36 WATER 



(b) Litmus-milk at 37 C. 



(c) Lactose-peptone litmus solution (in a double tube). 



(d) Peptone water (for indol production). 



(e) Saccharose peptone litmus solution (in a double tube). 

 Houston 1 has suggested and used very extensively a number 



of modifications which are valuable when a large series of samples 

 have to be examined. The B. colt organisms are isolated from 

 the positive gas tubes by inoculating sloped lactose, bile salt,, 

 neutral red peptone agar (called by Houston rebipelagar) in test 

 tubes instead of plates. As far as possible five red colonies are 

 in each case subcultivated and investigated. The confirmatory 

 tests selected are carried out with the following media : glucose 

 litmus gelatine, lactose litmus gelatine, saccharose litmus gelatine 

 and peptone water. The results are recorded after 24 hours' 

 incubation at 20 22 C., the data recorded being the production 

 of gas from the three sugars and the presence of indol. 



Houston recommends gelatine sugar media (i p.c. sugar, 

 2 p.c. peptone, I p.c. lemco, 7-5 p.c. gelatine and I c.c. of a 5 p.c. 

 potassium hydrate solution) for gas production testing, finding 

 such media more sensitive than the usual liquid media. 



Houston has introduced a number of modifications which 

 save time when dealing with large batches of routine samples 

 such as are examined daily in the Metropolitan Water Board 

 Laboratories. 



Labelling is reduced to a minimum by the use of coloured 

 cotton wool plugs. Five colonies from each rebipelagar tube 

 being subcultivated the media are tested in sets of five. Five 

 quite small test tubes (2" x J") filled with the same sugar gelatine 

 or other medium used are placed, in one large test tube, only the 

 latter having a cotton-wool plug, 



To expedite the inoculations the colony selected is picked 

 off by a sterile straight iron wire (sterilized in batches over 

 naked bunsen flame). The wire is then placed in a tiny test 

 tube containing a few drops of sterile salt solution. Into this 

 little tube are then introduced as many more sterile wires as 



1 For a more detailed account see Metropolitan Water Supply Itcport for January, 

 1907, pp. 46-52. 



