396 THE COLON BACILLUS 



In typical cases, the growth is more abundant and more opaque than that of 

 the typhoid bacillus. 



(y) Isolated colonies. As a rule, isolated colonies are small and lenticular 

 and their margins indented ; at first they are bluish and transparent but 

 later become white and opaque and are larger than those of the typhoid 

 bacillus. Frequently, however, the colonies remain transparent and preserve 

 the " iceberg " appearance already noted as characteristic of the typhoid 

 bacillus. 



Colonies which develop in the depth of the gelatin have the appearance 

 of small whitish opaque grains. 



3. Agar and coagulated serum. On these media the colon bacillus forms a 

 whitish layer with no characteristic feature. Gas-bubbles sometimes form 

 in the depth of the medium and increasing in size lift up the medium. 



4. Potato. As a rule, the growth is at first yellowish and then later becomes 

 brown, thick, raised and moist ; but some strains of the colon bacillus 

 give a thin colourless pellicle indistinguishable from the growth of the typhoid 

 bacillus. The quality and variety of the potato used have much to do with 

 the appearance of the growth. 



On Remy and Sugg's solid medium the colon bacillus invariably gives rise to an 

 abundant, thick growth which may be glairy or dry and which is always of a dirty 

 yellow or brown colour. 



5. Milk. Milk is coagulated in 24-30 hours when incubated at 37 C. 



SECTION III. BIOLOGICAL PROPERTIES. 

 1. Biochemical reactions. 



1. Action on carbohydrates. In both aerobic and anaerobic culture the 

 colon bacillus decomposes laevulose, lactose, saccharose, maltose, glucose, 

 erythrite, and mannite, with the formation of acid (formic, acetic, butyric, 

 lactic), gas (hydrogen, carbon-dioxide), and ethyl alcohol. These reactions 

 are invaluable for the purpose of identifying the organism. The technique 

 has been described in connexion with the action of the typhoid bacillus on 

 sugars (p. 373). 



Attention must be drawn to the fact that the colon bacillus under certain condi- 

 tions, particularly when it is grown in symbiosis with the typhoid bacillus (Remy), 

 may lose its power of splitting up sugars with the formation of acid and gas. Grim- 

 bert and Legros have found however, that in some cases where dysgonic influences 

 have affected the fermentation properties of the colon bacillus these properties though 

 markedly diminished, are not altogether lost ; they have been able to show, for 

 instance, that milk will be coagulated if in a shallow layer, and that lactose if present 

 in sufficient quantity is feebly, but nevertheless definitely attacked. 



(a) Action on lactose-broth containing calcium carbonate. When sown 

 on this medium and incubated at 37 C. for 12-20 hours the colon bacillus 

 decomposes the lactose with the formation of acids, which in turn attack the 

 calcium carbonate and give rise to numerous bubbles of carbon-dioxide. 



(fc) Action on litmus in presence of a carbohydrate fermented by the 

 organism. Litmus-lactose-gelatin and litmus-mannite-gelatin. The blue 

 colour of the litmus is first changed to red and later assumes a peculiar colour 

 somewhat resembling that of the skin of an onion. 



(c) Action on Grimbert and Legros' medium (p. 373). The colour of the 

 medium is rapidly changed to red. 



(d) Milk. Milk is rapidly coagulated (vide p. 373). 



[(e) Litmus milk. The litmus is first turned pink and subsequently 

 bleached ] 



