PROPERTIES USED FOR IDENTIFICATION 11 



IDENTIFICATION 

 General 



An organism is identified by a consideration of many 

 properties including its shape, staining reactions, biochemical 

 reactions, pathogenicity, etc. Many of the properties of an 

 organism tend to change with conditions of cultivation, such 

 as the nature of the growth medium, the age of the culture, 

 the temperature, the degree of aerobiosis, etc., and conse- 

 quently classification must be based as far as possible upon 

 stable properties tested under standard conditions. There 

 is no point in this book in attempting a detailed account of 

 the theory or practice of systematic classification, but the 

 non-bacteriologist requires some guidance concerning the 

 identification of particular organisms, so the following 

 represents a brief account of the properties which are 

 investigated for purposes of classification. 



Morphology 



Some bacteria are spherical, some rod-shaped, comma- 

 shaped, or twisted like a spiral, and all varieties of intermediate 

 shapes occur. The shape, where it is constant, is easily 

 observed through the microscope and formed the basis of 

 many of the earlier systems of classification. Thus spherical 

 organisms were called " Cocci," rod-shaped organisms 

 *' Bacilli," and spiral-shaped " Spirilla." This simple 

 morphological grouping has nowadays been complicated by 

 subdivision of the groups on a basis of other characteristics. 



Spore formation 



Some bacteria possess the capacity to produce spores which 

 are a resting or non-vegetative form considerably more 

 resistant to heat, desiccation, or unfavourable chemical 

 environments than the vegetative forms. It was thought at one 

 time that these organisms form spores when their environ- 

 ment becomes unsuitable for continued vegetative existence 

 but this is not necessarily the case, as it is known that some 



