1. Living pneumococcus cells contain a group of enzymes, the bacteriolytic system, capable of causing the lysis of heat-killed pneumococci (R and S variants irrespective of type derivation). This lysis expresses itself by a loss of the Gram staining reaction, a disintegration of the cell body, and a clearing of the bacterial suspension.

2. Under certain conditions of treatment with the bacteriolytic complex, it is possible to render the cocci Gram-negative without changing their characteristic morphology, or causing any appreciable clearing of the cell suspension.

3. The enzyme responsible for this change has been partially purified, and some of its properties described.

4. The cellular structure which is responsible for the Gram-positive reaction of pneumococci is resistant to proteolytic enzymes, and is still present when tryptic digestion has reduced the heat-killed cell to a body which has lost 75 per cent of its original weight, and contains only 8 per cent nitrogen.

5. The same enzyme preparation which attacks pneumococci is also capable of liberating reducing sugars from some acetyl amino glucose glucuronides of animal and bacterial origin. The possibility is considered, and discussed, that one and the same enzyme in the autolytic complex is capable of attacking both types of substrates.

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