QUALITATIVE BACTERIOLOGICAL ANALYSIS. 205 



fifth form exactly corresponds to the colonies described by Klie, 

 and Mayer also found that the same colonies were obtained in 

 a medium simply consisting of 3'3 per cent, gelatine and 0'5 per 

 cent, peptone, carefully neutralised with crystallised soda, or bi- 

 potassium phosphate. It therefore appears that the characteristic 

 growth of B. typhosus in Piorkowski's medium is chiefly 

 dependent on the small percentage of gelatine in the medium, 

 the temperature and length of the incubation, and the motility 

 of the bacillus. Mayer believes that if the characteristic form 

 appears, and the colony when planted out in glucose broth does 

 not; give rise to the production of gas, the diagnosis of B. 

 typhosus may be made. 



Nearly all the methods described aim at giving the B. typhosus 

 time to 'develop, but they all fail to prevent the growth of 

 the B. coli and its varieties. I have lately employed a modifi- 

 cation of Parietti's method combined with the use of glucose- 

 litmus-agar plates. Two litres of the water to be examined are 

 concentrated by pumping through a Berkefeld or Pasteur- 

 ChamberJand candle, and the deposit on the candle is then 

 diffused by means of a sterile brush in 10 c.c. of sterile water. 

 One c.c. of the concentrated water is then added to each of ten 

 broth tubes containing O05 per cent, carbolic acid. It has been 

 shown that the B. typhosus which has existed for several weeks 

 in sewage, may not develop in broth tubes containing more than 

 0'05 per cent, carbolic acid ; consequently it is important not to 

 exceed this amount of acid. It is true that many liquefying 

 organisms can develop in such a broth, but this disadvantage is 

 more than counterbalanced by the fact that the B. typhosus, no 

 matter how debilitated it may be, will always be able to grow in 

 the medium. The broth tubes after inoculation are incubated 

 at 37 C., and kept under observation for seventy-two hours. 

 All the broth tubes which show a diffused turbidity are then 

 plated out on glucose-litmus-agar. This medium contains two 

 per cent, glucose added to ordinary agar containing sufficient 

 aqueous extract of litmus to give the medium a light blue 

 colour. For use the medium is melted, cooled to 42 C., and 



then ^ NaHO added, so that each 10 c.c. of the medium has 



vr 



an alkalinity equal to 1'8 c.c. ^ alkali. The alkaline agar 



