372 



SCIENCE. 



[N. S. Vol. XVII. No. 427. 



From tlie results of these experiments it 

 appears that the colon bacillus is not nor- 

 mally present in the intestines of oysters, 

 and when present always indicates con- 

 tamination from some outside source. 



The Influence of Physical Conditions on the 

 Character of Colonies on Gelatin Plates: 

 A Preliminary Communication: Edward 

 K. Dunham, New Yorli University and 

 Bellevue Hospital Medical College, New 

 York. 



Attention was called to the influence of 

 physical conditions on the appearance of 

 colonies by two sets of observations: (1) 

 The same species of bacterium grown in 

 different lots of gelatin made with the 

 same ingredients and having the same re- 

 action frequently produced colonies of 

 widely divergent appearances ; (2) colonies 

 of different species often form colonies that 

 are indistinguishable in some gelatins, but 

 when grown in other lots of gelatin can be 

 readily recognized as different. These 

 variations were traced to differences in the 

 stiffness of the gelatins, and this led to a 

 study of the physical properties of nutrient 

 gelatin. The melting points, penetrabil- 

 ities and viscosities were determined and 

 compared with the appearance of colonies 

 on plates made with the gelatins. Atten- 

 tion was chiefly directed to the deep col- 

 onies, and the studies were confined to the 

 colon bacillus, bacillus typhosus, bacillus 

 dysenterise and a paratyphoid bacillus. 



If the gelatin is very stiff the colon 

 colony is lenticular in form and presents a 

 tendency to form multicontours. In a 

 somewhat softer gelatin the colony is spheri- 

 cal, with indications of concentric structure. 

 In still softer gelatin, budding or root-like 

 projections are formed on the surface. In 

 a very soft medium, not a single colony 

 but a federation of colonies, closely grouped 

 together, is produced. Similar variations 

 occur when typhoid colonies develop on 



plates. These may be small and spherical, 

 or more or less thready with delicate fila- 

 ments penetrating into the medium, accord- 

 ing to whether the gelatin is stiff or rela- 

 tively soft. In a very soft but still solid 

 gelatin, the typhoid bacilli may penetrate 

 the medium, disseminating themselves 

 throughout its mass. Such plates appear 

 sterile. 



Variations in the stiffness of the gelatin 

 may be produced by a reduction of its 

 original stiffness with heat or by incubat- 

 ing the plates at different temperatures. 

 A ten per cent, gelatin made with Compte 

 Fils's or Heinrich's 'Gold Label' gelatin, 

 cooked with an egg for thirty minutes and 

 sterilized three times for fifteen minutes 

 in the Arnold sterilizer, will melt at 29.5° 

 to 30.3° and have a viscosity between eight 

 and nine times that of water. Gelatin 

 plates made with this gelatin and incubated 

 at 27° will yield, e. g., colon and typhoid 

 colonies that can easily be distinguished 

 from each other and fished within twenty- 

 two hours. 



In the author's opinion the physical 

 properties of gelatin and temperature of 

 incubation should receive fully as much 

 attention as the ingredients and reaction 

 in the standardization and use of gelatin, 

 particularly when employed for plating 

 with reference to species. 



Milk-agar as a Medium for Demonstrating 

 the Production of Proteolytic Enzymes: 

 B. G. Hastings, University of Wisconsin. 

 If ten to twelve per cent, of sterile skim 

 milk is added to ordinary nutrient agar, 

 after the same has been allowed to cool to 

 50° C. after having been melted, an opaque 

 medium is obtained which, when allowed to 

 solidify in tubes in a sloping position, or 

 poured into Petri dishes, has some advan- 

 tages over gelatin for the determination 

 of the liquefying properties of bacteria, 



