42 AIDS TO BACTERIOLOGY 



having a horizontal surface, in which a shallow groove 

 has been made by means of a test-tube containing hot 

 water. 



Agar Plates. Tubes containing nutrient agar are 

 placed in a bath of boiling water until the contents are 

 completely melted. The bath is allowed to stand until 

 the temperature falls to nearly 45 C. The tubes are then 

 immediately inoculated and the contents poured into a 

 dish, as previously directed for the preparation of gelatin 

 plates. It is a good plan to warm the dishes or plates to 

 40 C. before pouring the agar, and, above all, to work 

 quickly, as the agar solidifies at 40 C., and after solidifica- 

 tion has begun to take place an even distribution of the 

 medium is no longer possible. Agar plates are usually 

 inverted in the incubator. If left upright, moisture 

 collects in drops on the medium and washes colonies into 

 one another. 



Character of Bacterial Colonies. Gelatin and agar 

 plates (incubated at 22 C. and 37 C. respectively) are 

 examined every day to ascertain the character of the 

 colonies. It occasionally happens that a colony contains 

 more than one species owing to the too close propinquity 

 of the original bacteria, but as a rule each isolated colony 

 is a pure culture. Some bacteria liquefy gelatin more or 

 less quickly, liquefaction being shown by a sinking and 

 the evident local destruction of gel. 



For further study of the colonies isolated on plates they 

 may be inoculated into liquid or solid media. Inocula- 

 tions on to solid media may take the form of a ' stab ' or 

 ' streak ' culture. 



* Streak ' Cultures. Nutrient gelatin or agar is solidi- 

 fied in an oblique position, so as to expose as much surface 

 as possible. The tube is held in a horizontal position to 

 prevent aerial organisms from falling in, and the plug 

 is carefully withdrawn with the third and fourth fingers 

 of the right hand, using the back of the hand. The plati- 

 num needle is sterilised by heating to redness in flame, 

 and given time to cool. A trace of the colony is taken 

 up on the point. The needle is carefully passed down the 

 tube so as not to touch the sides, and then gently drawn 

 along the centre of the medium, using a light but even 

 pressure. The noodle, on removal, is at once sterilised, 

 and the cotton- wool plug after flaming is returned to the 



