Plate Cultures 



205 







ever, that it would be a great omission not to describe the original 

 method in detail. 



Apparatus. Half a dozen glass plates, measuring about 6 by 4 inches, free 

 from bubbles and scratches and ground at the edges, are carefully cleaned, placed 

 in a sheet-iron box made to receive them, and sterilized in the hot-air closet. 

 The box is kept tightly closed, and in it the sterilized plates can be kept 

 indefinitely before use. 



A moist chamber, or double dish, about 10 inches in diameter and 3 inches 

 deep, the upper half being just enough larger than the lower to allow it to close 

 over it, is carefully washed. A sheet of bibulous paper is placed in the bottom, 

 so that some moisture can be retained, and a i : 1000 bichlorid of mercury solu- 

 tion poured in and brought in contact with the sides, top, and bottom by turning 

 the dish in all directions. The solution is emptied out, and the dish, which is 

 kept closed, is ready for use. 



A leveling apparatus is required. It consists of a wooden tripod with ad- 

 justable screws, and a glass dish covered by a flat plate of glass upon which a low 

 bell-jar stands. The glass dish is filled with broken ice and water, covered 

 with the glass plate, and then exactly 

 leveled by adjusting the screws under the 

 legs of the tripod. When level, the cover 

 is placed upon it, and it is ready for use. 

 Method. A sterile platinum loop is 

 dipped into the material to be examined, 

 a small quantity secured, and stirred about 

 so as to distribute it evenly throughout 

 the contents of a tube of melted gelatin. 

 If the material under examination be very 

 rich in bacteria, one loopful may contain a 

 million individuals, which, if spread out 

 in a thin layer, would develop so many 

 colonies that it would be impossible to sec 

 any one clearly; hence further dilation be- 

 comes necessary. From the first tube, 

 therefore, a loopful of gelatin is carried to 

 a second and stirred well, so as to distribute 

 the organisms evenly throughout its con- 

 tents. In this tube we may have no more than ten thousand organisms, and if 

 the same method of dilution be used again, the third tube may have only a few 

 hundred, and a fourth only a few dozen colonies. 



After the tubes are tnus inoculated, one of the sterile glass plates is caught by 

 its edges, removed from the iron box, and placed beneath the bell-glass upon the 

 cold plate covering the ice- water of the leveling apparatus. The plug of cotton 

 closing the mouth of tube No. i is removed, and to prevent contamination during 

 the outflow of the gelatin the mouth of the tube is held in the flame of a Bunsen 

 burner for a moment or two. The gelatin is then cautiously poured out upon the 

 plate, the mouth of the tube, as well as the plate, being covered by the bell-glass 

 to prevent contamination by germs in the air. The apparatus being level, the 

 gelatin spreads out in an even, thin layer, and, the plate being cooled by the ice 



beneath, it immediately solidifies, and in a few 

 moments can be removed to the moist cham- 

 ber prepared to receive it. As soon as plate 

 No. i is prepared, the contents of tube No. 2 

 are poured upon plate No. 2, allowed to spread 

 out and solidify, and then superimposed on 

 plate No. i in the moist chamber, being sepa- 

 rated from the plate already in the chamber 

 by small glass benches made for the purpose and previously sterilized. After 

 the contents of all the tubes are thus distributed, the moist chamber and 

 its contents are stood away to permit the bacteria to grow. Where each 

 organism falls a colony develops, and the success of the whole method depends 

 upon the isolation of a colony and its transfer to a tube of new sterile culture- 

 media, where it can grow unmixed and undisturbed. 



From the description it must be evident that only those culture-media that 



Fig. 51. Complete leveling ap- 

 paratus for pouring plate cultures, 

 as taught by Koch. 



Fig. 52. Glass bench. 



