Abstract
Detailed studies of the pH stability range of infectivity of plant viruses, summarized recently by Stanley, 1 show that for each virus studied there is a well defined region where infectivity is relatively stable. On both sides of this region infectivity is lost in a regular way so that the curve plotted to relate infectivity with pH of the medium is smooth and trapezoidal. Certain of the animal viruses 2 , 3 appear to behave in a like manner. It was entirely unexpected, then, when conditions were encountered under which the virus of equine encephalomyelitis (Eastern Strain) seemed to behave in a different way.
The experiments were made on virus propagated in chick embryos. 4 Virus-infected embryos were “harvested” when moribund and ground with sand in hormone broth to a 10% suspension. Centrifuged free of sand and gross tissue particles, the whole extract was mixed with composite buffer solution (0.05M) of various pH's. 2 , 3 Mixtures were maintained between 0°C and 5°C, and after various intervals, usually 1 hour, 1 day, and 1 week, portions were removed for test. The pH of each was readjusted approximately to neutrality and virus infectivity determined by titrations in decimal dilutions in mice. 5 pH was determined and frequently checked with the glass electrode.
Four such experiments covering the range from pH 1.0 to pH 12.0 have shown essentially uniform results, and one is summarized in Fig. 1. The curve after 1 hour is similar in contour to those of other viruses, the region of greatest stability lying between pH 3.5 and pH 11.5. After 1 week, maximum stability is at pH 7.5 to pH 8.5, while a second region of relative stability is apparent between pH 3.5 to pH 5.0.
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