Bacteriophage lytic to Desulfovibrio aespoeensis isolated from deep groundwater.

Viruses were earlier found to be 10-fold more abundant than prokaryotes in deep granitic groundwater at the Aspö Hard Rock Laboratory (HRL). Using a most probable number (MPN) method, 8-30 000 cells of sulphate-reducing bacteria per ml were found in groundwater from seven boreholes at the Aspö HRL. The content of lytic phages infecting the indigenous bacterium Desulfovibrio aespoeensis in Aspö groundwater was analysed using the MPN technique for phages. In four of 10 boreholes, 0.2-80 phages per ml were found at depths of 342-450 m. Isolates of lytic phages were made from five cultures. Using transmission electron microscopy, these were characterized and found to be in the Podoviridae morphology group. The isolated phages were further analysed regarding host range and were found not to infect five other species of Desulfovibrio or 10 Desulfovibrio isolates with up to 99.9% 16S rRNA gene sequence identity to D. aespoeensis. To further analyse phage-host interactions, using a direct count method, growth of the phages and their host was followed in batch cultures, and the viral burst size was calculated to be approximately 170 phages per lytic event, after a latent period of approximately 70 h. When surviving cells from infected D. aespoeensis batch cultures were inoculated into new cultures and reinfected, immunity to the phages was found. The parasite-prey system found implies that viruses are important for microbial ecosystem diversity and activity, and for microbial numbers in deep subsurface groundwater.

Viruses in granitic groundwater from 69 to 450 m depth of the Aspö hard rock laboratory, Sweden.

The objectives of this study were to determine if viruses exist in deep granitic groundwater and to analyse their abundance and morphological diversity. Fluorescent microscopy counts on 10 groundwater samples ranging from 69 to 450 m depth were in the range of 10(4)-10(6) TNC ml(-1) (TNC, total number of prokaryotic cells) and 10(5)-10(7) VLP ml(-1) (VLP, virus-like particles). A good positive correlation of VLP with TNC (r=0.91, P=0.0003) was found with an average VLP/TNC ratio of 12. Transmission electron microscopy revealed four distinct bacteriophage groups (polyhedral, tailed, filamentous and pleomorphic) with at least seven phage families of which some are known to be lytic. Our results suggest the presence of viruses in deep granitic groundwater up to 450 m depth. If they are active and lytic, they will constitute an important group of predators that might control the numbers of microorganisms in the analysed groundwater.