THE METHODS OF INOCULATING SOLID MEDIA. 69 



fragment of the contaminating material, care having been 

 taken beforehand to sterilize thoroughly the needle or 

 instrument. 



2. Gelatin culture-tubes are inoculated in one of three ways : 



a. Stab culture, made by puncturing the centre of the 

 solidified gelatin mass with a platinum needle previously 

 charged with the bacteria. 



b. Slant cultures, made by gently passing over the surface of 

 the medium the inoculating needle ; for this purpose the 

 gelatin is made to solidify with the tube in an inclined posi- 

 tion, so as to give a larger surface for inoculation. 



c. Plate cultures, made by inoculating the gelatin mass, 

 which has been previously liquefied by submitting it to a 

 temperature of 30 C., with a platinum needle or loop as 

 for liquid cultures, and pouring the liquid mass rapidly and 

 evenly on a sterilized glass plate, allowing it to solidify well 

 protected from dust. 



The plate method, introduced by Koch, is of great value 

 for the separation and isolation of bacteria. In this manner 

 each bacterium introduced into the liquefied gelatin is fixed by 

 the hardening of the gelatin and develops as a separate colony, 

 the number of colonies being, as a rule, equal to the number 

 of bacteria originally introduced. 



Each colony grows in its own peculiar way, because each 

 species has a definite way of growing in gelatin. This 

 method therefore not only serves for the separation of the 

 bacteria themselves, but also enables us to recognize one 

 species from another. Indeed, the classical method of Koch 

 for the separation and isolation of bacteria, though modified 

 in some particulars, has not been essentially changed since its 

 introduction. It is as follows : 



Three sterilized gelatin culture-tubes about a third full and 

 containing about 10 c.c. of the medium are liquefied by being 

 submitted to a temperature of 30 C. Tube I. is inoculated 

 with one or two loopfuls of the platinum needle from the 

 contaminated substance, its cotton plug replaced, and the 

 contents well shaken. After sterilizing the platinum needle 

 one or two loopfuls from tube No, I, are introduced into tube 



