MODES OF CULTURE 59 



varieties of growth. The two species selected are the cholera and the 

 anthrax bacteria. 



The anthrax colonies liquefy the gelatine slowly ; they are whitish and 

 round ; under a low power the edge of the colony appears filamentous, in 

 ' tresses ' and loops, not smooth ; in older colonies an irregular skein or 

 clump lies in the fluid, almost transparent, gelatine. The cholera colonies 

 liquefy the gelatine rapidly ; they are yellowish white, and magnified show 

 a granular texture and an undulating, not filamentous, edge ; the liquefied 

 gelatine immediately round the disintegrating granular clumps is turbid. 

 It must not be forgotten that many variations occur, and that closely 

 similar forms, such as the cholera vibrio and the aquatic vibrios or typhoid 

 bacilli and B. coli, are not distinguishable by their colonies alone. 



The gelatine stab-culture is made by thrusting vertically into a tube 

 of coagulated gelatine a platinum wire charged with the substance of a pure 

 culture. Species which need oxygen grow only in the upper parts of the 

 gelatine, anaerobic forms only in the deeper parts. Growth in filaments or 

 chains shows itself by delicate threads growing out into the surrounding 

 gelatine so that the path of the needle appears feathery or moss-like, 

 e. g. Anthrax. Bacteria whose cells are isolated are confined in their 

 growth to the path of the needle. Among species which liquefy the 

 gelatine great importance is laid upon the shape of the liquefied tract. 

 This may be equal all along the path of the needle, and have the appear- 

 ance of a straight liquid tube in the middle of the solid gelatine, or, if 

 liquefaction is more rapid towards the surface, a funnel-shaped well is 

 produced. Here again considerable variations take place, and closely allied 

 species are not distinguishable by these data alone. 



The surface of agar or gelatine that has been allowed to solidify in 

 tubes in a slanting position is used for ' streak ' cultures. These are made 

 simply by smearing the surface of the substratum with a platinum wire 

 charged with bacteria. The differences in colour, contour, lustre, and con- 

 sistence are similar to those of the colonies in plate cultures. Agar is not 

 liquefied by any bacteria. 



The modes of nutrition of particular biological groups among bacteria 

 will be considered in the subsequent chapters. 



