CHEMICAL COMPOSITION OF BACTERIA 107 



cell.- If placed suddenlj^ in a solution of higher osmotic pressure than 

 the one in which it has been, the cell contents shrink away from the cell 

 wall {plasmoli/sis) indicating that there exists a semipermeable mem- 

 brane 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, indicating that it is not absolutely semipermeable. Dif- 

 ferent bacteria behave differently, some bacteria not l)eing plasmolyzed 

 by solutions that plasmol^^ze others. As a nde, old bacteria plas- 

 mol^'ze more rapidly than young, and in some varieties there seems 

 to be a spontaneous plasmolysis, to which lias been attributed the 

 irregular staining of diphtheria and tubercle bacilli, the polar stain- 

 ing of plague bacilli, etc. Plasmolysis occurs only in living bacilli, 

 but does not necessarily cause death. The Gram-staining bacteria 

 cannot generally be plasraolyzed, and contain more water.^ 



AVlien bacteria pass from solutions of higher osmotic concentration 

 into solutions of lower concentration, tlie phenomenon of 2)^dsmoi)tysis 

 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 the 

 degree of permeability of their cell membranes for salts. The ex- 

 posure of the naked cell contents to the hypotonic fluid outside the 

 cells makes plasmoptysis more serious for bacterial life than plasmo- 

 lysis, 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 chemotaetic influences, in general being attracted by sub- 

 stances favorable for food, such as peptone, dilute potassium salts, 

 etc., and being repelled by harmful substances, such as strong acids 

 and alkalies. Attempts have been made to separate different organ- 

 isms 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 been demonstrated. (The chemotaetic influence of bac- 

 teria upon leucocytes is discussed in Chapter x.) 



CHEMICAL COMPOSITION 



This varies greatly, not only between different species, but even 

 in the same species grown on different media ; in this respect bacteria 



2 Literature, see Gotschlich. 7\olle and \YasFerinann's Handbuch, vol. 1. 

 sNicolle and Alilaire. Ann. Inst. Pasteur, 1000 (23), 547. 



