KOCH'S PLATE CULTURES 101 



For pure cultures at temperatures above 50 C. agar-agar cannot be used, 

 since it then begins to soften. For such (rare) cases, Miquel, when experiment- 

 ing with BaciUus th&rmophilus t replaced agar-agar in the nutrient solution by 

 2.5-3.0 per cent, of Caragheen moss (Irish moss), from Chondrus crispus. For 

 special purposes suitable indicators are also added to the nutrient media. For 

 example, if it is desired to separate merely the acid-forming species from a 

 bacterial mixture, then a little litmus is added to the medium before sterilising ; 

 the colonies of acid-forming bacteria in the subsequent plate culture will then 

 become surrounded by a red halo standing out conspicuously against the blue 

 background. For the same purpose BEYERINCK (V.) recommended an addition of 

 fine levigated chalk, which forms an opaque chalk nutrient medium, becoming, 

 however, clear at the parts of the plate culture occupied by acid-forming bacteria, 

 in consequence of their solvent action on the calcium carbonate. 



Certain organisms, such, for example, as the nitrifying bacteria, do not thrive 

 in the solidified nutrient media hitherto described. Therefore, in order to pre- 

 pare cultures of the same by the aid of the plate method, recourse is had to the 

 medium prepared from precipitated silica, proposed by W. KtiHNE (I.). Silica 

 precipitated from water-glass (alkali silicate) and carefully purified will, when used 

 as a 3.4 per cent, aqueous solution, set within an hour to a firm mass if mixed 

 with 0.25 per cent, of NaCl. The salt is added to a sterilised solution which also 

 contains the other requisite nutrient substances. In this solution is distributed 

 a small portion of the bacterial sample to be separated, the cells of which will, 

 when the medium has set, be fixed and develop into colonies. Further particulars 

 concerning the preparation of this silica medium will be found in the above- 

 mentioned treatise, as also in one by Winogradsky which will be referred to 

 later. 



The numerous nutrient media employed in practical mycology are more 

 fully described in the handbooks of Hueppe, Eisenberg, Tiemann-Gartner, and 

 Bernheim, but only one need be briefly noticed, viz., the potatoes employed for 

 the so-called potato cultures. The potatoes the better sorts used (in Germany) 

 for salad-making after being carefully cleaned externally, are steeped for an 

 hour in a i per mil solution of sublimate, then swilled with water and sterilised 

 in a wire basket by two hours' exposure in a current of steam. When this is 

 effected, and they are so far re-cooled as to be fit for handling (with disinfected 

 fingers), they are cut into halves by a sterilised knife and placed under a sterilised 

 bell-glass. When cold, inoculating streaks are drawn on the cut surfaces, and 

 subsequently develop into potato cultures. 



85. Koch's Plate Cultures 



are, as previously indicated, prepared by pouring out the liquefied and inoculated 

 medium (e.g. 5-8 c.c. in a test-tube) on to colourless glass plate s, rectangular in form, 

 about one-twelfth of an inch thick, 5 to 6 inches long, and 3 \ to 4 inches broad, 

 previously ^terilised in batches in a copper or iron box, from which they are taken 

 as required. The plates are laid on a plate-pouring apparatus arranged horizontally 

 as described and shown in the above-named handbooks and the distribution 

 of the stratum of gelatin or agar-agar is assisted by the aid of the rim of the test- 

 tube. To sterilise the latter, it should be held for a short time in the Bunsen 

 flame and allowed to re-cool sufficiently before proceeding to pour. When the 

 gelatin layer is set the plate is transferred to a sterile damp chamber, which is 

 placed in the thermostat and maintained at the constant temperature required. 



These plates are rather inconvenient to handle, since, in following up the 

 development of the growing colonies the plate must be frequently taken out of the 

 chamber. During each observation the mould spores in the air are liable to fall 



