test indicates absence of TB infection. Performing serodiagnostics will further confirm the absence of active infection. This is important to rule out TB before considering vaccination or isolation measures. Summary: A: BCG vaccination is not necessary as the boy shows a negative reaction to the Mantoux test. B: Repeating the test in a month is unnecessary as the initial test already showed a negative reaction. D: Isolating the boy is not warranted as there is no evidence of active TB infection.

Correct Answer: A

Rationale: The correct answer is A: Lipopolysaccharides. Endotoxins are a type of bacterial toxin found in the outer membrane of Gram-negative bacteria. They consist of lipid A, which is a lipid component, and polysaccharides, making them lipopolysaccharides. Lipoproteins (choice B) and glycoproteins (choice C) do not accurately describe the chemical structure of endotoxins. "None of the above" (choice D) is incorrect as lipopolysaccharides are indeed the chemical structure of endotoxins.