Multiple Choice: Which of the following characteristics are common to Shigella spp. and Salmonella enterica serovar Typhi?

Question

Multiple Choice: Which of the following characteristics are common to Shigella spp. and Salmonella enterica serovar Typhi?

BlackTom AI · Accepted Answer

Both Shigella spp. and Salmonella enterica serovar Typhi are human-restricted pathogens that spread primarily by the fecal–oral route, so humans serve as the reservoir and person-to-person transmission is the dominant mode of spread.

Relevant concepts:
- Reservoir: the organism’s natural host population where it persists. Many foodborne pathogens are zoonotic (animals are reservoirs), but some are human-restricted.
- Transmission: fecal–oral spread can occur through contaminated food/water or directly between people (hand-to-mouth).
- Epidemiology: S. Typhi and Shigella have adaptations for human infection and transmission dynamics (e.g., chronic human carriers for S. Typhi; low infectious dose and efficient person-to-person spread for Shigella).

Step-by-step reasoning:
1. S. Typhi: obligate human pathogen. It does not have a known animal reservoir; chronic human carriers (gallbladder colonization) can shed bacteria and transmit infection (classic “Typhoid Mary” concept).
2. Shigella spp.: also essentially human-restricted. Transmission is often direct person-to-person (e.g., within households, day-care centers) because of a very low infectious dose and survival on hands/fomites.
3. Compare to most foodborne pathogens (e.g., non-typhoidal Salmonella, Campylobacter, E. coli O157) which commonly have animal reservoirs (poultry, cattle, etc.) and typically cause outbreaks linked to contaminated animal-derived foods.
4. Therefore the shared characteristic is that humans are the primary reservoir and human-to-human spread is important.

Why the correct answer is correct:
- Epidemiologic and microbiologic evidence confirms both organisms rely on humans as reservoirs and frequently spread directly between humans, distinguishing them from many zoonotic foodborne agents.

Why other plausible statements would be incorrect:
- Statements claiming animal reservoirs or identical clinical syndromes are incorrect: S. Typhi causes systemic typhoid fever (with chronic carriage), while Shigella causes localized inflammatory dysentery; S. Typhi is motile (flagellated) whereas Shigella is non-motile—so not all features are shared.

Conclusion: The correct answer—“In contrast to most other foodborne pathogens, humans are the reservoir (human-human spread)”—is supported and confirmed.

Multiple Choice: Which of the following characteristics are common to Shigella spp. and Salmonella enterica serovar Typhi?单项选择题

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