Spot-on-lawn (halo) assay for screening enrichment cultures and
isolates for viruses—This protocol is based on Schleper et al.
(1992) as modified by Stedman et al. (2003). Gelrite plates are
preincubated ca. 10 min at 80°C to dry, then 10 mL of Sulfolobus
medium with ca. 0.2 % (w/v) Gelrite is boiled to dissolve
the Gelrite. This “softlayer” is allowed to cool slightly (to
ca. 80°C). Approximately 3 mL of softlayer are added to ca. 0.2
mL of exponentially growing host cells, generally Sulfolobus solfataricus,
and spread on a plate by swirling. After the Gelrite
solidifies, 1–2 μL of culture or supernatant to be screened is
spotted on the plate. For a positive control, 1 μL of a 0.01%
(v/v) Triton X-100 solution is spotted. Plates are incubated as
above for 2–3 d and plates examined for clearing around spots