58 VETERINARY BACTERIOLOGY 



easily isolated from salt fish. When grown in a test-tube, they 

 are sometimes sufficiently luminous, so that they may be photo- 

 graphed by their own light. 



FERMENTATION AND ENZYME PRODUCTION 



All microorganisms have their protoplasm bounded by the 

 ectoplast, a semipermeable membrane, as previously shown. The 

 cell wall, when present, seems to be a mechanical protection and 

 support, and is readily permeable to most substances in solution, 

 so may be disregarded in discussion. Whatever food or other mate- 

 rials are taken into the protoplasm must pass by diffusion through 

 the ectoplast. This membrane, on the other hand, must prevent 

 any valuable constituent of the cell from leaving by diffusion. 

 Its action is, therefore, selective. Microorganisms do not always 

 find the potential food materials with which they are surrounded 

 suitable for food, for they may be in a solid form or, if in solution, 

 of such a character that they cannot pass through the ectoplast. 

 Many organisms find it necessary to so change this food and 

 digest it that it may be assimilated. Once inside the cell, it 

 usually is not of such a character that it can be built up directly 

 into the protoplasm and further changes are necessary; or if the 

 material is used simply as a source of energy and not incorporated 

 into the cell substance, it is essential that the cell have some means 

 of developing this energy. All cells accomplish these changes by 

 means of enzymes (Gr. en, within, zyme, leaven). A dis- 

 tinction was once made between the so-called organized and 

 unorganized ferments. The former was held to be living cells 

 which could bring about a change or fermentation, the latter any 

 cell secretion which could bring about such changes. In other 

 words, jorgani/cd ferments were supposed to owe their activity 

 directly to the protoplasm, the unorganized, to substances secreted 

 by the protoplasm. This distinction is no longer maintained, 

 as it seems altogether probable that fermentative changes of 

 whatever kind are brought about by the secreted enzymes or 

 unorganized ferments. Enzymes may be intra- or extracellular. 

 The intracellular enzymes are extracted from the protoplasm 

 with difficulty, and during the life of the cell do not leave it. 

 Such an enzyme is that of bread or brewer's yeast (the zymase), 



