Multiple bacterial infection of Alexandrium catenella (Dinophyceae)

Chilean clones of Alexandrium catenella are shown to be infected with bacteria. The endocyto-plasmic bacteria have been isolated and grown in culture. Using biochemical characterization, they were tentatively identified as Aeromonas salmonicida, Flavobacterium breve, Pseudomonas diminuta, Pasteurella haemolytica, Proteus vulgaris, Pseudomonas putida, Pseudomonas versicularis and Moraxella sp.-like. In several instances, the analysis showed that A. catenella clones are simultaneously infected by different species of bacteria. Intracellular bacterial localization was demonstrated by confocal and electron microscope observations. Viability of intracellular bacteria was confirmed by using a specific dye that becomes fluorescent when it is reduced by electrons generated by respiration of live organisms. Thus, the bacteria are alive and appear to be dividing inside the cells. Antibiotic treatment of A. catenella did not generate bacteria-free cells, leading instead to the killing of the host cells. Of all bacterial strains isolated, only Moraxella sp.-like and P. diminuta are capable of producing small amounts of saxitoxin, as detected by HPLC. Toxic Moraxella sp.-like bacterium was isolated only from one A. catenella clone and confirmed by western blot analysis, suggesting that bacterial infection might be clone specific.