DESCRIPTION OF SPECIES 



409 



light. The diameter of the colony 

 is I to 2 mm. The colonies are 

 transparent, and sometimes irid- 

 escent, especially towards the 

 periphery, but at the centre and 

 over the entire surface in old 

 cultures an opacity due to a 

 greater thickness of the bacterial 

 growth is observed. 



It has been obser\'ed that species 

 derived from water grow in trans- 

 parent colonies, whereas those from 

 the alimentary canal or excreta may 

 show opacit)' of the colony, which 

 characteristic disappears if the 

 culture be passed through milk. 

 About the second or third day 

 the surface colonies attain a 

 diameter of 5 to 6 mm., and 

 become marked by concentric, or 

 radiating, or irregular markings. 

 The surrounding gelatine ver>' fre- 

 quently acquires a dull, cloudy, 

 faded appearance, and the edges 

 of the colony become more 

 crenated and thinner. The white- 

 ness of the colony turns to yellow. 

 There is no liquefaction of the 

 gelatine. 



In gelatine stab-cultures the 

 organism grows rapidly. On 

 the surface, in 24 hours, the growth 

 is often 2 to 3 mm. in diameter, 

 and closely resembles a surface 

 colony in a plate culture, though 

 more luxuriant. A thick white 

 growth extends along the whole 

 length of the track of the needle, 

 and not infrequently gas bubbles 

 or fissures ap{>ear. The gelatine 

 is not liquefied, even in old 

 cultures. 



In gelatine streak cultures 

 growth is also abundant. In 24 

 hours the elongated milky sur- 

 face colony may be 5 mm. in 

 diameter. It consists of a delicate 

 faintly-granular film with trans- 

 parent and irregular margrins. 



Down the centre longitudinally 

 the growth is thicker and there- 

 fore more opaque. Irregular thick- 

 enings, foldings, and corrugations 

 may occur in old cultures. Some- 

 times the film shows iridescence, 

 and the medixmi, though not lique- 

 fied, becomes clouded. The growth, 

 as on the plate cultures, is bluish- 

 white by reflected, and yellowish- 

 amber in colour by transmitted 

 light. 



/« 25 per cent, gelatine at 37° C. 

 In 48 hours the melted gelatine 

 remains clear, but a thick pellicle 

 forms on the surface (Klein). 



Gelatine shake cultures become 

 turbid, and within 24 hours at 

 20° C. are riddled with bubbles of 

 gas, which are generally more 

 numerous and larger towards the 

 foot of the tube. They increase in 

 size by the second day, sometimes 

 even forming fissures. The gas is 

 mainly carbonic acid. The presence 

 of a small per cent, of fermentable 

 sugar in the medium increases the 

 gas production. 



On potato-gelatine the colonies 

 of B. colt are similar in apf)ear- 

 ance to those occurring on 

 ordinary gelatine, except that they 

 grow more slowly, are more circum- 

 scribed, and of a characteristic 

 brown colour (Houston). 



On carbol-gelatine (-05 per cenL 

 of phenol) the growth does not 

 differ from ordinary gelatine cul- 

 tures except that it is delayed. 



Bouillon — In less than 12 hours 

 at 37" C. the medium becomes 

 uniformly turbid. It may be very 

 pronounced. Frequently there is 

 also at a later stage a marked 

 amorphous flocculent sediment con- 

 sisting of bacteria. Only a faint 

 film forms on the surface, which 

 rarely becomes a pellicle. There 

 is a foetid odour, and sometimes 



