54 



PURE CULTURE OF BACTERIA 



and away from the track of the needle must be noted 

 (Fig. 19), as well as the presence or absence of lique- 

 faction of the solid medium (Fig. 21). 



Ex. 22. Make stab-cultures in (i) gelatine and (2) agar tubes 

 of the bacteria which you have isolated. 



Incubate and make drawings to illustrate the development of 

 the colonies both on the surface and in the substance of the 



FIG. 21. Diagram of types of liquefaction following stabs in gelatine media. 



media. If liquefaction occurs, note the extent to which it pro- 

 gresses, and the form of space occupied by the liquefied medium. 



Ex. 23. Prepare some gelatine or agar " slants," and make 

 streak-cultures on these. Make drawings of the form of the 

 colonies along the streak, and note any peculiarities of colour, 

 dryness, form of growth, and any other points. 



Ex. 24. Make streak-cultures on solid media in Petri dishes. 



3. Media. There is no universal medium in or upon 

 which all kinds of bacteria will grow equally well. Some 

 of the organisms thrive best in beef broth, others in milk 

 or in solutions containing sugar, their requirements in 

 respect of nutrition being very varied. It is, therefore, 



