223 AIDS TO BACTERIOLOGY 



(c) Enumeration of bacteria capable of growth at blood- 

 heat, (d) Enumeration of streptococci, if present. 



B, Welchii is not often included among the bacteria 

 estimated, but cases occur where an estimation is de- 

 sirable. 



Enumeration of ' Cool ' Organisms. All media used 

 in counts should have a reaction of 4- 10 (Eyre's scale). 

 Nutrient gelatin is most often used for the ' cool ' organ- 

 isms, as it gives a relatively larger number of organisms 

 with a polluted water than does distilled water gelatin. 

 Conversely with an unpolluted water, a larger count is 

 usually obtained with distilled water gelatin than with 

 nutrient gelatin. So, although nutrient gelatin should 

 be used when only one gelatin medium is employed, the 

 use of both gives comparative figures of value. 



Three tubes of nutrient gelatin are melted in a water- 

 bath at blood-heat or else in a blood-heat incubator. 

 Three sterile Petri dishes are inoculated with 0-5, 0-3, 

 and 0-2 c.c. of the water respectively and labelled. After 

 wiping the adherent water from the outside and flaming 

 the mouth of the tube, the contents of a gelatin tube is 

 poured into each inoculated Petri dish, and the water 

 and gelatin mixed by tipping the dish back and forth a 

 few times. The plates are allowed to solidify on a flat 

 surface, and then placed in the cool incubator. Counting 

 should be done at the end of seventy-two hours, but 

 plates should also be inspected daily, as the count may 

 have to be made earlier should too many liquefying 

 organisms be present. Counting is done with the naked 

 eye, preferably in daylight, any doubtful colony being 

 determined with the aid of a lens or low-power objective. 



In a pure water it will generally be found that less than 

 10 per cent, of the organisms liquefy gelatin. With a 

 polluted water the ratio of the number of organisms 

 developing at room -temperature to those developing at 

 blood-heat approaches 10 to 1, and frequently becomes 

 10 to 2, 10 to 3, or even less. But, as Horrocks has 

 pointed out, the number of B. fluorescens liquefaciens and 

 non-liquefaciens may be abundant, and as they grow well 

 at blood-heat, not only may the ratio of liquefying to 

 non-liquefying organisms be misleading, but also the ratio 

 of ' blood-heat ' to ' cool ' organisms be rendered of no 

 value for an opinion. 



