Diversity of Flavobacterium psychrophilum and the potential use of its phages for protection against bacterial cold water disease in salmonids.

Flavobacterium psychrophilum causes rainbow trout fry syndrome (RTFS) and cold water disease (CWD) in salmonid aquaculture. We report characterization of F. psychrophilum strains and their bacteriophages isolated in Chilean salmonid aquaculture. Results suggest that under laboratory conditions phages can decrease mortality of salmonids from infection by their F. psychrophilum host strain. Twelve F. psychrophilum isolates were characterized, with DNA restriction patterns showing low diversity between strains despite their being obtained from different salmonid production sites and from different tissues. We isolated 15 bacteriophages able to infect some of the F. psychrophilum isolates and characterized six of them in detail. DNA genome sizes were close to 50 Kbp and corresponded to the Siphoviridae and Podoviridae families. One isolate, 6H, probably contains lipids as an essential virion component, based on its chloroform sensitivity and low buoyant density in CsCl. Each phage isolate rarely infected F. psychrophilum strains other than the strain used for its enrichment and isolation. Some bacteriophages could decrease mortality from intraperitoneal injection of its host strain when added together with the bacteria in a ratio of 10 plaque-forming units per colony-forming unit. While we recognize the artificial laboratory conditions used for these protection assays, this work is the first to demonstrate that phages might be able protect salmonids from RTFS or CWD.

Effects of Bacteriophages on the Population Dynamics of Four Strains of Pelagic Marine Bacteria.

Viral lysis of specific bacterial populations has been suggested to be an important factor for structuring marine bacterioplankton communities. In the present study, the influence of bacteriophages on the diversity and population dynamics of four marine bacterial phage-host systems was studied experimentally in continuous cultures and theoretically by a mathematical model. By use of whole genome DNA hybridization toward community DNA, we analyzed the dynamics of individual bacterial host populations in response to the addition of their specific phage in continuous cultures of mixed bacterial assemblages. In these experiments, viral lysis had only temporary effects on the dynamics and diversity of the individual bacterial host species. Following the initial lysis of sensitive host cells, growth of phage-resistant clones of the added bacteria resulted in a distribution of bacterial strains in the phage-enriched culture that was similar to that in the control culture without phages after about 50-60 h incubation. Consequently, after a time frame of 5-10 generations after lysis, it was the interspecies competition rather than viral lysis of specific bacterial strains that was the driving force in the regulation of bacterial species composition in these experiments. The clonal diversity, on the other hand, was strongly influenced by viral activity, since the clonal composition of the four species in the phage-enriched culture changed completely from phage-sensitive to phage-resistant clones. The model simulation predicted that viral lysis had a strong impact on the population dynamics, the species composition, and the clonal composition of the bacterial community over longer time scales (weeks). However, according to the model, the overall density of bacteria in the system was not affected by phages, since resistant clones complemented the fluctuations caused by viral lysis. Based on the model analysis, we therefore suggest that viral lysis can have a strong influence on the dynamics of bacterial populations in planktonic marine systems.

Bacterial Growth Rate and Marine Virus-Host Dynamics.

The dynamics of a marine virus-host system were investigated at different steady state growth rates in chemostat cultures and the data were analyzed using a simple model. The virus-host interactions showed strong dependence on host cell growth rate. The duration of the infection cycle and the virus burst size were found to depend on bacterial growth rate, and the rate of cell lysis and virus production were positively correlated with steady state growth rate in the cultures (r(2) > 0.96, p < 0.05). At bacterial growth rates of 0.02 to 0.10 h(-1) in the chemostats the virus burst size increased from 12 +/- 4 to 56 +/- 4, and the latent period decreased from 2.0 to 1.7 h. Resistant clones of the host strain were present in the cultures from the beginning of the experiment and replaced the sensitive host cells following viral lysis in the cultures. Regrowth of resistant cells correlated significantly (r(2) = 1.000, p < 0.02) with the lysis rate of sensitive cells, indicating that release of viral lysates stimulated growth of the non-infected, resistant cells. The constructed model was suitable for simulating the observed dynamics of the sensitive host cells, viruses and resistant clones in the cultures. The model was therefore used in an attempt to predict the dynamics of this virus-host interaction in a natural marine environment during a certain set of growth conditions. The simulation indicated that a steady state relationship between the specific viruses and sensitive and resistant bacterial clones may occur at densities that are reasonable to assume for natural environments. The study demonstrates that basic characterization and modeling of specific virus-host interactions may improve our understanding of the behavior of bacteria and viruses in natural systems.

Effects of viruses on nutrient turnover and growth efficiency of noninfected marine bacterioplankton.

The effects of virus infection and lysis of a marine Vibrio sp. on C, N, and P turnover and the growth efficiency of noninfected bacterioplankton were studied in a series of dilution cultures. The cultures were enriched with various sources of organic matter and N and P. The growth of the Vibrio host and the growth of the natural bacterioplankton were measured by immunofluorescence and 4(prm1),6-diamidino-2-phenylindole staining methods, respectively. Lysis products resulting from infection of the Vibrio sp. caused an increase in metabolic activity and cell production by the noninfected bacterioplankton. In P-limited cultures, the addition of viruses increased the uptake of dissolved organic carbon by 72% and the potential alkaline phosphatase activity by 89% compared with control cultures without viruses. Our data suggest that input of available phosphorus through virus-induced Vibrio lysates occurred, which caused an increase in the bacterial nutrient uptake. The growth efficiency of noninfected bacteria was reduced in the presence of viruses compared with the control without viruses (growth efficiencies, 0.08 (plusmn) 0.03 and 0.24 (plusmn) 0.02, respectively). We suggest that the decrease in growth efficiency may be explained by an increase in bacterial energy demand associated with extracellular degradation of polymeric organic nitrogen and phosphorus in cell lysates.

Attached and free-living bacteria: Production and polymer hydrolysis during a diatom bloom.

Abundance, production and extracellular enzymatic activity of free-living and attached bacteria were measured during the development and collapse of a spring bloom in a eutrophic lake. Free-living bacteria accounted for most of the total bacterial production during the first part of the bloom. Their production had a significant positive correlation to chlorophyll (P < .01) and polysaccharide concentration (P < .02) and to potential ß-glucosidase and aminopeptidase activity (P < .05), suggesting that algal release of dissolved polymeric compounds provided an important carbon source for bacterial production. As the bloom collapsed, we observed a change in the activity and structure of the microbial community. The mean contribution of attached bacteria to total bacterial production increased from 12% during the first part of the bloom to 26% at the end. Also, the extracellular enzymatic activity of attached bacteria increased as the bloom collapsed and constituted up to 75% of the total hydrolytic activity. An estimated disparity between hydrolytic activity and the corresponding carbon demand of attached bacteria suggested a net release of dissolved organic compounds from organic particles via polymer hydrolysis by attached bacteria.

Bacterioplankton Growth Yield: Seasonal Variations and Coupling to Substrate Lability and beta-Glucosidase Activity.

The seasonal variation in the carbon growth yield of pelagic bacteria in the eutrophic lake Frederiksborg Slotssø was studied. The growth yield was determined in dilution culture experiments, in which a substrate of dissolved organic carbon (DOC) from the lake was incubated with a natural bacterioplankton assemblage. Bacterial growth efficiency varied annually from 8 to 60% with an average (and standard deviation) of 41 +/- 11% (n = 29). Simultaneous measurements of growth yield, substrate lability (DOC(L)), chlorophyll and bacterial production, abundance, and extracellular enzymatic activity revealed new aspects of the regulation of bacterial DOC utilization. Growth yield correlated positively to DOC(L) and negatively to beta-d-glucosidase activity. These results indicated a close coupling between the substrate conditions and the physiological response of the bacteria. The large variations in yield within a few days and the close coupling to substrate availability showed that one single global carbon yield factor cannot be expected to apply in pelagic systems.