706 A MANUAL OF BACTERIOLOGY 



the Bacillus Welchii group, but in special instances it 

 may be desirable to do so. 



The bottle must be well shaken to mix the sample. 

 Before removing the stopper, it and the neck of the bottle 

 should be swabbed with absolute alcohol, which is then 

 ignited and allowed to burn away. 



MEDIA, TIME OF INCUBATION, ETC. For the gelatin 

 count ordinary nutrient gelatin is employed, the period of 

 incubation being seventy-two hours. In hot weather it 

 may be necessary to use 15-20 per cent, gelatin (unless 

 an incubator which can be cooled is available), but the 

 development of the colonies is slower. For the agar count 

 ordinary nutrient agar is used, the period of incubation 

 being forty to forty-eight hours. 



The media should preferably be recently prepared and 

 be standardised to a reaction of -f- 10. 



In addition to the actual numbers of organisms which 

 develop in the gelatin and in the agar, a comparison of the 

 ratio of the number of organisms developing in gelatin at 

 20 C. to those developing in agar at 37 C. also gives 

 useful indications. With a pure water this ratio is gene- 

 rally considerably higher than 10 to 1 ; with a polluted 

 water this ratio is approached, and frequently becomes 

 10 to 2, 10 to 3, or even less. The actual number of 

 organisms growing at blood-heat is of considerable value 

 apart from any question of ratio. 



In certain instances it is true that this ratio may be 

 unreliable. Thus with surface waters, especially in the 

 tropics (as pointed out by Horrocks) varieties of the 

 B. fluorescens liquefaciens and non-liquefaciens and B. 

 liquefaciens may be abundant and grow well at blood-heat. 



Distilled water gelatin and agar have also been recom- 

 mended, but since the organisms of polluted water develop 

 better in the ordinary nutrient media, the latter are 

 preferable for routine use, 



