THE CULTIVATION OF BACTERIA. 



95 



in culture-media. The growth, called a colony, which results 

 from the multiplication of a single bacterium, is in many cases 

 quite characteristic for the species. By the plate-method the 

 individual bacteria in a mixture are separated from one another 

 by distributing them through melted gelatin or agar in tubes. 

 They are fixed in the place where they chance to be when the 

 medium solidifies. They are allowed to grow, and from each 

 individual there forms a colony. It is usually possible to dis- 

 tinguish between colonies arising from different species when it 

 was not possible to distinguish between the individual bacteria 

 of these species. A convenient illustration has been suggested 

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

 very much alike, and considerable difficulty may be found in 

 distinguishing one from another with the eye. Let them be 

 sown, however, and let plants develop from them, and these 

 plants will easily be distinguished from one another.* 



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

 gelatin or agar. (There is some difficulty in keeping agar in a 

 fluid state while dilutions are being made. It is best to have 

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

 Let the melted tubes cool to a few degrees above 40° C. Take 

 a small portion of the material to be examined — pus, for exam- 

 ple — and introduce it with a sterilized platinum wire or loop 

 into one of the tubes. Stir it in carefully. Remove the needle, 

 sterilize it and replace the plug. Mix the material introduced 

 thoroughly with the melted culture-medium, taking care not 

 to wet the plug. Now remove the plug again, and, having 

 sterilized the platinum wire, insert it into the liquefied medium. 

 Carry three loopfuls in succession from this tube, which is No. 

 I, into tube No. 2; sterilize the needle; replace the plugs; mix 

 thoroughly by tilting the tube up and down, but avoid shaking 



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

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

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

 of one individual of the s^ime kind. 



