72 



The last test noted in the above table suggests that the ratio of the count 

 on lactose agar with and without the violet present may be a valuable diag- 

 nostic feature. Polluted waters show about 50 per cent, reduction of the total 

 count on violet media, while unpolluted water containing more of the sapro- 

 phytic violet positive organisms show a much greater reduction; 100 per cent, 

 in the case of the tap water at Lafayette. Gram stains of centrifuged speci- 

 mens of fresh sewage shows the ratio of Gram positive to Gram negative cells 

 to be anywhere from 1:5 to 1:100. This does not check the 50 per cent, reduc- 

 tion very closely but many factors of a variable nature enter into the two 

 tests. The significant point is that the majority of sewage organisms are 

 Gram negative and therefore may be expected to be violet negative. 



Further work is being done to determine the quantitative relations of 

 pure strains of typhoid and coli studied by this method and to test the effect 

 of attenuation of these forms in relation to the violet when held in suspension 

 in water under various conditions of temperature, light, etc. So far the 

 results seem to indicate that sojourn of a week or more has no selective inhibi- 

 tive effect; in fact, the violet media seems to be favored by the organisms 

 after this treatment. 



One interesting point has been brought out by this latter study. In 

 working with several strains of coli suspended in water, variation in counts 

 on the two media — lactose agar plus violet, and without the violet — was so 

 great that I decided to test the individual strains. I found one, No. 41 re- 

 ceived from the American Museum of Natural History and thoroughly tested 

 by myself, to be absolutely inhibited by the violet stain. A study of the 

 culture showed it to bo a motile. Gram negative bacillus, fcniienting l)ilc 

 rather weakly, not liciuefying gelatin after ten days, and giving otluM- char- 

 acteristics typical of coli. Churclnnan and Michael' have described work on 



'Jour. Exp. Med., Vol. XVI, No. 0, p. 822. 



