In the course of the studies on the i mutation of polioviruses it was found unexpectedly that mutants of poliovirus, forming minute plaque-m mutants were selected in some series of the serial passages of type 2 poliovirus (MEF-1) in tissue culture. Similarly m mutant of type 3 poliovirus (Saukett) was also obtained.
Selection of m mutants occurred usually in the course of several passages in tissue culture, but the factor or factors responsible for the selection of the m mutants remained undetermined.
The plaque size of m mutants was less than 0.5mm in diameter in HeLa, FL and monkey kidney cell monolayers, and certain factors such as PH and amount of agar overlay did not influence the plaque size.
m mutants were neutralized specifically by antiserum prepared against the respective m+ wild type viruses.
No apparent differences in both the cytopathogenic effect and the pattern of growth were observed between m+ and m mutants.
m mutants, after being purified repeatedly by limiting dilution technique, were proven to be stable through rapid serial passages in tissue cultures or mice.
m mutants of type 2 poliovirus were virulent for mice but a few of them were found to be less virulent than the wild type m+ virus.
m mutant, isolated after serial passages in tissue culture with inhibitory normal bovine serum, was found to have i character as well as the back mutant m+ i, obtained from the m i mutant. Furthermore i m mutant was obtained from i+ m mutant through serial passages in tissue culture with medium containing inhibitory bovine serum. Therefore it may be concluded that i and m mutation can occur independently from each other.
