240 BACTERIOLOGICAL EXAMINATION OF WATER. 



from cheese which form young colonies in gelatine with an 

 irregular margin, resembling in every way those of the true 

 cholera microbe. He regards this characteristic as of no 

 importance as a means of diagnosis. The rate of liquefaction 

 of the gelatine, once considered an important sign, has been 

 abandoned since Koch himself described a case of cholera in 

 which the vibrios liquefied the gelatine so slightly that the 

 colonies appeared in the form of shields. Also, according to 

 Metchnikoff, true cholera spirilla are met with, which liquefy 

 gelatine more rapidly than the typical forms and produce 

 funnels filled with masses of uniformly turbid culture. The 

 form of the spirilla is also very variable ; side by side with 

 thick curved forms others are found which are thin and 

 hardly curved at all. Friedrich described a Shanghai variety 

 which closely resembled a true bacillus, and a Malta variety 

 which appeared to be a cocco-bacillus. Koch, now, attaches 

 only a secondary diagnostic value to these characteristics, 

 and insists above all on the indol reaction and virulence for 

 animals. With regard to the cholera red reaction, he states 

 that none of the spirilla, which can possibly be confounded 

 with the cholera vibrios, produce indol and nitrous acid at 

 the same time in their cultures. Consequently he attaches 

 great value to this test, but points out that the test must 

 be performed according to certain well-defined rules. Above 

 all, a proper variety of peptone must be employed, as every 

 peptone does not give the reaction. Bleisch has shown that 

 the peptones which do not give a proper reaction contain 

 either too much or too little nitrates. The peptone to be 

 employed must first be tested with a known cholera vibrio 

 to see if it gives the characteristic reaction ; if the peptone 

 contains too little or no nitrates, these may be added in 

 proper amount. Bleisch recommends the following solution : 

 Witte's peptone 2'00 gramme, pure sodium chloride 0'5 gramme, 

 distilled water 100 c.c., pure potassium nitrate solution (0'08 

 per cent.) 30 to 50 drops. He also insists on the necessity 

 for using pure sulphuric acid, quite free from nitrous acid. 

 Broth, even though it contains peptone, does not give the 

 reaction so well as peptone and salt solution. 



With regard to the virulence test, Koch recommends that one 



