IO6 BACTERIOLOGY 



Methods of isolating bacteria, which are of more general appli- 

 cation, were introduced by Koch. The essential characteristic 

 of these methods is the dilution of the bacteria in a fluid medium 

 which quickly becomes solid so that each germ develops in a 

 definite fixed position in the medium. The great progress which 

 bacteriology has made during the last twenty years is largely 

 owing to these methods. 



It is impossible in most cases to distinguish between bacteria 

 of different varieties by microscopical examination alone. Bac- 

 teria of widely different species and quite unlike one another in 

 their properties may present similar appearances under the mi- 

 croscope. The differences which they exhibit are usually appar- 

 ent when they are grown in culture-media. The growth, called 

 a colony, which results from the multiplication of a single 

 bacterium, is in many cases very characteristic for the species. 

 By the plate-method, the individual bacteria in a mixture are 

 separated from one another by dilution. They are fixed in place 

 by the use of a solid medium. They are allowed to grow, and 

 from each individual there arises a colony. It is usually possible 

 to distinguish between colonies arising from different species when 

 it is not possible to distinguish between the individual bacteria 

 of these species. A convenient comparison has been suggested by 

 Abbott. A number of seeds of different sorts may appear very 

 much alike, and considerable difficulty may be found in distin- 

 guishing one from another with the eye. Let them be sown, how- 

 ever, and let plants develop from them, and these plants will 

 easily be distinguished from one another. 1 



Method of Making Plate -cultures. Melt three tubes of gela- 

 tin or agar. (There is some difficulty in keeping agar in a fluid 

 state while dilutions are being made. It is necessary to have 

 some form of water-bath with a thermometer for the purpose.) 

 Let the liquefied agar cool to 45 C. Gelatin may be used at a 



1 It must be understood that no close comparison can be drawn between higher 



"ally present in the 

 the progeny of one 



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plants, which simply complete the development of parts potentially present in the 

 seed, and colonies of bacteria, which are aggregates of individuals, 



individual of the same kind. 



