Predatory Behavior of Ixotrophy Predator in Response to Nutrient Availability

Ixotrophy is a unique epibiotic predation strategy employed by bacteria in the family Saprospiraceae , involving the capture of swimming prey via adhesive surface substances—analogous to “flypaper”—followed by prey lysis mediated by the Type VI Secretion System (T6SS). While ixotrophy predation mechanisms have been elucidated, the behavioral and molecular adaptation of these facultative predators under varying nutrient conditions remains poorly understood. This study investigates the regulation of the killing apparatus and the secretion of degradative enzymes by Saprospira sp. CCB-QB6, a potent predator of the aquaculture pathogen Vibrio parahaemolyticus that causes Acute Hepatopancreatic Necrosis Disease (AHPND). Microscopic observations revealed that CCB-QB6 utilizes contact-dependent predation, leading to rapid prey lysis and the release of cytoplasmic contents. Genomic analysis confirmed the presence of a T6SS subtype iv, phylogenetically related to the T6SS of the Aureispira sp. CCB-QB1. On the plaque assays, predatory activity was significantly suppressed by increasing concentrations of nutrients, such as tryptone or casamino acids. Gene expression analysis using RT-qPCR demonstrated that the expression of cis8 , encoding T6SS spike protein, was specifically induced by the presence of prey under nutrient limitation, but this response was abolished under nutrient-rich conditions. Furthermore, LC-MS/MS-based proteomic analysis of the secretome identified a more diverse secretion of hydrolytic enzymes under starvation than in nutrient-replete conditions. Many of these enzymes possess Type IX Secretion System (T9SS) sorting domains, highlighting the essential role of the T9SS in the saprotrophic and predatory lifestyle of CCB-QB6. Collectively, these findings provide novel insights into the foraging strategies employed by ixotrophic bacteria in response to environmental nutrient fluctuations.