| Summary: | Infection with the protozoan ectoparasite <i>Neoparamoeba perurans</i>, the causative agent of AGD, remains a global threat to salmonid farming. This study aimed to analyse the exoproteome of both an attenuated and virulent <i>N. perurans</i> isolate using proteomics and cytotoxicity testing. A disproportionate presence of proteins from the co-cultured microbiota of <i>N. perurans</i> was revealed on searching an amalgamated database of bacterial, <i>N. perurans</i> and Amoebozoa proteins. LC-MS/MS identified 33 differentially expressed proteins, the majority of which were upregulated in the attenuated exoproteome. Proteins of putative interest found in both exoproteomes were maltoporin, ferrichrome-iron receptor, and putative ferric enterobactin receptor. Protease activity remained significantly elevated in the attenuated exoproteome compared with the virulent exoproteome. Similarly, the attenuated exoproteome had a significantly higher cytotoxic effect on rainbow trout gill cell line (RTgill W1) cells compared with the virulent exoproteome. The presence of a phosphatase and serine protease in the virulent exoproteome may facilitate AGD infection but do not appear to be key players in causing cytotoxicity. Altogether, this study reveals prolonged culture of <i>N. perurans</i> affects the exoproteome composition in favour of nutritional acquisition, and that the current culturing protocol for virulent <i>N. perurans</i> does not facilitate the secretion of virulence factors.
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