356 PUBLIC HEALTH BACTERIOLOGY 



possessed by any of the bacteria, as these are found to 

 be depreciated by growth on the plate medium. 



(c.) From each growth on agar the following subcultures 

 are made : Gelatin stab, litmus milk, peptone water, and 

 seven tubes of MacConkey's broth, containing respec- 

 tively the following carbohydrates : Glucose, lactose, 

 saccharose, dulcite, mannite, adonite, inulin. 



The gelatin is incubated at 20 C. for eight to ten days. 

 The other tubes are incubated at 37 C. for seven days. 

 An organism which does not liquefy gelatin, and which 

 gives acid and clot in litmus milk, indol in peptone water, 

 and acid and gas in glucose, lactose, dulcite and mannite, 

 but not in saccharose, adonite, or inulin, is called Bacillus 

 coli communis. The nature of organisms giving varying 

 reactions from these may be established from the preced- 

 ing table, for which I am indebted to Dr. R. M. Buchanan. 

 Where a reaction here given contradicts that stated else- 

 where in the text, it serves to show that different results 

 have been observed. 



Houston adopts a different method of arriving at what 

 he calls a " typical B. coli " characteristic of excremental 

 pollution. 



He likewise uses MacConkey's glucose broth (rebipegluqua), 

 but incubates at 37 C. for two days (forty-eight hours). If 

 no gas developed, the result was considered negative. On 

 the other hand, if gas developed, he then made subcultures 

 from tubes showing acid and gas on to gelatin slopes, and 

 incubated for two days at 2o-22 C. If no colonies developed 

 resembling in any way that of B. coli, the result was considered 

 negative. If coli-like colonies were present, one of the most 

 typical looking colonies was chosen for subculture into glucose 

 gelatin, and a " shake " culture made in the customary way. 

 After twenty-four hours' incubation at 2o-22 C., if no gas 

 developed, the result was entered as negative ; just as if no 

 growth had occurred in the oblique gelatin cultures, or a 

 growth of microbes in no way resembling B. coli. 



But if gas production was noted, the result was recorded 

 as positive, and the other biological attributes of the coli-like 

 microbe were studied in neutral-red broth cultures for 

 fluorescence (fl) ; in lactose peptone cultures for acid and gas 

 formation (ag) ; in peptone water cultures for indol formation 

 (in) ; and in litmus milk cultures for acid clotting of the 

 medium (ac). The complete" combination of these positive 



