50 PURE CULTURE OF BACTERIA 



It will be found that many commonly distributed 

 bacteria have the power of liquefying nutrient gelatine 

 when they begin to grow.' When this occurs the 

 colonies soon mix and isolation becomes impossible. 

 Another disadvantage connected with a solid gelatine 

 medium is that, as ordinarily prepared, it melts at 22 to 

 24 C, and, therefore, cannot be used for growth of 

 organisms at higher temperatures. To avoid these 

 troubles a nutrient medium is used containing agar- 

 agar in place of gelatine. 



Agar-agar, or agar as it is generally called, is not 

 a protein like gelatine but a carbohydrate substance, 

 which is derived from certain species of Japanese and 

 eastern sea-weeds. It is not liquefied by bacteria, and 

 has the advantage of remaining solid at temperatures up 

 to nearly 80 C. 



The substance is sold usually in bars, sheets, or 

 powder, the latter being the most generally used. A 

 small amount (1.5 per cent.) is sufficient when dissolved 

 in water to form a solid moist " jelly " suitable for the 

 growth of many micro-organisms. 



Agar is very difficult to dissolve, requiring a tempera- 

 ture of near the boiling point of water to melt it, but 

 when dissolved its watery solution has the remarkable 

 property of remaining liquid and not gelatinizing until 

 the temperature has fallen far below the melting point, 

 namely, to near 39 C. 



On account of the sensitive nature of many kinds of 

 bacteria in regard to heat it is important to remember 

 that melted agar media should not be inoculated with 

 bacteria until the temperature has fallen to between 

 40 to 41 C. On the other hand, the temperature 

 should not be allowed to fall much lower than this, or 



