Development of anin situ assay to detect bacterial pathogens of the red algaGracilaria gracilis (Stackhouse) Steentoft, Irvine et Farnham

Since current protocols were found to be inadequate for the identification of bacteria pathogenic toGracilaria gracilis, an assay was developed which made use of ‘axenic’ algae in order to attribute disease symptoms directly to the presence of a specific bacterial isolate. However, this assay proved to be as unreliable as existing procedures and failed to satisfy Koch's postulates. A second pathogenicity assay was developed which proved more reliable in that each bacterial strain consistently induced a particular symptom in the infected algal thallus. The bacterial strain LS2i was identified as a possible pathogen ofG. gracilis using this assay. However, this assay did not satisfy Koch's criteria for establishing an unequivocal link between the observed disease symptoms and strain LS2i, since significant bacterial contamination of the test alga occurred. A third protocol generated consistent results and satisfied Koch's postulates, enabling the identification of six pathogens ofG. gracilis. All the pathogenic bacterial strains were agarolytic.