286 XXI. BACTERIA EAST. 



see spherical, regularly elliptic, clustered or otherwise shaped, 

 and lastly also entirely irregular colonies. In all gelatine 

 cultures it is necessary first of all to distinguish bacteria which 

 leave the gelatine solid, from those which liquefy it. In tube 

 cultures the bacteria of the first category soon form at the prick- 

 ing point a small projecting boss, so that the channel of the prick 

 together with this boss takes the form of a pin ; or they spread 

 upon the surface in the form of concentric rings or foliaceous 

 or grape-like figures ; or they grow more within the prick itself ; 

 or radiate from this into the surrounding medium. The colour 

 -of the colonies is very various, and often the gelatine also takes 

 on a definite colour, or changes in some way or another its appear- 

 ance. Characteristic odours often accompany all these processes. 



Just as for cultures in gelatine tubes, the gelatine plates can 

 be used as starting-points for cultures in suspended drops, in 

 moist chambers, or also upon certain sterilised solid nutrient 

 substrata. For these latter, disks of potato play the chief part. 

 As these show a slightly acid reaction, it is desirable before steri- 

 lising to lay them for a short time in a 1 per cent, solution of soda. 

 The potato disks are best sterilised in a somewhat tapering tube, 

 in which they cannot go to the bottom and therefore cannot be 

 exposed to the action of any condensed water. Sterilisation 

 results from exposure for fifteen or twenty minutes in a steam 

 chamber, or a culinary " steamer " with a vent pipe for steam 

 makes a fair substitute. Just as in sterilising gelatine, the tube 

 can be closed with a cotton-wool plug. In order in these 

 manipulations to avoid contamination from the hands, it is de- 

 sirable first to wash these thoroughly with soap, and then to 

 disinfect with 1 per cent, solution of corrosive sublimate, and also 

 even to previously dip in sublimate solution the finger with which 

 the objects will be touched. 



After any bacteriological investigation it is very desirable to 

 destroy all the material, so that it may not be a source of infection 

 for other cultures ; and the utensils used can be sterilised either 

 by laying for several days in 1 per cent, sublimate solution, by 

 boiling in water for half an hour, or exposure for a like time in the 

 steamer. 



Finally for the study of pathogenic bacteria inoculation 

 experiments upon healthy living animals have the utmost import- 

 ance, since by these means the infective properties of bacteria are 

 first established. The infection is made, with all needful pre- 



