1 86 ADDENDUM 



spread over 16 malachite green bile salt agar plates. After 

 incubation at 37 C. for 24 hours a large number of the colourless 

 colonies are picked off (Houston in his experiments examined 

 250, if so many were obtainable), subcultivated and investigated. 



The malachite green agar used by Houston differs from that 

 given in the appendix, and has the following composition : agar 

 2 per cent, peptone 2 per cent., bile salt 0*5 per cent., lactose, 

 saccharose, adonite, raffinose and salicin 0*2 per cent. each. 

 Neutral red is added in amount equal to 4 c.c. of a I per cent, 

 solution to each litre of medium. The medium is made of 

 this composition and stored. Just before it is used for pouring 

 into the Petri dishes malachite green is added in the proportion 

 of O'l gramme per litre of medium. 



This medium can also be used for the isolation of bacilli of 

 the Gaertner group. These bacilli as well as the typhoid bacillus 

 produce colourless colonies. 



In the utilization of methods for the isolation of the typhoid 

 bacillus from materials containing variable amounts of organic 

 matter it is important to realize that methods found satisfactory 

 for the isolation of this organism from excreta may be less suit- 

 able when water or sewage is the substance under investigation. 



Recent investigations by Krumwiede and Pratt 1 upon the 

 growth of bacteria in and on media containing various aniline 

 dyes throw light upon the relationship of the method to the 

 vehicle examined. 



They ascertained that in general the paratyphoid-enteritidis 

 types are highly resistant to the green dyes, the typhoid bacillus 

 less so but somewhat more resistant than the coli types, but they 

 also found that slight changes affected the action towards 

 B. typhosus. For example small additions of proteid substances 

 change the action of the dye. 



They directly tested the influence of faeces upon brilliant 

 green and the typhoid bacillus, and found that faeces reduced 

 the activity of the dye by about one-third. They add " when 

 we add faeces we are introducing a variable factor, and the 

 typhoid strains also vary somewhat in their resistance to the 

 dye. Success depends upon the right adjustment." 



1 Journ. of Exp. Med. 1914, xix, p. 501. 



