MICROORGANISMS AND FERMENTATIONS 



19 



has previously been melted, and then allowing this to set in a thin 

 layer. The isolated germs, fixed in position, will after some time 

 multiply into colonies visible to the naked eye. From the number 

 of colonies, the number of organisms per cubic centimetre of the 

 original liquid can be calculated. Care must be taken that the tem- 

 perature of the melted medium is not high enough to cause injury to 

 the organisms. For milk bacteria the best media are whey or the 

 solution recommended above, with the addition of 12 per cent, 

 of gelatine or 1J per cent, of agar. Gelatine may be regarded as 

 a protein, whereas agar, a product obtained from certain seaweeds, 

 is composed of carbohydrates known as pectins. Gelatine is to 

 be preferred, as it can be made to 

 yield clearer media than agar, but as 

 it melts slightly over 20 C. or lower 

 if it has been heated too frequently 

 or at too high a temperature, it 

 must be replaced by agar when deal- 

 ing with organisms which only thrive 

 at higher temperatures. If litmus 

 or chalk be added, acid-producing 

 organisms may at once be recognised. 

 Solid media are usually allowed to 

 set in Petri dishes, flat dishes with 

 lids, which have been sterilised dry 

 at 150 to 170 C. Organisms which 

 do not grow in presence of oxygen 

 are best cultivated in Burri's tubes, 

 which are closed at one end by a 

 rubber stopper and at the other by a 

 plug of cotton wool ; on removing 

 the stopper, the agar column may 

 be shaken out and examined for 



colonies. Some obligate anaerobes form characteristic surface 

 colonies ; to obtain these, the Petri dish must be kept in an 

 atmosphere of hydrogen or in a vacuum. The oxygen which 

 gradually leaks in may be absorbed by alkaline pyrogallol solu- 

 tion. The simplest plan is to use a vacuum desiccator, which 

 is tilted during exhaustion in such a way that the chemicals 

 (5 grams of pyrogallol and 50 c.c. of 10 per cent, caustic potash) 

 in the sulphuric acid container are only mixed when the desic- 

 cator is stood straight after evacuation. 



As a rule, the liquids to be examined contain so many germs that 

 they must be diluted with sterilised water before plating, in order 

 to avoid overgrowth. The number of colonies to be aimed a't is 

 40 to 200 per plate, and it will generally be necessary to make 



FIG. 21. Desiccator for Anae- 

 robic Cultivation. 



