Particle Counts of Meningopneumonitis Virus by Phase Microscopy.

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Abstract
A simple, rapid technique for counting meningopneumonitis virus particles has been developed. A suspension of polystyrene particles 0.81 [mu] in diameter was prepared in distilled water and diluted to contain 109.14 particles per ml as determined by counts in a bacteria counting chamber. This standard suspension was found to be stable for at least 2 months when stored at 4[degree]C. For counting, equal volumes of virus suspension and standard polystyrene suspension were mixed and thin smears were made with transfer loops on glass slides. After heat fixing, the smears were stained 5 minutes in basic fuchsin, decolorized briefly with 0.5% citric acid and washed with water. Dried smears were examined under oil immersion using phase contrast and the ratio of the 2 types of particles determined. A grid was placed in the microscope ocular to facilitate counting. The precision of the method was shown by repeated counts on the same virus suspensions. The validity of the determinations was supported by demonstrating constant relationships between virus counts obtained by this method and egg infectivity, total N content and electron microscope counts.
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