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

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.