# Unveiling Host Interactions and Evolutionary Constraints of a Novel Bacteriophage Infecting Xanthomonas hortorum pv. vitians

## TL;DR

A new bacteriophage that infects a lettuce pathogen was studied to understand how it interacts with its host and why resistance might not spread easily.

## Contribution

The study identifies a novel phage and reveals a fitness cost in bacteria that resist phage infection, offering insights for sustainable biocontrol.

## Key findings

- ΦXhv-1 targets LPS O-antigen side chains for host recognition.
- Phage-resistant mutants show reduced motility and virulence.
- Resistance to the phage may be naturally constrained due to fitness trade-offs.

## Abstract

Due to limitations in disease management strategies and the impact of climate change, phytopathogenic bacteria are threatening global crop production.
Xanthomonas hortorum
pv. vitians, the causal agent of bacterial leaf spot of lettuce, remains difficult to manage due to the lack of efficient treatment options. As an alternative, phage‐based biocontrol offers a promising solution, but its long‐term efficacy depends on a thorough understanding of phage–host interactions and the potential development of bacterial resistance. In this study, we isolated and characterised the lytic bacteriophage ΦXhv‐1, which defines a novel genus within the class Caudoviricetes. Using transposon insertion sequencing, we identified 36 bacterial genes essential for phage susceptibility, primarily involved in lipopolysaccharide biosynthesis and surface polysaccharide modifications. Targeted mutagenesis and fluorescence microscopy confirmed that ΦXhv‐1 adsorbs onto specific residues of the LPS O‐antigen side chains. Phage‐resistant mutants exhibited a decreased motility in vitro and a significant reduction in virulence in planta. These findings reveal a strong evolutionary trade‐off between phage resistance and bacterial fitness, suggesting that resistance emergence in the field may be naturally constrained. This study provides new insights into
X. hortorum
pv. vitians–phage interactions and supports the development of sustainable phage‐based biocontrol strategies against bacterial leaf spot of lettuce.

A transposon insertion sequencing approach revealed that the novel phage ΦXhv‐1 targets
Xanthomonas hortorum
pv. vitians through LPS O‐antigen recognition. Mutations in LPS conferring phage resistance reduce bacterial motility and virulence, highlighting a trade‐off that may naturally constrain resistance development and support the sustainable phage‐based control strategies.