58 METHODS OF CULTIVATION OF BACTEEIA 



Fig. 1 5. If the top of a plug be dusty it is best to singe it 

 before extraction. 



The Methods of the Separation of Aerobic Oe&anisms. 

 Plate Cultures. 



The general principle underlying the methods of separation 

 is the distribution of the bacteria in or on one of the solid media 

 so that the colonies formed by the individual organisms are 

 sufficiently far apart to allow their being examined separately. 

 For the purpose, circular shallow glass capsules, each fitted with 

 an overlapping glass cover, are almost universally used; these 

 are known as Petri dishes or capsules. The medium, after being 

 melted, is poured into a sterile capsule and allowed to solidify, 

 so as to form a thin layer ; in this way the colonies which after- 

 wards grow are readily accessible. In one method the material 



containing the bacteria is smeared 

 over the surface of the medium 

 after it has solidified in the 

 capsule, — " smear method." In 

 another method the organisms are 

 mixed with the medium when in 

 the melted state, and the mixture 

 Fig. le.^Petri's capsule. is then poured into the capsule 

 (Cover shown partially raised.) and allowed to solidify, — "dilu- 

 tion method." The former gives 

 the best results in the case of most pathogenic organisms. 



The smear method is the more convenient, and is that used 

 for the separation of typhoid and dysentery bacilli, meningococci, 

 etc. ; it is, in fact, capable of almost universal application. The 

 procedure varies according to the material to be examined. If 

 the organisms are on a swab, say from the naso-pharynx, con- 

 secutive strokes are made all over the surface of the medium, 

 always the same portion of the swab being brought into contact 

 with it. In this way the organisms are gradually wiped off the 

 swab, till in the later strokes they may be deposited at sufficiently 

 wide intervals to give separate colonies. Sometimes it is advis- 

 able to smear two plates consecutively with the same portion of 

 the swab. If the material to be examined is fluid, e.g., an 

 emulsion of faeces, the usual method is to place a loopful on the 

 surface of the medium, and then, with a sterile glass-rod bent at 

 a right angle, to smear the whole surfa.ce. If the organisms are 

 found, on microscopic examination, to be very numerous, say in 

 pus, it will be advisable to dilute with sterile saline before 



