The typhoid toxin of Salmonella Typhi - a new disease mechanism and a strategy for combatting drug-resistant typhoid and chronic carriage in humans

Antimicrobial resistance (AMR) is a defining problem facing society. This is epitomised by epidemics of typhoid fever caused by AMR lineages of the bacterial pathogen Salmonella Typhi that underlies 21 million typhoid cases and 168,000 deaths each year. For example, in Malawi, only 7% of typhoid cases were resistant to multiple drugs before 2010. By 2015, 97% of typhoid cases resisted multi-drug treatments. Thus, global organisations who lead the fight against S.Typhi, such as the Typhoid Vaccine Acceleration Consortium (TyVAC), aim to eradicate typhoid. However, there's a second problem: S.Typhi also causes chronic infections in humans without obvious clinical symptoms. These so-called 'chronic carriers' retain and transmit S.Typhi in the population. We know remarkably little about chronic carriage, which impedes global efforts to eradicate typhoid. To combat S.Typhi and develop pathogen eradication strategies, we must resolve the decisive mechanisms responsible for typhoid fever and chronic carriage.

Remarkably, the mortal symptoms of typhoid and chronic carriage are facilitated through a single S.Typhi virulence factor called the typhoid toxin, which causes damage to DNA in our cells and activates the cellular DNA damage response (DDR). How the toxin manipulates the DDR and how this mediates typhoid fever and chronic carriage is not understood.

My laboratory has made two major discoveries that have the potential to unlock the toxin mechanism and provide a new way to combat typhoid fever and chronic carriage.

We discovered that:

(i) The toxin drives infections by triggering senescence in human cells. The senescent cells released factors that induced senescence in neighbouring cells creating a domino-like effect throughout the population. To our surprise, the senescent cells have increased susceptibility to intracellular Salmonella infections, which reveals a new role for the toxin - inducing cellular senescence to drive infection.

(ii) The toxin induces senescence via a unique DDR Response Induced by a bacterial Genotoxin (RING). The RING phenotype signifies a new virulence mechanism driven by toxin-induced damage at sites of DNA replication, which underpins cell division.

The novelty and importance are clear. If the toxin facilitates typhoid fever symptoms and chronic carriage through the DDR, then we need to (i) resolve the DNA damage mechanisms underpinning the RING phenotype, and (ii) understand how senescence facilitates Salmonella infections in cells and animal models (Aims of the 4-year plan). By achieving the 4-year plan objectives, I will unveil the identity of cellular factors that are crucial to the toxin mechanism and reveal potential biomarkers that will help me address the problem of typhoid and chronic carriage in humans via my 7-year plan. This will be achieved by investigating the toxin mechanism in humans using clinical samples to reveal potential biomarkers that will help diagnosis and disease surveillance, and by screening for novel therapeutics, which counteract the toxin underlying typhoid and chronic carriage.

To maximise project impact and my leadership potential, I have designed a Fellow Development Plan that integrates with the research aims and builds towards the translational goals of the 7-year plan. The Development Plan includes: (i) experience in typhoid clinical trials and eradication programmes with TyVAC in the USA, (ii) field research on AMR S.Typhi in Vietnam, (iii) leading a typhoid-focussed research meeting in the UK, (iv) 'Take on Typhoid' public engagement experience, and (v) engaging with current leaders in the field who advise the UK government and the World Health Organisation on policy regarding typhoid.

The proposed UKRI FLF aims to contribute to global efforts against AMR S.Typhi by revealing how the toxin drives typhoid and chronic disease, which has the potential to improve human health by impacting typhoid control strategies and eradication programmes

Who will benefit from the proposed research?

1. Global health agencies and consortiums.
2. Pharmaceutical industry
3. The public and healthcare practitioners
4. Policy makers

How will they benefit from the proposed research?

1. Global health agencies and consortiums.
The potential impacts include new understanding of disease and treatments (+4-years).

The global effort against typhoid is driven in a large part by international groups including the 'International Typhoid Consortium', recently established by my external mentor Prof. Gordon Dougan, the Coalition against Typhoid (CaT) and the Typhoid Vaccine Acceleration Consortium (TyVAC) who have joined forces to 'take on typhoid'. Also, the World Health Organisation that published a new position paper on typhoid in 2018 and agencies who maintain public health via food security, e.g. Food Standards Agency (FSA).

These international groups use research to build public support and gain political momentum to bring about change and influence health policies. Their ability to prevent and control typhoid is largely reliant upon research advances in our understanding of the disease. The proposed work in the UKRI FLF application will have feed into the efforts of such global health agencies and consortiums by revealing the role of the typhoid toxin in disease mechanisms (4-year plan) and a new way to combat infections (7-year plan). Thus, discoveries from the proposed work will be exploited to strengthen public support, political momentum and to develop new avenues and awareness for typhoid prevention and control strategies.

2. Pharmaceutical industry
The potential impacts include new treatments against toxigenic Salmonella (+7-years).

Pharmaceutical companies will benefit from the proposed work by (i) revealing new disease targets for therapeutic intervention in the 4-year plan (e.g. toxin-induced DNA damage and senescence), and by (ii) identifying candidate FDA-approved drugs that target toxin-driven virulence mechanisms in the 7-year plan.

Antimicrobial resistance (AMR) is a defining problem facing society and the stalled development of antimicrobials impedes current prevention strategies. We need new therapeutics for multidrug-resistant typhoid, which causes treatment failures. For instance, only 7 percent of typhoid cases were resistant to multiple drugs in 2010. By 2014, 97 percent of typhoid cases resisted treatment from multiple drugs in Malawi, Africa. The repurposing of FDA-approved drugs offers an alternative approach for rapid identification of effective therapeutics to treat infectious diseases. Thus, discoveries arising from the proposed work will be of major significance to the pharmaceutical industry who will able to help maximise the potential of the project outputs by broadening the scope for therapeutic intervention in the long term.

3. The public and healthcare practitioners
The potential impacts include (i) improving public health through new treatments (see 1, 2), (ii) encouraging UK wealth by economic investment into public health and pharmaceuticals (+7-years), and (iii) the fostering of skills development in the UK (immediate impact). These include skills such as written communication, public speaking, computing, data analysis, management, laboratory skills etc, which can be applied in a range of employment sectors.

4. Policy makers
The potential impacts include new treatments/disease understanding that influences disease control and prevention strategies (+7-years).

It is important that key decision-makers in government understand the disease that impact their constituents and citizens, which can ultimately benefit the health and wealth of their respective countries. My research will equip policy makers with important advances that have the potential to influence key decisions on typhoid prevention and control in low-and middle-income regions where the disease is endemic, e,g. Sub-Saharan Africa, South East Asia.