Discovering the dormant lifestyle of Salmonella in intestinal cells

7 July 2020

Jakson Luk Chak Hon (Croucher Scholarship 2017) is close to completing four years of PhD research at the Unité Dynamique des Interactions Hôte-Pathogène, under the supervision of Dr Jost Enninga, and has already made his mark in the field of bacterial pathogens.

Luk’s work explores the complex interactions between host and bacterial pathogen at the cellular level, using Salmonella Typhimurium and human epithelial cells from the intestines as a model. Salmonella is one of the four major causative agents of diarrhoeal disease, and Salmonella Typhimurium is one of the standard models for investigating host-bacteria interactions.

Salmonella had been thought to adopt one lifestyle only. However, a breakthrough in 2010 suggested Salmonella could undertake different lifestyles, essentially fast-growing and slow-growing behaviours, Luk said.

Fast-growing bacteria create a huge population of bacteria, which subsequently spread inside the gut. Luk and his colleagues examine how the bacteria colonise and rapidly spread to neighbouring cells within our intestine and what genes influence or control this bacterial behaviour.

To better understand how the bacteria exhibit these different replication rates after invading the human cell, Luk and the team employ advanced dynamic imaging techniques to film the invasion of cells using innovative fluorescence microscopy. “It’s like making a movie,” he said.

Such dynamic images have significance for future medical research because understanding these interactions is critical for the discovery of new therapeutic approaches to treat infectious bacterial diseases. With increasing antibiotic tolerance and emerging pathogens, studying bacterial behaviours has become a priority in medical research.

By deciphering the molecular and cellular details of invasion strategies used by bacterial pathogens, it becomes possible to identify novel drug targets and provide fresh insight into fundamental cell biological processes.

Initially, Luk was tasked with following up on existing data of Salmonella Typhimurium to investigate how they can live inside human cells with two distinct behaviours. “They stay in different places and grow at different speeds inside human cells,” Luk explained.

However, previous data made available to him only covered a limited set of human genes, so Luk designed a new fluorescent reporter to examine all intrinsic Salmonella behaviours with all human genes.

I found something new, a third bacterial lifestyle. It was a new discovery.

To do so, Luk developed a tool that makes Salmonella emit different colours (fluorescence) when they exhibit a slow-growing or fast-growing lifestyle. The reporter combines three individual reporters. While undertaking this novel assay during his third year at the Institut Pasteur, Luk made a remarkable discovery.

The reporter programmes the bacteria to emit different colours during fast-growing or slow-growing behaviour, but a small Salmonella population (about 5 to 10 per cent) enters a dormant state and does not give either colour. These bacteria appear to have paused their metabolism within human cells.

Fast and slow moving bacteria as displayed by Luk's reporter

The development of this innovative method had allowed him to uncover this previously unknown behaviour. “In doing so, I found something new, a third bacterial lifestyle. It was a new discovery,” Luk said.

While it had recently been found that Salmonella in immune cells can enter a dormant state, Luk’s work was the first evidence of a dormant state in epithelial cells, the cells that first encounter the pathogen within human intestines.

It was a highly significant finding because antibiotics largely target active machinery, but dormant cells are much less susceptible to antibiotics because of their paused protein expression and bacterial division.

“To use a simple analogy, you cannot poison someone who is asleep because they will not eat or drink, no matter how much poison they are offered,” he explained.

Many research teams are working in areas of antibiotic resistance while dormancy is contributing to a phenomenon known as antibiotic persistence, where a fraction of antibiotic-susceptible bacteria exhibit elevated tolerance against antibiotics. Persisters are transiently antibiotic-tolerant cells that complicate the treatment of bacterial infections.

This was the first time that dormant Salmonella had been observed in human intestinal cells.

In his fourth year, Luk also identified the genes in the bacteria that regulate dormancy. “When Salmonella invade different cells, they encounter different micro-environments and we see different behaviours of dormant bacteria in immune cells and intestinal cells,” Luk said. This made the gene identification highly relevant.

Luk will be first author of the paper now being finalised.

The acute medical relevance of these discoveries is directly related to a better understanding of antibiotic persistence. Persistent bacteria cause relapses in infection and could be omitted by classical medical screening looking for antibiotic resistance in bacteria prior to antibiotic prescription.

With improved knowledge of dormancy and its links to persistence, the schedule of antibiotic treatment could be refined so dormant bacteria are killed as they awake and turn susceptible. Potentially, the breakthrough also allows for better use of existing therapies and the design of new strategies.

Luk wishes to further his research into the whole spectra of bacterial interactions and different strategies employed by bacterial pathogens in achieving a successful invasion of the host.

He is particularly interested in using his experience of dynamic imaging to examine changes in the micro-environment experienced by bacteria and to understand how that impacts bacterial behaviour and disease progression.

“This has been an incredible experience for me,” he said. “It feels like the science stories told to me at high school about scientists making new discoveries out of unexpected observations.”

Jakson LUK Chak Hon is a PhD candidate in the Département Biologie Cellulaire et Infection at the Institut Pasteur under the Pasteur-Paris University (PPU) International PhD Program, France. Jakson completed secondary school at St Joseph’s College in Hong Kong in 2011. He received his BSc in Biochemistry and Microbiology from the University of Hong Kong in 2014 and his MPhil in Life Science (Molecular Medicine) from the Hong Kong University of Science and Technology, under the supervision of Prof King L Chow. Luk joined Dr Jost Enninga’s unit at the Institut Pasteur in 2016 as a doctoral student, focusing on the interactions between host cells and bacterial pathogens. He received his Croucher Scholarship in 2017.

To view Jakson Luk’s Croucher profile, please click here