100 LABORATORY EXERCISES. 



49. STUDY OF BACTERIA. 



Materials for home work (A, B) : Three bottles (two provided 

 with stoppers), ice-box, thermometer; pint of milk; handful of dried 

 hay. 



Materials for class-room work (C, D, E) : Arnold steam sterilizer, 

 flask, Petri-dishes, cotton-wool, inoculator, compound microscope 

 (500 diameters), slide, cover-glass, eosin, carbol-fuchsin or Lbemers 

 blue, corrosive sublimate solution (1:1000), made by dissolving 

 one antiseptic tablet in a pint of water. 



Nutrient agar-agar is probably the best medium in which to grow 

 all kinds of bacteria. It can be readily prepared in the laboratory 

 or home kitchen from the following materials: 1000 cc water, 10 

 grams salt, 10 grams peptone, 10 grams Liebig's beef extract, small 

 amount of cooking soda, and 10 grams agar-agar (called, also, Japan 

 isinglass). If agar-agar cannot be obtained, 100 grams of the best 

 French gelatin may be used instead. 



Dissolve the beef extract in the 1000 cc. water. The agar, cut 

 into small pieces, is then added, together with the peptone and salt. 

 The mixture should be heated to cause the agar to dissolve, care 

 being taken that it is not allowed to burn. Just enough cooking 

 soda is added to cause red litmus paper dipped in the mixture to 

 turn blue, that is, the liquid should be faintly alkaline. Filtering 

 the hot agar sometimes involves more or less difficulty. The pro- 

 cess can be easily carried on. however, within the steam sterilizer. 

 A glass funnel should be put in the mouth of a Florence flask (used 

 commonly in a chemical laboratory) , and one or two layers of ab- 

 sorbent cotton placed within the funnel If the agar, flask, and 

 funnel are kept hot within the sterilizer, the liquid will readily pass 

 through the cotton. After filtering, close the mouth of the flask 

 with a plug of absorbent cotton, and boil in the cooker for half an 

 hour. The flask may be set aside as stock agar until needed for 

 use. (If the agar mixture is not clear, it should be filtered through 

 the same cotton a second time.) In case any bacteria or mould 

 colonies appear within the flask, it should be heated within the ster- 

 ilizer for half an hour on two or three successive days. 



A. Growth of bacteria in hay infusion (at home, . 

 i. Cut the hay into small pieces, place them in 

 a mason jar, and half fill it with water. 



