I06 BACTERIA IN RELATION TO PLANT DISEASES. 



Plates, tubes, and flasks containing pure cultures or designed for inoculation 

 should )iever be opened in the general laboratory on a windy day or in air currents. 

 Pour two uninoculated agar or gelatin plates in the proper way. Keep one covered 

 and uncover the other for a few moments in a current of air, /. ^., as long as the time 

 required to make a plate culture. Then keep the two plates together and com- 

 pare from time to time. A few experiments of this sort will convince the most 

 skeptical of the necessity of avoiding drafts. 



The person and clothing of the experimenter should be as clean and free from 

 dust as possible. White duck coats are ver^' convenient. They show at once when 

 they are soiled and need washing and ironing. 



Organisms which for some reason may be difficult to obtain in ordinary plate 

 cultures and which differ markedly from their associates in some particular wa}', 

 e. g.^ by more rapid growth, by indifference to heat, to acids, to thymol, to chloro- 

 form, to absence of air, etc., or which can use, as food, substances which will not 

 support the growth of most bacteria, may sometimes be isolated very readily by 

 providing conditions suited to their growth and unsuited to that of the bacteria with 

 which they are mixed. This is Winogradsky's principle of elective culture. As he 

 defines it, this is a culture "which presents conditions favorable only to a single 

 definite function or, more exactly, to a function as strictly limited as possible." Such 



Fig. 91.* 



media or methods are exactly the opposite of universal. Nutrient starch jelly and 

 nutrient silica jelly are good examples or such media. Nutrient fluids rich in acid 

 potassium phosphate or destitute of nitrogen are additional examples. 



Heat is often an excellent means of separation. Winogradsky separated his 

 Clostridhuii pastetirianum from all but two of the contaminating species by heating 

 ten minutes at 75° C. (Archives des Sci. Biol., Vol. Ill, p. 310). The isolation 

 of Streptococcus {Lcuconostoc) viesenterioidcs by L,iesenberg & Zopf and of Bacillus 

 hortiilanus by Sturgis are other examples of separation by heat. Om^lianski's 

 separation of his hydrogen-cellulose ferment from his methane-cellulose ferment by 

 exposure of the recently established methane ferment to 75° C. for fifteen minutes 

 is another good example. 



THE FINAL DISPOSAL OF INFECTIOUS MATERIAL. 



Diseased material should not be left around the laboratory any longer than is 

 necessary. When it has served its immediate purpose that which is not to be pre- 

 served permanently should be thrown into the furnace. Small amounts may be 

 sterilized by putting into beakers or jars and covering with cleaning mixture or 

 equal parts of crude sulphuric acid and water. Crude vegetable and animal sub- 



'FiG. 91. — Instrument for making puncturc-inuculations. It consists of a bone handle with a 

 metal-screw socket, into which a sewing needle is thrust. The needle is usually of small size — a 



No. 8 or 10. 



