MORPHOLOGY, REPRODUCTION, ETC. 25 



form, and the tliio bacteria contain insoluble sulphur in their proto- 

 plasm. 



The chemical constitution of bacteria is of considerable impor- 

 tance in connection with problems of immunization. It is not at all 

 sure that the antigenic substances in bacteria consist entirely of 

 proteins of the albumin-globulin variety. Extracts of bacterial 

 bodies, such as those we have, on a number of occasions, produced 

 with typhoid bacilli, tubercle bacilli, etc., contain very large amounts 

 of material, non-coagulable by heat, which come down on the addi- 

 tion of weak acetic acid in the cold, and which are soluble only in 

 dilute alkali, such as the antigenic proteins contained in many 

 plants. It is not impossible that these may be the decisive antigenic 

 constituents, rather than the albumin- globulin contents. 



Osmotic Properties of the Bacterial Cell. Like all other animal 

 and vegetable cells, the bacterial cell forms in itself a small osmotic 

 unit which reacts delicately to differences of pressure existing be- 

 tween its own protoplasm and the surrounding medium. The perfect 

 and normal morphology of a microorganism, therefore, can exist 

 only when the osmotic pressure within the protoplasm of the cell is 

 isotonic or equal to that of its own environment. The changes pro- 

 duced in the morphological relations of a cell when transferred from 

 one environment into another of varying osmotic pressure, depend 

 intimately upon the "permeability" of the cell membrane for dif- 

 ferent substances. When such a membrane is permeable for water 

 and not for substances in solution, it is technically spoken of as 

 "semi-permeable." Now, as a matter of fact, the bacterial cell 

 membrane is easily permeable for water, but its permeability differs 

 greatly in various species of bacteria for other substances. Thus, 

 for instance, the cholera vibrio shows great permeability for common 

 salt and B. fluorescens liquefaciens shows a lower permeability for 

 potassium nitrate than do many other bacteria. 38 



When a microorganism is suddenly removed from an environ- 

 ment of low osmotic pressure into one showing a high pressure, say, 

 from a dilute to a concentrated solution of NaCl, an abstraction of 

 water from the cell occurs, with a consequent shrinkage of the proto- 

 plasm away from the cell membrane. This process is spoken of as 

 "plasmolysis. " Cell death does not usually occur with plasmolysis, 

 but by slow diffusion of the salt itself into the protoplasm, the 



38 Gottschlich, in Fliigge, ' ' Mikroorganismen, " i, p. 91. 



