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Have you ever heard of the infamous Typhoid Mary, a cook responsible for spreading disease to over a thousand unlucky diners? In this lesson, we will learn about typhoid fever and the causative organism Salmonella enterica serovar Typhi.

The First Bioterrorist

The Northeastern United States was a dangerous place to live in the early 1900s if you were in the market for a live-in cook. Eerily similar to the plot from a medical thriller, a bioterrorist was moving around the region, taking brief jobs as a cook and infecting her employers and their families with a deadly disease before changing her name and starting all over again somewhere new.

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This bioterrorist not only knew she was infecting people, but she stubbornly, and often violently, refused to stop. She was eventually caught and thrown in jail, but her stay was short-lived. After three years in isolation, she was able to gain enough sympathy from the public that officials agreed to release her in exchange for the assurance she would no longer serve as a cook.The terrorist swore she had learned her lesson and would not only stop cooking, she would check in with the authorities frequently and regularly. Upon her release, she immediately disappeared, only to be found several years later up to her old tricks: cooking, infecting and killing.

This time, the authorities were less lenient. The terror was thrown in jail for the next 25 years, until her death. All told, she infected around 1,300 people in her lifetime, but officially, only three deaths were conclusively linked to her, though some estimates put that number above 50.This is the story of Typhoid Mary, and it is 100% true. Mary Mallon was a cook that was a confirmed carrier of typhoid fever.

She knew she was infecting people, she violently resisted arrest and refused a cure, and she belligerently kept cooking despite the warnings and deaths. Mary Mallon wasn’t necessarily a bad person, she just didn’t fully understand the consequences of her actions and refused to give up her livelihood.What is most startling is that Mary wasn’t unique; any person could find themselves a serial carrier and distributor of infection, even today. So it is better to educate yourself now, so the next typhoid fill-in-the-blank doesn’t include your name.

Salmonella Enterica Serotype Typhi

Typhoid Mary was infected by Salmonella enterica, a Gram-negative, motile, facultative aerobic, rod-shaped bacteria. The species Salmonella enterica is split into many subspecies, also called serotypes.

S. enterica serovar Typhi is the serotype that causes typhoid fever.’Gram-negative’ refers to Salmonella having a cell wall with a thin peptidoglycan layer and an outer membrane, making these cells appear red in the classic Gram stain. Being a facultative aerobe, Salmonella can survive with or without oxygen. Humans are the only host for S. enterica serovar Typhi, which lives in the anaerobic human small intestine, blood and gall bladder.

Outside of the host, the bacteria are able to survive in the oxygen-rich environment and are very resistant to environmental stresses. These bacteria are most often found in water, sewage and foods.People are exposed to S. enterica serovar Typhi by the fecal-oral route. Typhoid Mary was infecting her victims by serving them food she had touched with her hands contaminated by her own fecal material.

In areas of the world with poor sanitation, this is still a frequent source of infection. Drinking and cooking water contaminated with sewage is the other major source.

Typhoid Fever

Typhoid fever is a potentially fatal foodborne infection caused by Salmonella enterica serovar Typhi. Remember, a foodborne infection is a disease that results from ingesting actively growing bacteria. The symptoms are a direct result of the growth and metabolism of the bacteria in the body.Typhoid Mary was serving Salmonella to her employers. After they consumed the bacteria, the microbes passed through the stomach, where they are protected from the stomach acid by the food bulk and began to invade the lining of the small intestine.

In a process not fully understood, the bacteria are able to pass through the intestinal cells and enter the lymphatic vessels without triggering any host immune response. Once in the lymphatic system, the bacteria invade the blood, spreading throughout the body. While in the bloodstream, the lipopolysaccharides on the bacterial cell wall, also called endotoxins, induce a massive host immune response, leading to the development of symptoms.Symptoms of typhoid fever include intestinal ulcers, bloody stools and abdominal pain from the initial intestinal invasion.

This is followed by a high fever, often up to 104 degrees Fahrenheit, lethargy and delirium. These symptoms represent the massive immune response to endotoxin. About 30% of patients will develop a faint rash on their chest, called ‘rose spots.’ These symptoms can last for three to four weeks.

Without treatment, most people will recover, but between 15 and 20% will die.About 5% of those people that recover become a chronic carrier of S. enterica serovar Typhi, with the bacteria hiding in the gallbladder.

These people shed the bacteria for years without showing any symptoms themselves. In the case of Typhoid Mary, there is no evidence that she ever developed any symptoms with her initial exposure, which might help explain why she was so resistant to changing her lifestyle.She also refused to have her gallbladder removed, which would likely have stopped her from infecting others. This is where her legacy comes into play. Today, chronic carriers are required to register and retest with the health department regularly.

These people are legally barred from working in any food service or childcare industry to try and limit their infection potential.

Diagnosis and Treatment

Typhoid Mary was diagnosed with Salmonella by a direct microscopic examination of her feces, which was teeming with Salmonella bacteria. Today, blood cultures are performed.

A sample of blood is spread on a growth medium, and any visible colonies are microscopically examined to identify the bacteria. This is always combined with a patient history to determine if the person has eaten anything suspect or traveled to a typhoid fever hotspot.The United States has about 5,500 cases of typhoid a year, but about 85% of those are people who have recently returned from trips to areas where typhoid is more common. Worldwide, typhoid infects about 21 million people a year. Without treatment, 15-20% of infected people will die.

With treatment, the number drops to about 1%. The antibiotic ceftriaxone has been very successful recently, but resistant strains of S. enterica serovar Typhi have been popping up more frequently.


Typhoid fever has nearly been eliminated in the United States due mostly to water sanitation and monitoring of chronic carriers. For people traveling to areas with poor sanitation or no water treatment, several vaccines are available.

These vaccines are only about 50-80% effective, so other measures should be taken while traveling.Before you eat or drink anything in a typhoid risk area, the Centers for Disease Control and Prevention suggest you follow this philosophy: Boil it, cook it, peel it or forget it. If you can’t personally boil the liquid, cook the food or peel the fruits and vegetables, you are much better off not consuming it. Drinks are much safer if they are bottled and carbonated. You should always pass on the ice, which could be contaminated itself. As always, make sure you practice good personal hygiene.

Lesson Summary

It’s time to review. Salmonella enterica serovar Typhi is a Gram-negative, motile, facultative aerobic, rod-shaped bacteria that causes the foodborne infection typhoid fever. The bacteria only infects humans and is acquired by the fecal-oral route. The most common sources of infection include sewage-contaminated water and foods contaminated by human handlers.Once eaten, the bacteria pass through the walls of the small intestine into the lymphatic system before invading the bloodstream.

The resulting immune response causes fever, lethargy and delirium, lasting for three to four weeks. Most people will recover, but 5% will remain a chronic carrier, spreading the bacteria for up to several years. In the U.S.

, these carriers are required to maintain contact with the local health department and are barred from working in food service or childcare.Treatment involves the antibiotic ceftriaxone, and its use reduces fatalities from 20% to about 1%. Fortunately, typhoid fever is not common in the United States. People traveling to areas where typhoid is common can get vaccinations to prevent infection but should still be vigilant and avoid eating or drinking anything that appears risky.

Learning Outcome

Once you have seen this video lesson, you should be able to:

  • Describe the characteristics of Salomella enterica seroyar Typhi
  • Understand the signs and symptoms of typhoid fever
  • List the available treatments
  • Explain how to prevent infection

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