14 BACTERIOLOGY FOR NURSES 



those species which can grow only within or upon a living host 

 (plant or animal) are known as strict parasites. A hard and fast 

 line cannot be drawn between these two groups because there 

 exist certain organisms which are ordinarily saprophytes but 

 which may grow in living tissues and cause disease. Such forms 

 are spoken of as facultative parasites. Other species exist which 

 grow best as parasites in living tissue but which can be cultivated 

 on non-living material. The pathogenic organisms which can be 

 grown on culture media belong to this class. They are known as 

 facultative saprophytes. 



Moisture. Water is essential for bacterial growth. The dif- 

 ferent species vary in the degree of their need. The cholera spiril- 

 lum, if deprived of moisture, will die in from two to three hours; 

 the bacillus of diphtheria may live under the same conditions several 

 days. Spores are much more resistant to drying than vegetative 

 forms ; they will germinate after remaining in a dry condition for 

 years. It often happens, however, that organisms exposed to such 

 harmful influences, even though they survive, lose some of their 

 original properties. 



Osmosis. A certain degree of dilution is necessary for food 

 substances in solution. When bacteria suddenly find themselves 

 in a concentrated fluid they cannot readily adjust themselves, 

 and if the difference is too sudden or too great death may speedily 

 result. An illustration of this is the keeping qualities of a thick 

 syrup as compared with the rapid fermentation of a dilute sugar 

 solution. The best development of an organism takes place when 

 the osmotic pressure is the same in the surrounding fluid as that 

 within the cell itself. If the fluid is too concentrated water is 

 drawn from the bacterial cell and the protoplasm shrinks from its 

 outer covering ; the condition is spoken of as " plasmolysis." 

 If on the other hand the new fluid has a lower pressure the cell 

 absorbs more water and may burst. This latter condition is 

 termed " plasmoptysis." 



Oxygen. The free oxygen of the air is absolutely necessary 

 for the growth of the majority of organisms ; there is, however, 

 a small group which cannot live when it is present. Pasteur 



