SPECIAL EXAMINATIONS 349 



{b.) Stab and surface cultures in gelatin. These may- 

 be done in the same tube. 

 (c.) Litmus milk incubated at 37 C. 

 (d.) Glucose litmus medium. 

 (e.) Lactose litmus medium. 

 (/.) Peptone water for indol reaction. 



The Committee consider an organism to be typical B. coli 

 when it conforms to the following characteristics : Small 

 motile bacillus, non-sporing, decolorized by Gram, which 

 grows well at 37 C. and at room temperature ; never liquefies 

 gelatin ; produces permanent acidity in milk ; curdles milk 

 within seven days at 37 C. ; ferments glucose and lactose, 

 with formation of acid and gas ; grows in smooth thin surface 

 on gelatin (not corrugated) ; and in gelatin stab grows well 

 to the bottom of stab. This typical B. coli generally also 

 forms indol, gives a thick yellowish- brown growth on potato, 

 changes neutral-red, reduces nitrates to nitrites, and in fer- 

 menting glucose half of the gas produced is absorbed by KOH. 

 It sometimes ferments saccharose. 



The method here described is that used in the City 

 Bacteriological Laboratory, Glasgow. 



I. Enumeration of Bacteria. Add o-i c.c. and 1 c.c. 

 of sample by sterile pipette to 10 c.c. of liquefied gelatin 

 and agar. Mix thoroughly by rotation and plate. Incubate 

 the gelatin plates at 20 C. ; inspect daily, and count after 

 three days, unless necessary earlier. 



Incubate the agar plates at 37 C, and count after two 

 days. 



The ordinary water bacteria grow best on gelatin, 

 whereas the intestinal forms grow best on the agar at 

 blood heat. Hence, in a pure water the gelatin count 

 should be much the greater ; and in an impure water the 

 difference between the counts becomes less marked the 

 more impure the water. The ratio between the two 

 counts is also noted. This ratio of the number of organisms 

 developing at room temperature to the number at blood- 

 heat = 10 : 1 in pure water and = 10 : 2 or 3 or 5, etc., in 

 polluted waters. The ratio is unreliable in surface waters 

 in tropical countries, because B. liquefaciens, B. fluorescens 

 liquefaciens, and B. fluorescens non-liquefaciens are com- 

 monly present, and all grow well at 37C., and are harmless. 



