Molecular Diversity of Aquatic <i>Pseudomonas</i> spp. With Inhibitory Activity Against <i>Saprolegnia</i> Water Mould (Oomycota)

Several Pseudomonas bacterial species demonstrate potential for biological control. We examined 24 aquatic Pseudomonas isolates mainly from fish farms and natural waters in Hungary, with siderophore production and inhibitory effects on Saprolegnia water moulds. We identified six Pseudomonas species (P. paracarnis, P. shahriarae, P. salmasensis, P. canadensis, P. chlororaphis subsp. aurantiaca and P. danubii) distributed across five distinct phylogenetic clusters based on 16S ribosomal RNA and RNA polymerase sigma factor, rpoD gene sequences. The MALDI-TOF MS protein profiles showed species distribution patterns and clustering of the most inhibitory strains. In vitro experiments revealed that three strains of P. danubii, one P. chlororaphis subsp. aurantiaca and one unidentified Pseudomonas sp. B379-HU demonstrated the highest inhibitory activity in both bacterial culture and supernatants. Partial inhibition caused hyphae branching and nodule formation, whereas complete inhibition led to fragmentation and death of hyphae. This study is the first to document inhibitory activity in these Pseudomonas species, suggesting potential for water mould control. For fish health safety and to mitigate the risk of antibiotic resistance, utilising the extracellular inhibitory substances offers the most promising approach for future application.