PLATES AND PETRI DISHES 145 



margin of the culture tube over a flame. Lift up the lid of the Petri 

 dish a little, but be careful that it protects the dish from the micro- 

 organisms falling into it from the air. As soon as the fluid is trans- 

 ferred from the culture tube cover the Petri dish again with its lid. 

 Do this with all three tubes and allow the agar in the dishes to cool 

 as rapidly as possible. This is best accomplished by cooling them 

 in the refrigerator before they are used. When the agar has set in 

 the Petri dishes these can be labelled as Nos. 1, 2, and 3, corresponding 

 to the tubes, and placed in the incubator, lids down, and the dish 

 proper, containing the agar on top. This is done so that the con- 

 densed water which forms in the incubator will collect in the lid and 

 not on the agar, where it would do harm. 



Object of Preparing Petri Dishes. This is explained by the follow- 

 ing considerations : Tube No. 1 has been inoculated with pus from the 

 abscess, and it would be reasonable to assume that several thousand 

 cocci which are the cause of the suppurative abscess and a few hun- 

 dred contaminating bacteria have been introduced. The pus has been 

 diluted in the agar of tube No. 1, and when No. 2 has been inoculated 

 from No. 1 there will be perhaps in No. 2 several hundred cocci and 

 twenty to thirty of the contaminating organisms, while in tube No. 3, 

 inoculated from the very dilute pus of No. 2, there will be only twenty 

 cocci and one or two of the contaminating organisms. As soon as 

 the agar has become solid, each individual bacterium, or perhaps a 

 small group of individual bacteria which have clung together in spite 

 of the energetic shaking and mixing, is fixed in a definite place, and 

 at these places colonies of bacteria, composed of many millions, 

 which can be seen either with the naked eye or with a hand lens or low 

 magnification of the microscope, have developed from the single bacte- 

 rium or small group within the next twenty-four to forty-eight hours. 

 Petri dish No. 1, or, as it is often called, plate No. 1, will develop so 

 many colonies in the incubator within the next twenty-four hours 

 that they soon become confluent and cannot be picked up individually. 

 Plate No. 3 is generally the best one. It contains perhaps ten to twenty 

 colonies of the coccus and one or two colonies of the contaminating 

 microorganism. The two different types can often be distinguished 

 by the naked eye from the appearance of their colonies, but at times 

 it may be necessary to make microscopic examinations before their 

 character can be recognized. It must not be supposed that the organ- 

 ism present in twenty colonies is always the cause of the pathologic 

 lesion, and that the one represented by one or two colonies is the 

 contaminating microbe. Sometimes this is not the case, hence it is 

 often necessary to find out by animal experiments which is the causa- 

 tive and which the contaminating bacterium. On the other hand, 

 it is often possible to decide immediately which of the two is really 

 the cause of the pathologic lesion. For instance, when examining 

 the pus of an abscess as described above, and colonies of Staphylo- 

 coccus pyogenes aureus and colonies of the hay bacillus are found, it 

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