Quantifying relative virulence: when µmax fails and AUC alone just is not enough.

A challenge in virology is quantifying relative virulence (VR) between two (or more) viruses that exhibit different replication dynamics in a given susceptible host. Host growth curve analysis is often used to mathematically characterize virus-host interactions and to quantify the magnitude of detriment to host due to viral infection. Quantifying VR using canonical parameters, like maximum specific growth rate (µmax), can fail to provide reliable information regarding virulence. Although area-under-the-curve (AUC) calculations are more robust, they are sensitive to limit selection. Using empirical data from Sulfolobus Spindle-shaped Virus (SSV) infections, we introduce a novel, simple metric that has proven to be more robust than existing methods for assessing VR. This metric (ISC) accurately aligns biological phenomena with quantified metrics to determine VR. It also addresses a gap in virology by permitting comparisons between different non-lytic virus infections or non-lytic versus lytic virus infections on a given host in single-virus/single-host infections.

Host-Dependent Differences in Replication Strategy of the Sulfolobus Spindle-Shaped Virus Strain SSV9 (a.k.a., SSVK1): Infection Profiles in Hosts of the Family Sulfolobaceae.

The Sulfolobus Spindle-shaped Virus (SSV) system has become a model for studying thermophilic virus biology, including archaeal host-virus interactions and biogeography. Several factors make the SSV system amenable to studying archaeal genetic mechanisms (e.g., CRISPRs) as well as virus-host interactions in high temperature acidic environments. Previously, we reported that SSVs exhibited differential infectivity on allopatric vs. sympatric hosts. We also noticed a wide host range for virus strain SSV9 (a.k.a., SSVK1). For decades, SSVs have been described as "non-lytic" double-stranded DNA viruses that infect species of the genus Sulfolobus and release virions via budding rather than host lysis. In this study, we show that SSVs infect hosts representing more than one genus of the family Sulfolobaceae in spot-on-lawn "halo" assays and in liquid culture infection assays. Growth curve analyses support the hypothesis that SSV9 virion release causes cell lysis. While SSV9 appears to lyse allopatric hosts, on a single sympatric host, SSV9 exhibits canonical non-lytic viral release historically reported SSVs. Therefore, the nature of SSV9 lytic-like behavior may be driven by allopatric evolution. The SSV9-infected host growth profile does not appear to be driven by multiplicity of infection (MOI). Greater stability of SSV9 vs. other SSVs (i.e., SSV1) in high temperature, low pH environments may contribute to higher transmission rates. However, neither higher transmission rate nor relative virulence in SSV9 infection seems to alter replication profile in susceptible hosts. Although it is known that CRISPR-Cas systems offer protection against viral infection in prokaryotes, CRISPRS are not reported to be a determinant of virus replication strategy. The mechanisms underlying SSV9 lytic-like behavior remain unknown and are the subject of ongoing investigations. These results suggest that genetic elements, potentially resulting from allopatric evolution, mediate distinct virus-host growth profiles of specific SSV-host strain pairings.