Growth and adaptation of Zika virus in mammalian and mosquito cells

The recent emergence of Zika virus (ZIKV) in the Americas coincident with increased caseloads of microcephalic infants and Guillain-Barre syndrome has prompted a flurry of research on ZIKV. Much of the research is difficult to compare or repeat because individual laboratories use different virus isolates, growth conditions, and quantitative assays. Here we obtained three readily available contemporary ZIKV isolates and the prototype Ugandan isolate. We generated stocks of each on Vero mammalian cells (ZIKV(mam)) and C6/36 mosquito cells (ZIKV(mos)), determined titers by different assays side-by-side, compared growth characteristics using one-step and multi-step growth curves on Vero and C6/36 cells, and examined plaque phenotype. ZIKV titers consistently peaked earlier on Vero cells than on C6/36 cells. Contemporary ZIKV isolates reached peak titer most quickly in a multi-step growth curve when the amplifying cell line was the same as the titering cell line (e.g., ZIKV(mam) titered on Vero cells). Growth of ZIKV(mam) on mosquito cells was particularly delayed. These data suggest that the ability to infect and/or replicate in insect cells is limited after growth in mammalian cells. In addition, ZIKV(mos) typically had smaller, more homogenous plaques than ZIKV(mam) in a standard plaque assay. We hypothesized that the plaque size difference represented early adaptation to growth in mammalian cells. We plaque purified representative-sized plaques from ZIKV(mos) and ZIKV(mam). ZIKV(mos) isolates maintained the initial phenotype while plaques from ZIKV(mam) isolates became larger with passaging. Our results underscore the importance of the cells used to produce viral stocks and the potential for adaptation with minimal cell passages. In addition, these studies provide a foundation to compare current and emerging ZIKV isolates in vitro and in vivo.