144 PURE CULTURES FROM PATHOLOGIC MATERIAL 



7. The platinum loop is once again brought into use to make 

 some smears on the slides or cover-glasses which have been in readi- 

 ness for this purpose. When working around animals it is generally 

 better to use slides, because they can be handled more easily than 

 the fine delicate cover-slips, and the subsequent preparation by air 

 drying, fixing, and staining is the same for either. If only one species 

 of bacterium were present in the abscess and the work of cleansing 

 the surface and opening the abscess cavity was aseptically performed, 

 pure cultures should be obtained in the two test-tubes inoculated. 



Plates and Petri Dishes. The procedure just described will give 

 results, but a much better method is to prepare plates. This was 

 first practised by Robert Koch, but his method was somewhat com- 

 plicated and easily leads to contamination, hence it has been largely 

 replaced by the use of the Petri dish. The latter 

 FlG - 72 is a small, circular glass vessel about four to six 



inches in diameter, one-half inch high, and pro- 

 vided with a cover. These Petri dishes must be 

 sterilized before use in the hot-air sterilizer, and 

 it is advantageous when they have to be taken to 

 Petri dish. a distant place to wrap them in paper which has 



been sterilized with them and which is only ic- 

 moved just before they are to be used. The paper, of course, is 

 not necessary when the dishes, after cooling, are taken from the 

 sterilizer and used at once in the laboratory. 



Method of Pouring Plates or Preparing Petri Dishes. This 

 method must always be used when there is any chance of contami- 

 nation of the material to be examined, and is at all times better than 

 simply inoculating two tubes, as previously described. The method 

 of preparing Petri dishes is practised as follows: 



Take four agar tubes (if gelatin tubes are taken the procedure 

 is the same, except as to the temperature figures given) and melt 

 the contents in a water bath. Open one of the tubes and introduce 

 a thermometer. This tube is not to be used for inoculation but 

 merely for the control of the temperature. After the agar has been 

 melted in all the tubes, allow it to cool down to about 50 C. Take 

 the tubes out of the water bath. Flame the upper ends, remove the 

 cotton plugs, and place the three tubes in a slanting position on the 

 table. This can be done with the aid of an ordinary small slide box, 

 paper box, or small wire rack. Inoculate tube No. 1 with platinum 

 loop (properly sterilized) three times from the pus. Shake tube No. 1, 

 always keeping it in a slanting position, so that microorganisms from 

 the air cannot fall into it. Inoculate tube No. 2 three times with the 

 platinum loop (previously sterilized) from tube No. 1. Shake the 

 contents of No. 2 well and finally inoculate No. 3 from No. 2 in the 

 same manner. Shake No. 3 well. Now pour the still fluid contents of 

 the three tubes into three sterile Petri dishes ready for the purpose. 

 This pouring of the Petri dishes is done as follows: Heat the upper 



