Isolation and identification of freshwater bacteria antagonistic to Giardia intestinalis cysts.

We have isolated three freshwater bacterial strains that demonstrate the ability to degrade Giardia intestinalis cysts. These strains have been identified by 16S rRNA sequencing and phylogenetic analysis as belonging to the Flavobacterium columnare clade of the Cytophaga-Flavobacterium group. While the cyst degradation mechanism is unclear, two different effects on the cysts were observed: non-viability and lysis. Cysts exposed to bacterial strains BR1 and SC1 were generally non-viable, but remained structurally intact. In contrast, cysts exposed to strain SR1 were clearly lysed. Increases in bacterial densities with a concomitant decrease in cyst viability suggest that these bacterial strains are capable of using the cysts to enhance their growth. We propose that the presence of bacterial strains such as SR1, BR1 and SC1 may play a role in reducing the viability of G. intestinalis cysts in natural waters.