318 



McKenney — On Luminous Bacteria. 



and the open end of the tube sealed. The tube was then shoved through 

 the cotton plup so that the bulb was just a little distance above the 

 culture fluid in the bottom of the flask. The tube and bulb with the 

 contained reagent were found light enough to be held in place by the 

 cotton plug. The arrangement is shown in Fig. 1. 



The whole, containing culture media and 

 reagent was sterilized in the usual way, the 

 cotton plug slightly raised to permit the in- 

 sertion of the needle and the media inocu- 

 lated. When the culture is luminous and 

 the reagent is to be applied, one presses on 

 the end of the tube (a) and the bulb (b) is 

 pressed against the bottom of the flask and 

 shattered, bringing the reagent in direct 

 contact with the bacteria. In this method, 

 the only danger of contamination is that 

 which is usually incidental to inoculation, 

 . and this experience shows to be extremely 

 small. 



The acids used in these experiments in- 

 clude mona-, di-, and tribasic members of 

 both the inorganic and organic series. Since 

 these representative acids all destroyed light 

 emission and often the life of the organism, 

 it seems probable that all acids would react 

 in the same way. My results, then, are op- 

 posed to the conclusion of Beijerinck. I find 

 all acids to be injurious to light production, lactic and malic (which 

 Beijerinck distinctly labels photogenic) fully as much as citric and acetic 

 (which he classes as indifferent and injurious respectively). 



A consideration of Beijerinck's methods may explain his results. In 

 the auxanogram method which he used, the reagent was dropped on a 

 nutrient gelatine plate containing a rich bacterial growth. The acid 

 diffused in radiating fashion from the point of contact, and as the dif- 

 fusion circles widened, the reagent came in contact with the bacteria. 

 Now the effect noted could not have been that of the free acid, since 

 the moment the acid came in contact Avith the gelatine it would react 

 with the contained alkali and form a salt. Consequently the effect 

 noted must have been that of a probably neutral salt and not that of 

 the free acid. The beneficial effect of the salts of certain acids noted 

 by Beijerinck, I have been able to confirm. It is clear then that the 

 error in Beijerinck's account, as far as acids are concerned, is not one 

 of result but of interpretation of these results, since his experiments 

 did not show the effect of the free acid, but rather of its salts. 



The few experiments conducted with the hydrates of sodium and 

 potassium show that while the photobacteria thrive in and, in fact, need 

 a slightly alkaline nutrient medium, still the maximum point is rather 

 sharply defined, and but a slight excess over that is even more fatal than 

 an excess in the other direction. 



Fig. 1. Diagram of culture 

 flask ana bulb for introduc- 

 tion of sterile reagents. 



