438 BACTERIAL FLORA OF THE SYDNEY WATER SUPPLY, 



bacteria are the number of species per cubic centimetre. In a 

 pure water he considered that this should not exceed 10. 



The media generally employed for growing the colonies are 

 either meat-peptone-gelatine or meat-peptone-agar. In Sydney 

 water many organisms rapidly liquefy the former and utterly 

 prevent the appearance of the more slowly growing colonies. 

 There are others that form amoeboid colonies on the surface 

 of agar, and rapidly spreading over the plate, obliterate the 

 fixed colonies. In the beginning of my investigation it 

 became evident that these media were unsuited for separating 

 the bacteria; still in working with the agar media portions of the 

 plates could sometimes be obtained in which the amoeboid activity 

 of the colonies had been restrained. Agar also throws up the 

 colour of the surface colonies, and this is of some assistance in 

 picking out species. The agar was improved to a certain extent 

 by omitting, in the preparation of the medium, the peptone and 

 the common salt and adding 2% dextrin or gum acacia which 

 tends to restrain the amoeboid growth of the colonies. 



Hesse and Niedner employ an agar medium containing agar 

 1-25 grm. " Nahrstoff Heyden " 0-75 grm., and water 98 c.c. 

 Abba advises a medium containing Liebig's meat extract 6grms. 3 

 gelatine 150 grins, and distilled water 1000 c.c. This is after 

 solution made neutral to phenolphthalein and rendered alkaline 

 by the addition of - 5 grm. amhydrous sodium carbonate. The 

 plates are incubated for 10 days at 18-19° C. He claims that 

 more colonies develop in this medium than when an agar medium 

 (such as Hesse and Niedner's) is employed. I have confirmed 

 this, but I have also found that the bacteria of Sydney water 

 grow so freely in this medium that the colonies have in four days 

 so liquefied the gelatine that observation under the microscope 

 is impossible, and that the plate is almost entirely liquefied in six 

 days. A temperature of 15° C. is more suitable for separating 

 bacteria, but even at this temperature the colonies cannot be 

 allowed to grow for longer than four days. With 30 colonies per 

 plate liquefaction had proceeded so fast in six days that separa- 

 tion was impossible. 



