206 BACTERIOLOGICAL EXAMINATION OF WATER. 



mixture is then poured into Petri dishes and allowed to solidify. 

 Three plates are prepared for each turbid broth tube, one loop- 

 ful from which is stroked in a ring form, without recharging, 

 over the surfaces of all three plates. The colonies of B. typhosus 

 appear on the plates as small round transparent drops of -a bluish 

 tint, the colonies of B. coli are larger, more opaque, and red in 

 colour, also the medium around them shows the same red colour. 

 The bacilli which belong to the fluorescent group (B. fluorescens 

 liquefaciens and non-liquefaciens), also some of the Proteus 

 species, which are able to develop at 37 C., give rise to colonies 

 which have a deep blue colour, and are readily distinguished 

 from the colonies of B. typhosus and B. coli. The sewage 

 streptococcus produced colonies which by the naked eye cannot 

 be distinguished from the colonies of the typhoid bacillus. The 

 transparent blue colonies are then fished and examined in a 

 hanging-drop ; if motile bacilli like the B. typhosus are seen, 

 I am in the habit of preparing 150 and 1500 dilutions of 

 anti-typhoid serum, and then adding a loopful of the 1-50 

 dilution to the hanging-drop already prepared, and a loopful of 

 the 1-500 dilution to another hanging-drop made from the 

 same colony. If agglutination of the bacilli is observed a 

 portion of the colony is then planted out on an agar-slope, and 

 the growth which results is then tested on all the various media 

 described under the cultural reactions of B. typhosus. 



The special alkalinity of the glucose-lit mus-agar recommended 

 above was arrived at by testing the amount of acid or alkali 

 produced by B. typhosus, B. coli and its varieties, and the 

 various water bacteria. I thought at first that about 6 per 



cent, of ^ alkali (0*6 c.c. per 10 c.c. tube) would suffice to 

 neutralise the acid produced by B. typhosus, and still leave B. 

 coli markedly acid. In litmus-whey tubes 6 per cent. ^ alkali 



was sufficient to neutralise the acid, but on glucose-litmus-agar 

 poured into Petri dishes the typhoid colonies showed a marked 

 red colour in the presence of this amount of alkali. By gradually 



increasing the amount of alkali it was found that 1'8 c.c. y^ 

 NaHO. per 10 c.c. tube of lit mus-agar sharply differentiated 

 B. typhosus from most of the varieties of B. coli. There are a few 

 varieties of B. coli which are also neutralised by this amount of 



