A patient with fever and diarrhea had a stool sample that revealed Gram-negative rods that fermented lactose. What is the most likely causative agent?

Correct Answer: A

Rationale: The correct answer is A: Escherichia coli. E. coli is a Gram-negative rod that ferments lactose. It is a common cause of gastroenteritis presenting with fever and diarrhea. Salmonella typhi does not ferment lactose and causes typhoid fever. Shigella dysenteriae causes dysentery and does not ferment lactose. Proteus vulgaris does not typically cause gastroenteritis and is not known for lactose fermentation.

Correct Answer: D

Rationale: The correct answer is D: iatrogenic infections. Iatrogenic infections refer to infections caused by medical or surgical treatment, including procedures that introduce microorganisms into the body. In this case, infections of the reproductive tract caused by medical procedures fall under this category. A: Chronic infections do not specifically refer to infections caused by medical procedures. B: Endogenous infections arise from organisms already present in or on the body, not introduced through medical procedures. C: Systemic infections affect the entire body, not specific to infections caused by medical procedures. Therefore, the term "iatrogenic infections" is the most appropriate choice in this context.

Correct Answer: C

Rationale: Nigrosin is used in negative staining, where the background is stained instead of the cells themselves. This technique helps highlight the morphology and structure of cells that may be damaged by traditional staining methods. Nigrosin is negatively charged and repels the negatively charged bacterial cells, resulting in a clear halo around the cells. This process is particularly useful for observing capsules and determining cell size and shape. The other options are incorrect because simple staining uses a single dye to color the entire cell, gram staining involves a series of dyes to differentiate between gram-positive and gram-negative bacteria, and acid-fast staining is used to detect acid-fast organisms like Mycobacterium.

Correct Answer: D

Rationale: The correct answer is D because all three bacteria listed (Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa) can produce endotoxins that can lead to septic shock. Endotoxins are lipopolysaccharides found in the outer membrane of Gram-negative bacteria like E. coli and P. aeruginosa. Staphylococcus aureus, a Gram-positive bacterium, can also produce endotoxins called superantigens that can trigger a massive immune response leading to septic shock. Therefore, all of the above choices are correct. Choices A, B, and C alone are incorrect because they do not account for the fact that all three bacteria listed produce endotoxins that can cause septic shock.

Correct Answer: A

Rationale: Correct Answer: A Rationale: 1. Enteropathogenic E. coli (EPEC) attach to enterocytes via a type III secretion system. 2. Once attached, they form attaching and effacing lesions on the microvilli, leading to microvillus destruction. 3. This mechanism allows EPEC to cause diarrhea by disrupting the normal absorptive function of the intestines. Summary: - B (They are invasive and produce enterotoxin) is incorrect because EPEC are non-invasive and do not produce enterotoxins. - C (Cause disease clinically similar to bacterial) is vague and does not provide specific information about EPEC pathogenesis. - D (None of the above) is incorrect as A is the correct explanation for EPEC pathogenicity.

Correct Answer: A

Rationale: The correct answer is A (25-37 °C) because mesophilic bacteria thrive best at moderate temperatures typical of most environments, making 25-37 °C their optimum range. This range allows for optimal enzyme activity and growth. Choice B (55-65 °C) is incorrect as it is more suitable for thermophilic bacteria. Choice C (15-18 °C) is too low for mesophiles to thrive efficiently. Choice D (38-45 °C) is too high and is more suitable for thermophiles. Ultimately, the correct answer aligns with the typical temperature range where mesophilic bacteria are most active and abundant.

Correct Answer: B

Rationale: The correct answer is B: Clostridium perfringens. In this case, the patient's presentation of infected wound with tissue crepitation, foul odor, and Gram-positive bacteria with centrally located spores that cannot grow in the presence of oxygen is indicative of gas gangrene caused by Clostridium perfringens. This anaerobic bacterium produces various toxins that lead to tissue destruction and gas production in the affected area. Clostridium tetani (choice A) causes tetanus, characterized by muscle stiffness and spasms. Clostridium botulinum (choice C) causes botulism, leading to muscle paralysis. Clostridium difficile (choice D) is associated with antibiotic-associated diarrhea. These choices are incorrect as they do not match the clinical presentation described in the question.

Correct Answer: C

Rationale: The correct answer is C. Bacterial spores are NOT used for cell division. Bacterial spores are dormant, resistant forms of bacteria that are formed in response to harsh environmental conditions. They are not involved in cell division but rather serve as a survival mechanism for the bacteria. A: They can deform the cell or not - This is true. Bacterial spores can deform the cell or not depending on the conditions. B: They can be terminal, subterminal, and central - This is true. Bacterial spores can be located at different positions within the bacterial cell. D: They are formed for 6-8h - This is incorrect. The formation of bacterial spores can take variable time periods depending on the species and environmental conditions.

Correct Answer: C

Rationale: The correct answer is C because Endo, Gassner, and Levin are all examples of differential media used in microbiology. These media contain specific components that allow for the differentiation of different types of microorganisms based on their metabolic characteristics or biochemical reactions. Lowenstein-Jensen and Petraniani (choice A) are selective media for mycobacteria, not differential. Apoholate-citrate agar and Brilliant-green phenol-red Lactose agar (choice B) are selective media for Salmonella and E. coli, respectively, not differential. Therefore, the correct choice is C as it includes examples of differential media commonly used for microbial identification.