82 MICRO-ORGANISMS AND DISEASE [chap, 



seen that the London waters must have been considerably 

 polluted, and at the same time imperfectly filtered. 



for the detection of the cholera vibrio in water the 

 peptone salt method is the simplest and best. A stock 

 solution of lo per cent, best peptone and 5 per cent, 

 common salt in distilled water is made ; this is made 

 faintly alkaline and sterilised by boiling. To each 90 cc. 

 of the water to be examined, contained in a sterile flask, 10 

 cc. of the above peptone solution are added, so as to 

 make the mixture in reality a i per cent, peptone o'5 per 

 cent, salt solution. The flask is then incubated at 37" C. for 

 twelve to twenty-four hours. 



The cholera vibrio grows well and rapidly in a i per cent, 

 peptone, i per cent, salt solution (Dunham), and is un- 

 doubtedly for this reason the best means of detecting the 

 vibrio. Such a peptone solution shows already, provided 

 cholera vibrios had been present, after twelve hours' distinct 

 turbidity, and if of the top layer a droplet is removed and 

 examined fresh, briskly moving (revolving) comma bacilli 

 will be found ; they are easily recognised as commas if 

 a drop of the top layer of the cultivation fluid is deposited 

 in the centre of the cover-glass and without spreading it 

 out is dried, stained, and mounted. After twenty-four hours 

 the turbidity is much more pronounced, and cholera vibrios, 

 whether in pure or impure condition — i.e., without or with 

 admixture of other microbes, notably bacillus coli or proteus 

 vulgaris — can be isolated by gelatine or Agar plates in the 

 usual way, and then subjected to the various tests for 

 cholera vibrios (see cholera). 



The two microbes which next to the cholera vibrio grow 

 fairly well in the above peptone-salt solution, are the bacillus 

 coli and the proteus vulgaris ; for the detection of the latter in 

 water the peptone method is excellent, since a large quantity 



