A patient with acute food poisoning had fecal cultures grown on Endo agar, resulting in colorless colonies. What microorganism is most likely responsible?

Correct Answer: A

Rationale: Step 1: Endo agar is a selective and differential media commonly used to isolate gram-negative bacteria. Step 2: Colorless colonies on Endo agar indicate the absence of lactose fermentation. Step 3: Salmonella is a gram-negative bacterium that does not ferment lactose, leading to colorless colonies on Endo agar. Step 4: Therefore, the correct answer is A: Salmonella. Summary: B: Escherichia coli ferments lactose, producing pink colonies on Endo agar. C: Staphylococcus is a gram-positive bacterium and would not grow on Endo agar. D: Shigella is a gram-negative bacterium but ferments lactose, resulting in pink colonies on Endo agar.

Correct Answer: A

Rationale: The correct answer is A: Commensalism. In this relationship, the skin microflora benefits from the host without causing harm or benefitting it. This is evident in the mutualistic interaction where the microflora receive nutrients and a place to live, while the host remains unaffected. Choice B, Infection, is incorrect because it implies harm to the host. Choice C, Parasitism, is also incorrect as it involves one organism benefiting at the expense of the other. Choice D, No correct answer, is not applicable here as Commensalism accurately describes the relationship between the macroorganism and skin microflora.

Correct Answer: D

Rationale: The correct answer is D: all of them. Malignant disease weakens the immune system, making individuals more susceptible to infections. Cytostatic therapy suppresses the immune system, increasing the risk of infections. Premature birth can lead to underdeveloped immune systems, making babies more prone to infections. Therefore, all three choices are risk factors for the development of infections. Other choices are incorrect because each individual risk factor listed contributes to an increased susceptibility to infections.

Correct Answer: B

Rationale: The correct answer is B: Insect. A vehicle transmission refers to the mode by which a disease spreads from one host to another. Insects can transmit diseases, making them a vehicle for transmission. Airborne, foodborne, and bodily fluid transmissions are well-known modes of disease spread, while insects are also known vectors for diseases like malaria and Zika.

Correct Answer: C

Rationale: The correct answer is C: Direct transfer of DNA between two bacterial cells. In bacterial conjugation, genetic material is transferred directly from one bacterial cell to another through a conjugative pilus. This process involves physical contact between the cells, followed by the transfer of plasmid DNA or chromosomal DNA. This mechanism allows for the exchange of genetic information between bacteria, leading to genetic diversity. Explanation of why other choices are incorrect: A: DNA transfer via bacteriophage - Bacteriophage is a virus that infects bacteria and transfers its genetic material, but this is not the mechanism involved in bacterial conjugation. B: Uptake of naked DNA from the environment - This process is known as transformation, where bacteria take up free DNA from the environment, but it is not specific to bacterial conjugation. D: Binary fission - Binary fission is a form of asexual reproduction in bacteria where a single cell divides into two identical cells, and it is not related to the process

Correct Answer: C

Rationale: The correct answer is C (7.2-7.4) because most pathogenic bacteria and viruses thrive in a neutral pH environment close to 7.0. This pH range is optimal for their growth and survival. Choice A (5.0-5.5) is too acidic for most pathogens. Choice B (8.0-9.6) is too alkaline, which is not conducive for their survival. Choice D (4.2-4.4) is also too acidic and would likely inhibit the growth of pathogenic bacteria and viruses. Thus, a pH range of 7.2-7.4 provides the most suitable conditions for their proliferation.

Correct Answer: A

Rationale: The correct answer is A: Acid resistance. Tuberculous bacilli are acid-fast bacilli, meaning they resist decolorization by acid-alcohol after staining with Ziehl-Neelsen stain. This property is due to the high lipid content in the cell wall of Mycobacterium tuberculosis. Alkali resistance (B) and alcohol resistance (C) are not specific properties of tuberculous bacilli. Capsule formation (D) is a characteristic of some bacteria but is not a defining feature of tuberculous bacilli.

Correct Answer: A

Rationale: The correct answer is A: Anthrax. Anthrax is caused by the bacterium Bacillus anthracis, which can form spores that are highly resistant and can survive in the soil for long periods. These spores can remain dormant until they come into contact with a susceptible host, leading to infection. Diphtheria, viral hepatitis, and pertussis are caused by different types of pathogens (bacteria and viruses) that do not have the same ability to persist in soil as anthrax spores. Therefore, the causative agents of these diseases are less likely to exist in the soil for extended periods compared to anthrax spores.

Correct Answer: C

Rationale: The correct answer is C because the case specifically mentions "enteropathogenic bacteria and viruses," indicating a focus on these specific microorganisms. Choice A is too broad and includes all bacteria, which is not specified in the case. Choice B mentions opportunistic pathogenic bacteria, which are not necessarily relevant to the case. Choice D specifies colibacilli, which may or may not be relevant to the case, but it is not as specific as choice C which clearly matches the information provided in the case.