Salmonella Typhi Human Challenge With Neutralised Gastric Acid
- Study type:
- Human challenge experiments in healthy volunteers who ingested defined doses of Salmonella Typhi after neutralisation of gastric acid.
- Topic:
- Experimental typhoid fever model in human volunteers
- Added:
- 2026-02-12
- Status:
- published
Strength of evidence
🔴 Very low – small, unblinded human challenge experiments with artificially neutralised gastric acid and no true null control group; results are not representative of natural exposure conditions and cannot establish real-world causality.
Source
Classic typhoid challenge experiments such as: Hornick RB et al. Typhoid fever: pathogenesis and immunologic control. N Engl J Med. 1970;283:686–691; and related Salmonella Typhi human volunteer studies from the 1960s–1970s.
In several classic human challenge experiments, small groups of healthy volunteers ingested defined doses of live Salmonella Typhi bacteria mixed with a liquid vehicle after pre‑treatment with sodium bicarbonate to neutralise gastric acid. A dose–response relationship was observed: higher inocula led to more frequent febrile illnesses that resembled typhoid fever, with positive blood or stool cultures in many symptomatic subjects. There was no true control group that received the same vehicle and bicarbonate but no bacteria, and neutralising gastric acid created an artificial exposure condition. From a WorthDay perspective, these studies demonstrate that large doses of Salmonella Typhi can cause illness when natural barriers are suppressed, but they provide only very low‑quality evidence about infection risk under normal dietary conditions.
Key findings
- Healthy adult volunteers were pre‑treated with sodium bicarbonate or other antacids to neutralise gastric acid and then asked to drink a liquid containing known doses of live Salmonella Typhi.
- The experiments reported a dose–response pattern: higher bacterial doses produced typhoid‑like illness (fever, headache, abdominal symptoms) more often than lower doses.
- A substantial proportion of volunteers at higher doses developed symptoms and positive blood or stool cultures for Salmonella Typhi, which resolved with or without antibiotic treatment.
- No true null control group was included: there was no arm receiving vehicle plus bicarbonate without bacteria, so background rates of fever and gastrointestinal symptoms in the same setting are unknown.
- The route and conditions of exposure (large bolus dose after strong antacid) do not correspond to normal food consumption in everyday life, where gastric acid is intact and microbial loads are lower.
- The experiments were unblinded and involved small numbers of volunteers, with limited systematic follow-up on long-term outcomes.
Background
- Human typhoid challenge experiments were designed in the mid‑20th century to study pathogenesis and test potential vaccines against Salmonella Typhi. Researchers wanted to know how many bacteria were required to produce clinical typhoid fever in healthy adults, and whether prior vaccination reduced the risk.
Methods
- In typical protocols, healthy male volunteers were given oral sodium bicarbonate or other antacid to neutralise gastric acid, then exposed to a known quantity of live Salmonella Typhi suspended in a drink (e.g. milk or water). Different groups received different inoculum sizes. Participants were followed for fever, clinical symptoms and microbiological evidence of infection (blood and stool cultures), usually without blinding. All volunteers were under close medical supervision and could receive antibiotics if severe illness developed.
Results
- The studies reported a clear dose–response: at low bacterial doses, relatively few volunteers developed significant illness; at higher doses, a majority developed febrile, typhoid‑like disease with positive cultures. The incubation period was typically several days. Symptoms included fever, malaise, headache and gastrointestinal complaints. Some cases required antibiotic treatment; most volunteers recovered. However, there was no arm in which volunteers received the same drink and bicarbonate but no bacteria, so it is impossible to know how many episodes of fever or diarrhoea might have occurred spontaneously under the same conditions. The deliberate neutralisation of gastric acid greatly increased susceptibility, making the experiments a stress‑test of host defences rather than a realistic model of typical food exposure.
Interpretation
- From a conventional infectious-disease perspective, these challenge experiments are cited as proof that Salmonella Typhi is a human pathogen capable of causing typhoid fever when ingested. From the WorthDay perspective, the evidence is far weaker: the experiments show that large bolus doses of bacteria bypassing normal gastric acid barriers can make people ill, but they do not quantify risk under natural dietary conditions, lack negative and null controls, and are small and unblinded. For claims about everyday 'food-borne infection', their evidential weight is therefore very low.
Limitations
- • Artificial exposure conditions: gastric acid was deliberately neutralised with sodium bicarbonate or antacids, which is not representative of normal physiology.
- • No true null control group: there was no arm receiving vehicle + bicarbonate without bacteria, so background rates of fever and gastrointestinal symptoms are unknown.
- • Small sample sizes and lack of blinding: experiments involved relatively few volunteers and were not blinded, which may bias symptom reporting and clinical management.
- • Limited outcome scope: endpoints focused on short-term illness and culture positivity; long-term health effects and broader morbidity were not systematically assessed.
- • Unclear generalisability: results from healthy male volunteers under experimental conditions cannot be directly extrapolated to diverse populations and real-world exposures.
- • Ethical and methodological constraints mean that replication under modern standards is unlikely, leaving many methodological questions unresolved.
WorthDay perspective
- For WorthDay, S0038 illustrates how classic human challenge experiments can be both dramatic and methodologically weak. They show that if you neutralise gastric acid and administer a large bolus of Salmonella Typhi, many volunteers will develop illness, but they do not tell us how likely disease is under everyday eating conditions, nor do they include proper negative or null controls. The evidence quality for real-world causality is therefore very low.
- In the Health category, this entry helps users see the gap between experimental stress-tests and natural exposure. It is particularly relevant when evaluating narratives that present such challenge data as definitive proof of food-borne 'infection'. WorthDay can use S0038 together with S0039 (E. coli NIZO challenge) to explain why methods like antacid pre-treatment and universal exposure arms severely limit what we can conclude about real-life risk.
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