Prophage {Phi}ESI promotes competitive dominance in the emergent Salmonella serovar Infantis lineage

This study identifies the temperate bacteriophage {Phi}ESI as a key competitive factor in the emergent, multidrug-resistant *Salmonella* Infantis lineage, demonstrating that its prophage-mediated lysis of susceptible strains facilitates the lineage's ecological dominance and global spread.

The Gist

Imagine a bacterial world that looks a lot like a crowded city. In this city, there are two rival gangs of Salmonella bacteria (the kind that makes you sick from bad chicken).

One gang is the "Old Guard" (non-emergent strains). They've been around for a while, they're okay, but they aren't taking over the whole city.

The other gang is the "New Bosses" (the emergent Salmonella Infantis lineage). These guys are super successful. They are everywhere, they are resistant to almost all antibiotics, and they are taking over the food supply chain globally. Scientists have known for a long time that the New Bosses carry a giant "backpack" of superpowers called the pESI plasmid. This backpack gives them resistance to drugs and helps them survive.

But here's the mystery: The backpack explains why they are tough, but it doesn't fully explain how they are so good at crushing their rivals and becoming the dominant gang.

Enter the secret weapon: ΦESI (Phi-ESI).

Think of ΦESI not as a backpack, but as a hidden spy drone living inside the New Bosses' headquarters (their DNA).

The Spy Drone's Strategy

1. The Dormant Spy: Most of the time, this spy drone (a virus called a bacteriophage) is asleep inside the New Bosses. It's integrated into their DNA, like a secret file in their computer. It doesn't hurt them; in fact, it protects them.
2. The Attack Mode: When the New Bosses are in a crowded room with the Old Guard, the spy drone wakes up. It starts building thousands of tiny, invisible missiles.
3. The Selective Strike: These missiles fly out and look for the Old Guard. When they find an Old Guard bacterium, they crash into it and blow it up (lysis).
4. The Immunity Shield: Here's the clever part: The New Bosses have a shield. Because they carry the spy drone's blueprint, they are immune to the missiles. The Old Guard, however, has no idea what's coming and gets wiped out.

The "Turf War" Experiment

The scientists in this paper set up a little arena to watch this happen.

• They put the New Bosses (with the spy drone) and the Old Guard (without it) in the same test tube.
• Result: At first, they grew at the same speed. But then, the New Bosses started releasing the spy missiles. The Old Guard population crashed. The New Bosses took over the test tube completely.
• The "No Drone" Test: When they tested the New Bosses without the spy drone, they couldn't beat the Old Guard. The spy drone was the key to their dominance.

Why This Matters for the Whole World

The researchers looked at thousands of Salmonella samples from all over the globe. They found a pattern:

• The New Bosses almost always have this spy drone (or a very similar version of it).
• The Old Guard almost never has it.

It's like finding that every member of a successful criminal organization has a specific, rare tattoo, while the regular citizens don't. This suggests that the spy drone didn't just happen to show up; it was a major factor in helping this specific lineage of bacteria conquer the world.

The "Double Trouble" Connection

There's another twist. The "New Bosses" also carry that giant "superpower backpack" (the pESI plasmid). The paper suggests these two things work together:

• The Backpack makes them tough against antibiotics.
• The Spy Drone makes them aggressive against other bacteria.

Together, they create a bacterial super-villain that is hard to kill and good at killing its competition.

The Big Picture

This study teaches us that when a disease-causing bacteria becomes a global threat, it's not just about one thing (like antibiotic resistance). It's often a "team effort" between different genetic tools.

In this case, a hidden virus living inside the bacteria acts like a biological weapon, clearing the path for the "New Bosses" to dominate the ecosystem. Understanding this helps scientists realize that to stop these super-bugs, we might need to figure out how to disable their "spy drones" or break the alliance between the drone and the backpack.

In short: The Salmonella that is taking over the world isn't just tough; it's armed with a secret, self-replicating weapon that specifically targets and destroys its competition, allowing it to win the battle for survival.

Technical Summary

1. Problem Statement

The global emergence and spread of multidrug-resistant (MDR) Salmonella enterica serovar Infantis, particularly the lineage carrying the pESI megaplasmid, pose a significant public health and economic threat. While the pESI megaplasmid is known to confer antimicrobial resistance, virulence, and heavy metal tolerance, the specific genetic factors driving the ecological dominance and competitive success of this lineage over non-emergent strains remain poorly understood. The authors hypothesized that additional mobile genetic elements, specifically bacteriophages, might provide a competitive advantage by eliminating susceptible competitors.

2. Methodology

The study employed a multidisciplinary approach combining experimental microbiology, genomics, and bioinformatics:

• Phage Isolation & Characterization:

  • Source: A temperate bacteriophage (ΦESI) was isolated from a coculture of an emergent pESI-positive strain (PM57) and a non-emergent strain (DR006).
  • Morphology: Transmission Electron Microscopy (TEM) was used to determine the virion structure.
  • Genomics: Whole Genome Sequencing (Illumina) was performed on the phage. The genome was assembled, annotated (Pharokka), and analyzed for taxonomic classification (VICTOR, VIRIDIC) and homology (BLASTn).
  • Integration Site: Comparative genomics between PM57 and DR006 identified the prophage integration site within the host chromosome.

• Global Bioinformatic Analysis:

  • Dataset: 36,499 Salmonella Infantis genomes from Enterobase were screened. A subset of 20,429 non-clonal genomes was analyzed for the presence of ΦESI genes.
  • Classification: Genomes were categorized based on the number of ΦESI Coding Sequences (CDSs) present: ΦESI-positive (>86 CDSs), ΦESI-like-positive (73–85 CDSs), and ΦESI-negative (<73 CDSs).
  • Structural Analysis: 146 complete closed chromosomes were analyzed to map insertion loci and detect large-scale chromosomal inversions relative to the reference strain (PM57).
  • Phylogenetics: A recombination-free core-SNP maximum likelihood tree was constructed to correlate prophage presence with phylogenetic clades and geographic distribution.

• Experimental Competition & Susceptibility Assays:

  • Competition Assays: PM57 (ΦESI+) and DR006 (ΦESI-) were grown in monoculture and coculture. Bacterial loads were quantified via colony counting and strain-specific qPCR. Phage titers (PFU/mL) were measured via double-layer agar assays.
  • Efficiency of Plating (EOP): 19 diverse Salmonella Infantis strains were tested for susceptibility to ΦESI-mediated lysis to determine if resistance correlates with prophage carriage.

3. Key Contributions & Results

A. Characterization of ΦESI

• Morphology: ΦESI is a siphovirus with a long flexible tail (~165 nm) and an elongated head.
• Genome: It possesses a 46,490 bp genome with 49.3% G+C content and 88 CDSs.
• Taxonomy: It represents a novel genus within the class Caudoviricetes, with low nucleotide identity to known phages.
• Integration: In the emergent strain PM57, ΦESI is integrated as a prophage at the 3'-end of an Arg-tRNA gene. It is absent in the non-emergent strain DR006.
• Lytic Activity: Upon induction, ΦESI forms clear lytic plaques on DR006 but not on PM57 (lysogenic immunity).

B. Global Distribution and Genomic Context

• Lineage Specificity: There is a statistically significant association ($p < 0.0001$) between ΦESI/ΦESI-like prophages and the emergent pESI-positive lineage.
  • 92.5% of emergent (pESI+) genomes harbor ΦESI/ΦESI-like prophages.
  • Only 7.7% of non-emergent genomes carry them.
  • The odds ratio is 147.2, indicating a tight linkage.
• Insertion Profiles & Inversions: Analysis of complete chromosomes revealed two conserved insertion loci (~1.45 Mbp and ~3.05 Mbp).
  • Chromosomal Inversions: In 39% of ΦESI-positive genomes, a large-scale (1.5 Mbp) chromosomal inversion occurs between these two loci. This inversion flips the intermediate region of the chromosome.
  • Geographic Clustering: Profile B (full prophage at both loci) is enriched in the Americas, while Profile C (fragmented prophage with inversion) is associated with Europe.

C. Competitive Advantage Mechanism

• Coculture Dynamics: In coculture, PM57 (ΦESI+) outcompetes DR006 (ΦESI-).
  • DR006 growth is significantly reduced starting at hour 4, with a difference of ~0.96 log at 24 hours.
  • Phage titers in coculture increase exponentially, reaching $5.04 \times 10^7$ PFU/mL.
  • The released ΦESI particles selectively lyse DR006 while PM57 remains protected via superinfection exclusion.
• Superinfection Resistance: All ΦESI-positive isolates were resistant to ΦESI-mediated lysis (EOP = 0). Most ΦESI-negative isolates were susceptible. Notably, some strains with partial ΦESI gene sets (73 CDSs) showed intermediate or full resistance, suggesting specific CDSs (likely CDS 006-012, 017, 027, etc.) mediate immunity.

D. Interaction with pESI Megaplasmid

• The study hypothesizes a synergistic relationship: The pESI megaplasmid carries a psiB homolog (anti-SOS response), which may prevent inadvertent prophage induction (lysis) during stress, thereby stabilizing the ΦESI prophage within the host. Conversely, ΦESI eliminates competitors that might otherwise acquire the pESI plasmid, restricting the plasmid's spread to the emergent lineage.

4. Significance

• Mechanism of Emergence: This study identifies ΦESI as a critical "competitive weapon" that drives the ecological success of the emergent Salmonella Infantis lineage, complementing the role of the pESI megaplasmid.
• Ecological Niche Defense: The prophage functions as a "replicative toxin," selectively eliminating non-lysogenic competitors and securing the niche for the emergent lineage.
• Genomic Stability: The discovery of large-scale chromosomal inversions flanked by prophage insertion sites highlights how mobile elements can drive major structural rearrangements in bacterial genomes without necessarily disrupting gene dosage balance.
• Public Health Implication: Understanding that prophage-mediated competition contributes to the persistence of MDR pathogens suggests that phage dynamics should be considered in models of pathogen emergence and potential control strategies. The tight linkage between the plasmid and prophage suggests that targeting one element might destabilize the other.

In conclusion, the paper demonstrates that the global dominance of MDR Salmonella Infantis is not solely due to antibiotic resistance genes but is significantly bolstered by a temperate bacteriophage (ΦESI) that provides a potent competitive advantage through the selective lysis of susceptible strains.