SUMMARY: During the course of adaptation to the mouse lung of the Rob strain of influenza B, ‘pure’ clones of virus were isolated from bronchial washings at succeeding passages by the limiting infective dilution technique in chick embryos. The clones were studied for various and properties. A progressive dominance was obtained in the virus population of virus particles causing extensive pulmonary consolidation. Those particles which produced extensive consolidation became resistant to inhibition of heated virus haemagglutinin by sheep mucin. A complete correlation was found between the appearance of resistance to sheep mucin inhibitor and a decrease of enzymic activity on sheep mucin.

From the heterogeneous population of particles found in the early course of mouse passage a virus was isolated which exhibited all of the tested properties of the adapted virus. In comparison to a non-pathogenic virus form, these properties included: () the production of extensive pulmonary consolidation, high mortality of mice, and multiplication of virus on serial mouse passage; () the production of a more rapid growth rate, and the ability to multiply to a greater extent; () the inability of heated virus haemagglutinin to be inhibited by sheep mucin and ovomucin; () a decrease of enzymic activity on sheep mucin and mouse lung inhibitors under certain experimental conditions; and () a high position in the fowl red cell receptor gradient.

The above data offer presumptive evidence that one of the processes operative in the adaptation process of Rob virus to the mouse lung is a selection of mutants found in the unadapted heterogeneous virus population.


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