148 BACTERIOLOGY. 



glass, which is then passed through the flame, and stained in the- 

 usual manner. 



Parietti's Method. As typhoid fever bacilli are apt to be 

 crowded out by more rapidly growing micro-organisms, some method 

 had to be devised for restraining the growth of the latter, and 

 Chantemesse and Widal suggested the use of carbolic acid. Parietti 

 put this into practice by the method he introduced. This consists in 

 adding to tubes of broth about five drops of a mixture composed of 

 sterilised water (100 parts), hydrochloric acid (4 parts), and carbolic 

 acid (5 parts). The tubes are first tested by incubation, and are 

 then ready for use. A few drops of the suspected water are added 



FIG. 76. APPARATUS FOR COUNTING COLONIES IN A ROLL CULTURE. 



to the broth, and if it becomes turbid in a day or two the typhoid 1 

 fever bacillus is present in the form of a pure- cult tire. 



An excess of bacteria in a fresh sample indicates an excess of 

 organic matter, and points to possible contamination with sewage. 

 Where there is such contamination we are very likely to find 

 pathogenic bacteria ; and moreover impure water is a constant 

 source of danger, for if the contagia of infectious diseases ar& 

 introduced they will retain their vitality in such water for a long 

 period, and will in some cases even multiply, whereas the same 

 organisms introduced into pure water would in a short time perish. 



The actual number of bacteria in water is not of very great 

 importance, and it must be remembered that if a sample is set aside 

 for a few days there will be an enormous increase in the number 

 of bacteria present ; but in dealing with perfectly fresh samples it 



