Effects of phosphorus limitation on viral population dynamics

Background/Question/Methods

A surprising number of macroecological patterns are correlated with organismal phosphorus content; however few studies have experimentally investigated the short-term (ecological) and long-term (evolutionary) nature of these patterns. We empirically investigate the effect of phosphorus on both the ecological and evolutionary dynamics of West Nile Virus (WNV). We first developed a stage-structured model of the intra-host population dynamics of a flavivirus, where the rates of viral attachment, assembly and release are affected by the concentration of phosphorus in the extracellular matrix. We then conducted an in vitro serial passage experiment to quantify the effects of phosphorus on the growth of a 2006 isolate of WNV from Indian River County, FL, grown in Vero cells. We record the growth rate of the cells non-destructively, using image analysis (ToupView). We quantify the number of virus particles that are either attached to the cells or floating in the matrix using focus-forming unit (plaque) assays. We present results from an experimental evolution study in which we allow the virus to evolve under three levels of phosphorus for approximately 100 generations.

Results/Conclusions

We found that the growth rate of Vero cells is limited by the concentration of phosphorus in the cell culture medium: at high levels of phosphorus (120 mg/L), cell replication rate is doubled. At low levels of phosphorus (15 mg/L), cell replication rate is less than half the rate observed in normal phosphorus conditions (50-75 mg/L). This has strong implications for the rate of viral replication, which is tightly coupled with cell replication rate. Previous studies have shown that the growth of WNV in vitro is affected by temperature and pH; however to date no studies have determined how rate-limiting nutrients can affect the rates of growth and mutation of any of the flaviviruses. Furthermore, there is evidence that the concentration of phosphorus within an organism can vary substantially according to dietary concentrations. We discuss how our findings can be used to better understand the ecology and evolution of viruses. These findings can be useful for controlling viruses such as WNV, which is transmitted by mosquitos that reproduce in variable nutrient environments.