In the first paper of this series, a description was given of a cell-free extract, obtained from autolysates of a particular strain of a soil bacillus, which selectively inhibits the growth of all the Gram-positive microorganisms so far tested, and exerts on them a bactericidal effect in vitro.

In the present study it is shown that the same agent protects white mice against infection with large numbers of virulent pneumococci. It also exerts a curative effect when administered to mice several hours after injection of the infecting organisms.

The degree of protection afforded, and the minimal effective dose of bactericidal agent, are approximately the same for all virulent pneumococci, irrespective of type specificity.

The bactericidal agent is entirely ineffective against infection with virulent Friediänder bacilli (type B). This agrees with the fact that the agent does not affect Gram-negative bacilli in vitro.

The protective action exerted by the bactericidal agent against experimental pneumococcus infection depends upon the same mechanism which determines its bactericidal effect in vitro.

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