184 CLINICAL BACTERIOLOGY. 



wound thread. In the further course of the growth not before 

 several weeks the gelatin is completely liquefied. 



On agar-plates growth is not so characteristic as in gelatin- 

 plates. The superficial colonies present a peculiar, light gray- 

 ish-brown, transparent appearance. 



Agar streak- cultures exhibit a grayish-white, moist, glistening 

 coating. 



Blood- serum is gradually liquefied. 



Potatoes. In spite of the usually acid reaction of potatoes 

 the cholera-vibrios generally thrive upon this culture-medium, 

 but only at temperatures above 21 C. (69.8 F.). They then 

 form a grayish or grayish-brown, thin, translucent coating. 

 Upon some varieties of potatoes, however, the comma-bacilli do 

 not thrive, but they can readily be cultivated if the potatoes are 

 rendered slightly alkaline by means of a solution of soda, or if 

 they are boiled in a three per cent, solution of sodium chlorid. 



Milk is coagulated by cholera-bacteria obtained in certain 

 epidemics, but not by others. The latter appears to be the rule. 



Bouillon is rendered turbid, and in the majority of cases a 

 superficial membrane forms at body-heat. Like the majority 

 of vibrios and spirilla, the cholera-vibrios also possess the prop- 

 erty of multiplying with especial activity in quite dilute bouillon 

 (from six to eight times). A one per cent, aqueous solution of 

 peptone, with addition of one-half per cent, sodium chlorid, 

 also favors the development of the comma-bacilli. If the pep- 

 tone is not alkaline originally, the last-mentioned nutrient 

 medium must be rendered so by addition of soda. 



Cholera-red Reaction. If a few drops of pure dilute hydro- 

 chloric acid or sulphuric acid are added to cholera-cultures that 

 have grown in nutritive media containing peptone, a rose-red or 

 purple-red coloration appears within a short time. Bouillon- 

 cultures yield this reaction after exposure for twelve hours in 

 the thermostat. This so-called cholera-red reaction is nothing 

 more than an ordinary nitroso-indol reaction. The comma- 

 bacilli possess the property of forming indol and of converting 

 into nitrites the nitrates always present, at least in small amount, 

 in nutrient solutions. In addition to the comma-bacilli, there 

 are other vibrios that likewise yield the nitroso-indol reaction : 

 as, for instance, the vibrio Metschnikoff, and the vibrio bero- 

 linensis found in the water-supply of Berlin. (See water-bac- 

 teria, Appendix.) The Finkler-Prior bacillus and the Denecke 

 cheese-bacillus also form indol, but no nitrite, so that addition 

 of pure acids free from nitrous acid does not cause a red colora- 

 tion with them. The one per cent, aqueous solution of peptone 

 and sodium chlorid previously mentioned is especially suited 

 for the formation of cholera-red. The reaction is at times want- 

 ing in bouillon when too much or too little nitrate is present. 



