Harmful Aeromonas spp. associated with the pathogenic tilapia parasite Gyrodactylus cichlidarum – Double trouble?

Co-infections involving monogenean parasites and pathogenic bacteria frequently result in increased fish mortality. Among the most problematic pathogens affecting tilapia aquaculture are monogenean parasites, such as the cosmopolitan Gyrodactylus cichlidarum, as well as opportunistic bacteria like Aeromonas spp. Bacterial infections are assumed to be opportunistic, facilitated by the parasite's breaching the fish host's first line of defense (mucus-covered skin) with its attachment hooks and through feeding. Here, using scanning electron microscopy, metataxonomy, microbiology and matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) we characterized bacterial communities associated with farmed tilapia and G. cichlidarum. We demonstrate that bacterial communities associated with G. cichlidarum were significantly different and independent from those collected from fish or the environment and enriched in members of the genus Aeromonas; in particular, the pathogenic species Aeromonas veronii and A. caviae. The strong and specific association between Aeromonas and this monogenean suggests that G. cichlidarum might in effect be both a pathogen itself and a vector for a secondary pathogen – a hypothesis yet to be tested.