# Conjugative Transfer of Disease‐Encoding Plasmid Variants in Serratia spp. Alter Production of Enzymes and Virulence Properties

## TL;DR

This study shows that plasmids in Serratia bacteria change how the bacteria behave, making them less fit and altering their disease-causing abilities.

## Contribution

The study experimentally demonstrates coevolution between Serratia plasmids and their hosts through conjugation and transcriptomic analysis.

## Key findings

- Transconjugant Serratia strains show erratic disease phenotypes and reduced fitness compared to plasmid-free and native plasmid-bearing strains.
- Transcriptomic analysis reveals differential expression of virulence-related genes in transconjugants.
- Results support that Serratia plasmids have speciated and coevolved with their host chromosome.

## Abstract

Some strains of
Serratia entomophila, S. proteamaculans
and
S. quinivorans
(Enterobacterales: Yersiniaceae) are entomopathogens of the New Zealand pasture pest Costelytra giveni (Coleoptera: Scarabaeidae). Virulence is encoded by variants of the amber disease‐associated plasmid (pADAP), collectively termed Serratia transmissible adaptive megaplasmids (STAMPs), whose diverse insect‐active complexes impart hypervirulence to chronic pathotypes. An estimated 40%–60% of New Zealand Serratia are plasmid‐free non‐virulent conspecifics to STAMP‐carrying entomopathogens, implying a complex evolutionary relationship between the plasmid, host and disease. To further define this relationship, plasmids from chronic and hypervirulent pathotypes were conjugated into recipient strains, allowing experimental comparison of virulence relative to donor and naïve strains. Through competitive bioassays and plate‐based enzyme assays, transconjugants (strains selectively conjugated with donor plasmids) showed altered enzymatic activity and variable disease phenotypes. Transconjugants were also found to have reduced fitness, outcompeted by naïve plasmid‐free and native plasmid‐bearing strains within the host and in vitro cultures, suggesting a degree of coevolution. Transcriptomic analysis comparing naïve strains and transconjugants revealed differentially expressed genes associated with virulence, including plasmid‐encoded anti‐feeding prophage (Afp) genes and chromosomal chitinases and proteases. Results further support that STAMPs have speciated to their host chromosome and that naturally occurring Serratia plasmid‐containing isolates have coevolved accordingly.

Analysis of plasmid transconjugants relative to plasmid‐free and plasmid‐carrying Serratia spp. demonstrates erratic disease phenotypes and reduced fitness. Transconjugants also show reduced ability to degrade substrates associated with virulence, supported by transcriptomic data. These results further support that Serratia plasmids have speciated to their host chromosome and have coevolved.