THE USE OF CULTURE MEDIA. 51 



upright after solidification. Thus agar is commonly used in 

 such tubes (less frequently gelatin is also "sloped"), and this 

 is the position in which blood serum is inspissated. Tubes, 

 especially those of the less commonly used media, should be 

 placed in large jars provided with stoppers, otherwise the 

 contents are apt to evaporate. A tube of medium which has 

 been inoculated with a bacterium, and on which growth has 

 taken place, is called a "culture." A "pure culture" is one 

 in which only one organism is present. The methods of 

 obtaining pure cultures will presently be described. When a 

 fresh tube of medium is inoculated from an already existing 

 culture, the resulting growth is said to be a "sub-culture" of 





Fig. 16. — Platinum wires in glass handles. 



a. Straight needle for ordinary puncture inoculations, d. " Platinum loop.'' t. Long needle for 



inoculating "deep " tubes. 



the first. All manipulations involving the transference of 

 small portions of growth either from one medium to another, 

 as in the inoculation of tubes, or, as will be seen later, to 

 cover-glasses for microscopic examination, are effected by 

 pieces of platinum wire (Nos. 24 or 27 Birmingham wire 

 gauge — the former being the thicker) fixed in glass rods 

 8 inches long.^ Every worker should have three such wires. 

 Two are 2^- inches long, one of these being straight (Fig. 16, a), 

 and the other having a loop turned upon it (Fig. 16, b). The 

 latter is referred to as the platinum " loop " or platinum " eyelet," 

 and is used for many purposes. " Taking a loopful " is a phrase 

 constantly used. The third wire (Fig. 16, c) ought to be 4^ inches 

 long, and straight. It is used for making anaerobic cultures. 

 Cultures on a solid medium are referred to (i) as "puncture" 

 or " stab " cultures (German, Stichkultur), or (2) as " stroke " 

 cultures (Strichkultur), according as they. are made (i) on tubes 

 solidified in the upright position, or (2) on sloped tubes. 



1 Aluminium rods are made which are very convenient. The end is split with a 

 knife, the platinum wire is inserted and fixed by pinching the aluminium on it in a vice. 



