14 General Bacteriology. 



EXERCISE XI. CARE OF CULTURE HEDIA. 



When sterile culture media (or test-tube cultures) are to be kept for some time they 

 must be protected from evaporation and stored in a dark, cool place. Evaporation may 

 be checked to a considerable extent, (1) by storing them iu tin cans, e. g. quinine cans. 

 Care must be taken, however, that these do not become too damp, in which case the 

 mould fungi frequently grow through the cotton plugs; (2) flasks and test-tubes may 

 be sealed by removing the plugs, dipping same in melted paraffin (melting point about 

 50 C.) and then replacing them; (3) by cutting off the projecting cotton and 

 drawing over the mouth of the vessel a rubber cap (made for the purpose) which has 

 been sterilized in a solution of mercuric bichloride (1: 1,000, spoken of in the lab- 

 oratory as "sublimate solution") ; or (4) By use of a cap of tin-foil. In this case the 

 foil should be put on as soon as the tubes are filled, and sterilized with the medium. 



All media should be carefully examined every day for a week or more, and if spots or 

 the least cloudiness appears, the medium is not sterile and the process of sterilization must 

 be repeated. 



EXERCISE XII. PLATINUM NEEDLES. 



GENERAL DIRECTIONS. These are made by C \ 



fusing a piece of No. 27 platinum wire (5 cm. 

 long) into a glass rod or tube (18 cm. long) . (Fig. 



6.) Each student should have two such needles; FlG ' 8 - platinum needles - 



in one the wire should be straight (designated "needle") and the other bent to form 

 a "loop". This loop should be formed around a No. 10 wire. These instruments must 

 be sterilized shortly before and immediately after use by heating the wire to a glow in 

 the gas flame. The handle should also be passed through the flame two or three times. 

 Cool before using. If the habit of sterilizing is thoroughly acquired much trouble will 

 be avoided and possible danger prevented. These needles will be in constant use. 



REFERENCES. A. 125; M. & B. 58; N. 172; P. B. C. 33, foot note. 



EXERCISE XIII. TEST-TUBE CULTURES. 



EXPLANATORY. The extreme minuteness and slight variation in the form of dif- 

 ferent bacteria render a thorough study of them by direct microscopic observation a dif: 

 ftcult and well nigh impossible task. In their study, therefore, it is necessary to depart 

 from the usually accepted rules that govern the determination of the life history of other 

 forms of life and resort to special methods. The most successful of these are those 

 known as culture methods. According to these methods the bacteria are sown on vari- 

 ous food substances and upon these they develop forming masses easily visible to the 

 naked eye. The manner of their growth and the changes which they produce in these 

 media make it possible to detect differences which would otherwise escape attention. 

 The most common culture media, bouillon, gelatin, agar and potato have already been 

 prepared, and others will be described as needed. 



Cultures maybe made either in test-tubes (streak or stab cultures), or on glass 

 plates, as plate cultures. The plate culture is especially important and is used (a) to 

 obtain pure cultures; and (6) for ascertaining the character of the colonies as an aid to 



