STRUCTURE AND PHYSICAL PROPERTIES 105 



Plasmolysis and Plasmoptysis. Under conditions of 

 altered osmotic pressure the bacterial cell behaves quite similarly 

 to the plant cell . l If placed suddenly in a solution of higher 

 osmotic pressure than the one in which it has been, the cell con- 

 tents shrink away from the cell wall (plasmolysis) indicating 

 that there exists a semipermeable membrane through which 

 water passes more rapidly than salts. If the change in osmotic 

 pressure is gradual, the bacteria accommodate themselves to it by 

 the slow diffusion of the salts through the cell membrane, in- 

 dicating that it is not absolutely semipermeable. Different 

 bacteria behave differently, some bacteria not being plasmolyzed 

 by solutions that plasmolyze others. As a rule, old bacteria 

 plasuiolyze more rapidly than young, and in some varieties 

 there seems to be a spontaneous plasmolysis, to which has been 

 attributed the irregular staining of diphtheria and tubercle 

 bacilli, the polar staining of plague bacilli, etc. Plasmolysis 

 occurs only in living bacilli, but does not necessarily cause 

 death. 



When bacteria pass from solutions of higher osmotic concen- 

 tration into solutions of lower concentration, the phenomenon 

 of plasmoptysis is produced. The cell contents swell until the 

 cell wall gives way at some point, and then exude as glistening 

 drops, which may become detached from the wall and escape 

 free into the fluid. Plasmoptysis is shown best by bacteria 

 that have been grown on salt-rich media before being placed in 

 the salt-free fluid. Not all varieties of bacteria can be made to 

 undergo this change, depending probably upon a greater perme- 

 ability of their cell membranes for salts. The exposure of the 

 naked cell contents to the hypotonic fluid outside the cells 

 makes plasmoptysis more serious for bacterial life than plasmol- 

 ysis, but how often either process plays a part in the resistance 

 of infected animals against bacteria is unknown. 



Chemotaxis. Just as with unicellular animal organisms, 

 bacteria respond to chemotactic influences, in general being 

 attracted by substances favorable for food, such as peptone, 

 dilute potassium salts, etc., and being repelled by harmful sub- 

 stances, such as strong acids and alkalies. Attempts have been 

 made to separate different organisms in mixed cultures by means 

 of their response to chemotaxis, but without striking success. 

 It is possible that chemotaxis may play a part in the localization 

 of bacteria from the blood stream in favorable localities, just as 

 leucocytes are attracted to points of injury, but this has not 



1 Literature, see Gotschlich, Kolle and Wassermann's Handbuch, 1903, vol. 

 1, p. 62. 



