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Due to theoretical and practical applications in biomedical, environmental, and industrial microbiology, robust metrics for quantifying the virulence of pathogens is vital. For many virus–host systems, multiple virus strains propagate through host populations. Each strain may exhibit a different virulence level. Likewise, different hosts may manifest different levels of host resilience to infection by a given virus. Recent publications have assessed metrics for quantifying virulence (VR) from growth curve data. Regardless of the metric used, a feature that most methods have in common is focus on the exponential growth phase of virus–host interactions. Often ignored is mortality phase. Following a report introducing the Stacy–Ceballos Inhibition Index (ISC), a robust metric to quantify relative virulence (VR) between viruses, we have turned attention to quantifying relative resilience (RR) between hosts in single-virus/single-host (SVSH) experimental infections. Although resilience during viral infection impacts the entire host growth curve, RR has particular biological significance during the mortality phase. In this report, we argue that calculating RR using a modified ISC provides a robust metric for comparisons between SVSH infections. Wet lab data from fusellovirus infections in Sulfolobales, bacteriophage infections in Mycobacteriales, and simulated infected-host growth profiles form the basis for developing this metric, RR, for quantifying resilience. 

Bacteriophages are a key part of natural environments, and they have a powerful ability to shape bacterial populations. To understand how individual phages interact with slow-growing bacterial hosts such as actinomycetes, an easy and reliable method for quantifying long-term bacterial growth in the presence of phages is needed. Spectrophotometric microplate readers allow for high-throughput repeated measurements, but incubating a small volume for an extended time can present technical challenges. This procedure adapts a standard 96-well microplate to allow for the co-culturing of phages and bacteria without sub-sampling for 96 h, with the bacterial growth recorded every 8 h using spectrophotometric absorbance values. These optical density values are analyzed using R to yield infection metrics, including the percent growth inhibition, relative virulence, and the Stacy-Ceballos index. The methods outlined here provide an effective way to conduct and analyze extended- duration microplate growth curve experiments and includes modifications to reduce evaporation and lid condensation. These protocols facilitate microplate-based assays of interactions between slow-growing bacterial hosts and their bacteriophages. 

ABSTRACT Chop, DelRio, and GrandSlam are phage with a Siphoviridae morphotype isolated from soil in Arkansas using the host Gordonia terrae 3612. All three are temperate, and their genomes share at least 96% nucleotide identity. These phage are assigned to cluster DI based on gene content similarity to other sequenced actinobacteriophage.