Salmonella Typhi bacteria can invade the intestines and blood and lead to a disease called typhoid fever – a disease that affects around 11 million people each year globally.
Despite the availability of know-how for improved water and sanitation, and the recent availability of new vaccines (which are on the verge of roll out), the disease still causes nearly 130,000 deaths, most of which occur in low and middle income countries (LMICs).
New research published in Nature Medicine in July 2019, led by Professor Andrew Pollard – Director of the Oxford Vaccine Group at the University of Oxford – uncovers some new insights into how the typhoid bacteria cause disease.
The mystery of the typhoid toxin
There are lots of different types of Salmonella that infect people, and many of them only affect the gut, causing diarrhoea. Salmonella Typhi (S. Typhi) is a type of the bacteria that can cause a more serious disease, leading to a blood infection called typhoid fever.
One unique factor that distinguishes S. Typhi from less harmful types of Salmonella is its ability to make a small molecule called typhoid toxin.
Bacterial toxins can play several roles that support the growth and spread of bacteria, for example helping them enter peoples’ bodies or hiding from the immune system. Researchers think that typhoid toxin must be important, but the exact role it plays is still a mystery.
Research involving animals hasn’t been able to get to the bottom of the enigma, because the disease that Salmonella causes in mice is different from the human infection. To shed more light on the toxin’s role, Professor Pollard’s team used a human infection challenge study – where volunteers are intentionally exposed to Salmonella in a safe way – to find out more about how it affects people.
The toxin isn’t critical for rapid-onset disease
The study involved 41 healthy volunteers, and the researcher who carried out the work, Dr Malick Gibani, explains how they helped uncover some important results about the toxin.
“To really understand the function of the typhoid toxin, we needed to compare bacteria that could make the toxin to bacteria that couldn’t (but were otherwise identical). So, working with our collaborators at Yale University, we took a natural strain of S. Typhi and genetically engineered it, so the bacteria couldn’t make the toxin.”
The volunteers were given a thorough explanation of what the study involved, what to expect and about any risks of taking part – after providing consent, they were given a drink containing either natural S. Typhi or the genetically engineered S. Typhi bacteria that couldn’t make the typhoid toxin.
“We used a strain that’s very sensitive to antibiotics, to make the study as safe as possible,” says Dr Gibani. “Some people didn’t have any symptoms at all. If a volunteer showed signs of developing the disease – a fever for more than 12 hours or bacteria detected in a blood sample – then they were given antibiotics to kill the bacteria. And at the end of the study everyone got antibiotics, even if they had no symptoms.”
The results were surprising. The S. Typhi bacteria lacking the typhoid toxin showed little difference from the natural strain when it came to causing disease.
“The number of people who developed signs of disease was similar, and the symptoms themselves were the same. The number of volunteers who could potentially transmit the infection was the same, as were the levels of bacteria in their poo samples.
“In short, loss of the typhoid toxin didn’t affect the ability of the bacteria to infect people, cause disease, or spread. If anything, there was a small trend towards the bacteria lacking the toxin being more harmful.”
However, there was an important clue to be uncovered from the volunteers’ blood samples. The researchers spotted differences in the immune reaction caused by infection with toxin-free S. Typhi.
The results from characterising the immune cells armed to fight the bacteria and the chemical signals they sent out suggests that the typhoid toxin might play a role in dampening down the immune reaction. This would work in the bacteria’s favour by protecting it from being killed and cleared from our systems.
Could the toxin be responsible for persistence?
The findings from this study have two main implications, according to Dr Gibani.
“It’s tempting to suggest that developing treatments or vaccines that target the typhoid toxin to protect an individual might not be a good strategy, because according to our data it won’t reduce the capacity of the bacteria to cause disease. However, it’s likely that the reality is a lot more nuanced.
“The typhoid-toxin is a very complicated molecule – a mixture of toxins found in other bacteria – and it’s hard to see why the bacteria would make the toxin if it had no role in disease. It’s very likely to be doing something in natural disease – the questions is what exactly, and what effect would neutralising it have?”
The results also hint that the role of typhoid toxin might be to protect the bacteria from our immune systems. And this could be important for tackling the persistence of disease in populations.
One of the barriers to reducing typhoid fever cases is the pool of ‘carriers’ – these are people who don’t completely clear the bacteria from their system after infection. The carriers harbour the bacteria long-term in their intestines and continue to spread the infection to others via contaminated food or water, perpetuating the disease. Yet scientists don’t understand why this persistent infection occurs.
“One theory is that the typhoid toxin might be a factor in the inability of our immune cells to eliminate the bacteria, and lead to long-term bacterial persistence,” says Dr Gibani.
“Alternatively, the toxin might have a role in some of the severe features of typhoid fever. But we can’t test either of these theories using a human infection challenge study, because it wouldn’t be safe or ethical to expose volunteers to severe symptoms or long-term infections. There might be clues in the samples that we’ve collected in the study though, and we’re busy exploring these leads.”
More research is needed to understand the precise role of the typhoid toxin and its role in disease. Shedding light on what it does could be a step towards preventing the bacteria causing severe disease, or helping reduce the spread of the infection by shrinking the number of people carrying the bacteria in their intestines. Ultimately, the goal is to prevent deaths from this widespread infection.