A persistent, productive, and seasonally dynamic vibriophage population within Pacific oysters (Crassostrea gigas)

Abstract

In an effort to understand the relationship between Vibrio and vibriophage populations, abundances of Vibrio spp. and viruses infecting Vibrio parahaemolyticus (VpVs) were monitored for a year in Pacific oysters and water collected from Ladysmith Harbor, British Columbia, Canada. Bacterial abundances were highly seasonal, whereas high titers of VpVs (0.5 x 10(4) to 11 x 10(4) viruses cm(-3)) occurred year round in oysters, even when V. parahaemolyticus was undetectable (< 3 cells cm(-3)). Viruses were not detected (<10 ml(-1)) in the water column. Host-range studies demonstrated that 13 VpV strains could infect 62% of the V. parahaemolyticus strains from oysters (91 pairings) and 74% of the strains from sediments (65 pairings) but only 30% of the water-column strains (91 pairings). Ten viruses also infected more than one species among V. alginolyticus, V. natriegens, and V. vulnificus. As winter approached and potential hosts disappeared, the proportion of host strains that the viruses could infect decreased by approximately 50% and, in the middle of winter, only 14% of the VpV community could be plated on summer host strains. Estimates of virus-induced mortality on V. parahaemolyticus indicated that other host species were required to sustain viral production during winter when the putative host species was undetectable. The present study shows that oysters are likely one of the major sources of viruses infecting V. parahaemolyticus in oysters and in the water column. Furthermore, seasonal shifts in patterns of host range provide strong evidence that the composition of the virus community changes during winter.

FIG. 1.

Temperature (⧫) and salinity (⋄) during May 2000 to May 2001.

FIG. 2.

Bacterial and viral abundances from May 2000 to May 2001. Closed symbols indicate abundances within oysters, and open symbols indicate water-column abundances. (A) Bacterial abundances: water-column total presumptive Vibrio spp. (□), oyster total presumptive Vibrio spp. (▪), and oyster V. parahaemolyticus (•). (B) Viral abundance in the oyster (▴). Values are means of duplicate samples; error bars indicate the ranges of duplicates and are smaller than the width of the symbol if not visible.

FIG. 3.

Total number of bacterial strains infected (sensitivity) by each viral strain from oysters in summer. Viruses are organized in increasing isolation date and correspond to those in Table 4.

FIG. 4.

Plating efficiency of summer versus winter oyster viral communities on strains of V. parahaemolyticus from oysters in summer. Host strains and source dates correspond to those in Table 4. Efficiencies are indicated on the figure for clarity and are relative to the control strain (E1-80). Values are means of duplicate samples; error bars indicate ranges of duplicates and are smaller than the width of the symbol if not visible.