Pretto, T. | bioRxiv Science

bioRxiv2026

Variation of anti-oomycete activity in Pseudomonas spp.: phenotypic characterization and comparative genomics

Pathogenic aquatic oomycetes Aphanomyces astaci and Saprolegnia parasitica represent a major threat to biodiversity and aquaculture production, but their interactions with host-associated microbes remain poorly understood. From a collection of bacterial isolates (n = 328) obtained from fish and crayfish hosts, we focused on Pseudomonas spp. (n = 66) and confirmed their previously reported strong inhibitory potential against A. astaci and S. parasitica. However, our results also revealed substantial inter- and intra-species variation in antagonism. To capture this variation, we selected eight isolates belonging to different P. fluorescens, P. putida, and P. syringae species groups and displaying contrasting levels of anti-oomycete activity for further phenotypic assays and comparative genomic analysis. Across these isolates, mycelial inhibition was markedly stronger against A. astaci than against S. parasitica, indicating species-specific differences in susceptibility. Comparative genomic analysis revealed substantial variation in biosynthetic gene cluster (BGC) repertoires among the analysed strains. Strongly inhibitory isolates carried candidate BGCs with similarity to characterised bioactive pathways, including pyoluteorin, rhizoxin, pyrrolnitrin, DAPG, and orfamide, alongside with multiple uncharacterised clusters that were either shared among inhibitory isolates or restricted to individual strains. All analysed genomes also contained clusters related to siderophore and HCN biosynthesis. However, in vitro assays showed that siderophore production was not clearly associated with inhibitory activity and that inhibition was mediated mainly by diffusible rather than volatile compounds. Altogether, our results suggest that Pseudomonas anti-oomycete activity is species- and strain-dependent and likely reflects different combinations of multiple, predominantly diffusible metabolites rather than a single conserved mechanism. In conclusion, this study provides a foundation for future work aimed at resolving mechanisms underlying microbial antagonism toward aquatic oomycete pathogens.

microbiology↗

bioRxiv2024

The complete genome sequence of the crayfish pathogen Candidatus Paracoxiella cheracis n.g. n.sp. provides insight into pathogenesis and the phylogeny of the Coxiellaceae family

The Coxiellaceae bacterial family, within the order Legionellales, is defined by a collection of poorly characterized obligate intracellular bacteria. The zoonotic pathogen and causative agent of human Q fever, Coxiella burnetii, represents the best characterized member of this family. Coxiellaceae establish replicative niches within diverse host cells and rely on their host for survival, making them challenging to isolate and cultivate within a laboratory setting. Here we describe a new genus within the Coxiellaceae family that has been previously shown to infect economically significant freshwater crayfish. Using culture-independent long-read metagenomics, we reconstructed the complete genome of this novel organism and demonstrate that the previously referred to as Candidatus Coxiella cheraxi represents a novel genus within this family, herein denoted Candidatus Paracoxiella cheracis. Interestingly, we demonstrate that Candidatus P. cheracis encodes for a complete, putatively functional Dot/Icm type 4 secretion system that likely mediates the intracellular success of this pathogen. In silico analysis defined a unique repertoire of Dot/Icm effector proteins and highlighted homologues of several important C. burnetii effectors including a homologue of CpeB that was demonstrated to be a Dot/Icm substrate in C. burnetii. IMPORTANCEUsing long-read sequencing technology we have uncovered the full genome sequence of Candidatus Paracoxiella cheracis, a pathogen of economic importance in aquaculture. Analysis of this sequence has revealed new insight into this novel member of the Coxiellaceae family, demonstrating that it represents a new genus within this poorly characterized family of intracellular organisms. Importantly, the genome sequence reveals invaluable information that will support diagnostics and potentially both preventative and treatment strategies within crayfish breeding facilities. Candidatus P. cheracis also represents a new member of Dot/Icm pathogens that rely on this system to establish an intracellular niche. Candidatus P. cheracis possesses a unique cohort of putative Dot/Icm substrates that constitute a collection of new eukaryotic cell biology manipulating effector proteins.

microbiology↗