Isolation and characterisation of poliovirus mutants resistant to heating at 50 degrees Celsius for 30 min

Poliovirus is heat-labile; on heating at 50 degrees Celsius for 30 min its infectivity decreases drastically and its antigenicity reverts from N to H. However, mutants resistant to heating at 50 degrees Celsius for 30 min from the Sabin 1 and 2 viruses were isolated by repeating the process of incubation of the virus stock at 50 degrees Celsius for 30 min and multiplication of the remaining virus in a cell culture. The isolated mutants were stable genetically, and maintained the rct and d markers of the parent virus. On electron microscopical examination, the mutants were observed to retain the intact morphology after being heated at 50 degrees Celsius for 30 min, while the parent virus was converted to empty particles devoid of RNA under the same conditions. On determination of the nucleotide sequence of the P1 region, a single nucleotide sequence substitution was detected at nucleotide no. 2741, resulting in an amino acid change from valine to alanine at the 87th position of VP1. This amino acid might be associated with the heat-resistance of the mutants. Furthermore, it was found that the thermostable mutants obtained in this study, which are resistant to "high" temperature (50 degrees Celsius) for a short time (30 min), were not stable against heating at the ambient temperature (37 degrees Celsius) for a long time (5 or 7 days). This suggests that the inactivation at high temperature for a short time and that at ambient temperature for a long time involve different mechanisms.