Hemolytic mechanism and virulence of Antarctic psychrophilic Pseudomonas.

Polar warming is expected to release and redistribute psychrophilic microorganisms, thereby increasing their likelihood of encountering warmer environments and potential hosts. However, it remains uncertain whether psychrophilic polar bacteria maintain pathogenic properties relevant to mammalian hosts during such transitions. Here, we investigated two hemolytic Antarctic Pseudomonas isolates, 3S6 and NS21, assigned to P. antarctica and P. yamanorum, respectively. Both isolates exhibited hemolytic activity, biofilm formation, and siderophore production, although these phenotypes were weaker than those of P. aeruginosa. Functional analyses indicated that their cell-associated hemolytic activity was dependent on the ExlBA two-partner secretion system (TPS), in which the outer-membrane transporter ExlB secretes the pore-forming cytotoxin ExlA: deletion of exlB abolished hemolysis, whereas genetic complementation restored this phenotype, and this activity was temperature-sensitive, consistent with repression of ExlBA expression under elevated temperatures. Temperature elevation from 20 °C to 28 °C reduced hemolytic activity, exlBA transcription, and intracellular cAMP levels, while strongly inducing cpdA, implicating a CpdA-cAMP/Vfr-associated regulatory mechanism. Elevated temperature also triggered a transition from a motile to a biofilm-associated lifestyle in strain 3S6, potentially reflecting adaptive resource allocation toward survival rather than virulence under thermal stress. Under host-relevant conditions, both isolates caused ExlBA-dependent membrane damage in mammalian cells and shortened Caenorhabditis elegans survival. In mice, they caused transient bloodstream invasion and acute lung inflammation without mortality or sustained colonization. Collectively, these findings indicate that psychrophilic hemolytic polar Pseudomonas possess restricted, temperature-responsive host-damaging capacity under exposure conditions associated with environmental warming.