Characterisation of novel Campylobacter jejuni Type VI secretion system (T6SS) effectors and exploration of the roles of the C. jejuni T6SS in bacterial antagonism and human host cell interaction

This study characterizes the *Campylobacter jejuni* T6SS as a critical virulence factor that mediates contact-dependent antagonism against competing bacteria via two novel effectors (CJ488_0980 and CJ488_0982) and enhances interaction with, invasion of, and survival within human intestinal epithelial cells.

The Gist

Imagine the human gut as a bustling, crowded marketplace. In this market, different types of bacteria are constantly trying to set up shop, grab resources, and survive. One of the main players in this market is Campylobacter jejuni, a germ that causes food poisoning. For a long time, scientists thought this germ was a bit of a "sleeper agent"—it caused trouble, but it didn't seem to have the heavy weapons that other bad bacteria use to fight off rivals.

However, this new study reveals that Campylobacter actually has a secret, high-tech weapon hidden in its arsenal: a Type VI Secretion System (T6SS).

Here is the breakdown of what the researchers found, using simple analogies:

1. The "Harpoon Gun" (The T6SS)

Think of the T6SS as a microscopic harpoon gun or a spring-loaded spear.

• How it works: When Campylobacter bumps into a rival bacterium (like E. coli or another strain of Campylobacter), it fires this harpoon.
• The Payload: The harpoon doesn't just poke the enemy; it injects a toxic "poison dart" directly into their cell.
• The Result: The rival cell gets poisoned and dies. This allows the Campylobacter to clear the neighborhood of competition and take over the resources.

2. The "Poison Darts" (The New Effectors)

For years, scientists knew the gun existed, but they didn't know exactly what kind of poison darts were inside. This study is the first to identify two specific darts, named CJ488_0980 and CJ488_0982.

• The Discovery: The researchers found these darts hidden in the bacteria's genetic code. They look like "restriction enzymes," which are basically molecular scissors that cut DNA.
• The Analogy: Imagine the poison dart isn't just a toxin, but a pair of molecular scissors that snips the enemy's DNA, causing their factory to shut down and the cell to collapse.
• The Test: When the scientists removed these specific darts from the bacteria, the "gun" still fired, but it was much less effective. It was like taking the warheads off a missile; the missile still flew, but it didn't destroy the target. This proved these two specific proteins are the main weapons used to kill rivals.

3. "Bumping into You" (Contact-Dependent)

The study showed that this weapon only works if the bacteria touch.

• The Analogy: It's like a game of tag where you have to physically tag someone to eliminate them. If you put a glass wall (a membrane) between the attacker and the prey, the attacker can't fire the harpoon, and the prey survives.
• Why it matters: This means the bacteria are constantly scanning their immediate environment. They only attack when they are right up against a competitor.

4. The "Double Agent" Effect (Human Cells)

Here is where it gets really interesting. The T6SS isn't just for fighting other bacteria; it also changes how Campylobacter interacts with human cells (the lining of our intestines).

• The Invasion: When the bacteria have a working T6SS, they are better at sticking to human cells and sneaking inside them (invading). It's like having a better set of grappling hooks to climb the walls of a fortress.
• The Twist: However, once the bacteria are inside the human cell, having the T6SS active actually makes them less likely to survive long-term.
• The Analogy: Think of it like a spy. The spy uses a special tool (the T6SS) to break into a building (the human cell). But once inside, that same tool makes them noisy and suspicious, so they get kicked out or killed faster. If the spy doesn't use the tool, they are quieter and can hide inside the building for much longer.

Why Does This Matter?

This study is a big deal for a few reasons:

1. New Weapons Identified: It's the first time we've named the specific "poison darts" Campylobacter uses to fight.
2. Competition is Key: It shows that Campylobacter is a fierce competitor in the gut, using these weapons to wipe out other bacteria, which helps it survive and cause infection.
3. Human Health: Understanding how this system helps the bacteria invade our cells helps scientists figure out how to stop the infection. If we can jam the "harpoon gun" or neutralize the "molecular scissors," we might be able to stop the bacteria from taking hold in the first place.

In short: Campylobacter is a tough customer in the gut. It uses a spring-loaded harpoon to shoot toxic scissors at its rivals to clear the way, and it uses the same system to help it climb inside human cells, though that same system eventually makes it harder for the bacteria to hide once it's inside.

Technical Summary

Title

Characterisation of novel Campylobacter jejuni Type VI secretion system (T6SS) effectors and exploration of the roles of the C. jejuni T6SS in bacterial antagonism and human host cell interaction.

1. Problem Statement

Campylobacter jejuni is the leading global cause of bacterial foodborne gastroenteritis. While the Type VI Secretion System (T6SS) is a well-known virulence factor in other Gram-negative pathogens, its specific roles in C. jejuni remain poorly characterized.

• Knowledge Gap: Although T6SS prevalence has been noted in specific C. jejuni clonal complexes (e.g., ST-353), no specific T6SS "cargo" effectors (toxic proteins delivered by the system) have been experimentally characterized in Campylobacter species.
• Functional Ambiguity: Previous studies linked the T6SS to host colonization and stress resistance, but these phenotypes were often studied using structural mutants (e.g., tssD knockout), making it difficult to distinguish the role of the secretion machinery from the specific effectors it delivers.
• Objective: This study aims to identify and characterize novel T6SS cargo effectors in the C. jejuni 488 strain (ST-353) and define the system's role in interbacterial competition and interaction with human intestinal epithelial cells (IECs).

2. Methodology

The study utilized the C. jejuni 488 strain (T6SS-positive) and generated specific isogenic mutants to dissect T6SS function.

• Strain Construction:
  • Mutants: Created cj0980 and cj0982 mutants (putative effectors) and a tssD mutant (structural component, rendering T6SS non-functional) using Splicing by Overlap Extension (SOE) PCR and allelic exchange.
  • Controls: Generated a tssD complemented strain and a GFP-expressing wild-type strain for competition assays.
• Bioinformatic Analysis: Identified putative effectors CJ488_0980 and CJ488_0982 within the CJPI-1 Pathogenicity Island. Both were predicted to contain Tox-REase-7 domains (restriction endonuclease family).
• Interbacterial Competition Assays:
  • Setup: Co-cultured "attacker" C. jejuni strains with "prey" strains (C. jejuni 81-176, 11168H; C. coli; E. coli; E. faecium).
  • Metrics: Calculated Competitive Indices (CI) based on Colony Forming Units (CFUs) before and after co-incubation.
  • Contact Dependency: Used a 0.2 µm membrane to physically separate attackers and prey to test for contact-dependent killing.
  • Density Dependence: Tested various attacker-to-prey ratios (1:1, 5:1, 50:1).
• Host-Cell Interaction Assays:
  • Cell Lines: Human intestinal epithelial cells (T84 and Caco-2).
  • Assays: Measured bacterial adhesion (interaction), invasion (gentamicin protection assay), and intracellular survival (extended gentamicin treatment).
  • Gene Expression: qRT-PCR to measure tssB, tssC, and tssD expression during infection.
• Protein Analysis: Western blotting to confirm TssD secretion in whole-cell lysates vs. supernatants.

3. Key Contributions

• First Characterization of C. jejuni Cargo Effectors: This study provides the first experimental evidence of specific cargo effectors (CJ488_0980 and CJ488_0982) in Campylobacter.
• Mechanism of Action: Identified that these effectors possess Tox-REase-7 domains, suggesting a nuclease-based killing mechanism similar to other bacterial toxins.
• Dual Role of T6SS: Demonstrated that the C. jejuni T6SS functions not only as a weapon for interbacterial competition but also as a modulator of host cell invasion and intracellular survival.

4. Key Results

A. Bacterial Antagonism and Competition

• T6SS-Mediated Killing: The wild-type C. jejuni 488 strain (and tssD complement) exhibited strong contact-dependent antagonism against T6SS-negative strains of C. jejuni, C. coli, E. coli, and the Gram-positive E. faecium.
• Contact Dependence: The killing phenotype was abolished when a membrane separated the attacker and prey, confirming the T6SS requires direct cell-to-cell contact for this specific antagonism.
• Density Dependence: Predation efficiency was density-dependent; higher attacker-to-prey ratios were required to kill fast-growing species like E. coli.
• Role of Effectors:
  • Mutants lacking cj0980 showed a severe reduction in competitive ability (approx. 2-log decrease in CI) against C. jejuni 81-176.
  • Mutants lacking cj0982 showed a moderate reduction in competitive ability.
  • Neither single mutant completely abolished killing (unlike the tssD mutant), suggesting synergistic action between the two effectors and/or the presence of additional, uncharacterized effectors.

B. Host Cell Interaction

• Gene Upregulation: tssB, tssC, and tssD genes were significantly upregulated in C. jejuni 488 upon infection of T84 and Caco-2 cells.
• Adhesion and Invasion:
  • The tssD mutant (non-functional T6SS) showed significantly reduced adhesion and invasion in T84 cells compared to the wild-type.
  • This phenotype was not observed in Caco-2 cells, indicating cell-line specificity (likely due to T84 resembling colonocytes vs. Caco-2 resembling enterocytes).
• Intracellular Survival: Paradoxically, the tssD mutant displayed enhanced intracellular survival compared to the wild-type in both cell lines. This suggests the active T6SS may trigger host defense mechanisms or metabolic costs that reduce long-term survival, or that the system is downregulated once inside the cell to promote persistence.

5. Significance

• Virulence Determinant: The study establishes the T6SS as a critical virulence factor for C. jejuni, enhancing its ability to colonize the gut by outcompeting resident microbiota (including Gram-positives) and facilitating invasion of human intestinal cells.
• Ecological Fitness: The ability to kill E. faecium and C. coli suggests the T6SS provides a competitive advantage in the complex polymicrobial environments of the chicken and human gut.
• Novel Effector Class: The identification of Tox-REase-7 domain effectors expands the known repertoire of T6SS toxins, suggesting a mechanism of DNA/RNA degradation in prey cells.
• Therapeutic Implications: Understanding the specific effectors and the dual nature of the T6SS (killing competitors vs. modulating host survival) could inform strategies to disrupt C. jejuni colonization or develop targeted antimicrobials.

Conclusion: This research bridges a critical gap in Campylobacter pathogenesis by moving from structural characterization to functional effector analysis, revealing a sophisticated T6SS system that drives both interbacterial warfare and host cell manipulation.