KOCH'S PLATE METHOD loi 



originally devised by Koch. This consists in the exposure of 

 a plate of nutrient agar or gelatine for a given time to the 

 air of which examination is desired, in such a manner that the 

 organisms floating, or borne upon dust particles therein, will by 

 gravity fall upon the plate, and, finding themselves in a suitable 

 environment, start growth thereon. It will be found in practice 

 that nutrient agar is better for the purpose than nutrient gelatine. 

 Greater latitude is obtained both in point of temperature and length 

 of incubation, and the result is uncomplicated by the, at times, very 

 rapid liquefaction of the gelatine by liquefying organisms. Care 

 should be taken in preparing the plates to allow them to cool on a 

 perfectly level surface, and at least 1 5 cc. of the medium should be 

 [employed for each Petri dish in order to ensure a perfectly even 

 ^surface and sufficient depth all over the plate. After exposure the 

 )lates are, under ordinary circumstances, best left at room tempera- 

 Tture during the development of the colonies, but if it is desired to 

 1 examine the bacteria alone it will be found well to favour the 

 pgrowth of these at the expense of the moulds by first incubating 

 jthe dish at a temperature of 37° C. for, say, eighteen hours. In any 

 [case the plate should be shielded from light, as othervvise many of 

 the chromogenic organisms will not assume their typical coloration. 

 ^Should it be desired to photograph the plates in order to obtain 

 [.a permanent record, the growth should be arrested and the 

 [organisms killed about the third or fourth day of incubation. The 

 [best method of doing this is to reverse the dish, and to pour upon 

 |a piece of blotting-paper placed on the inner surface of the lid, 

 /hich will now be undermost, a sufficient quantity of Formalin 

 to saturate it. 



We have ourselves in this method employed special dishes of 

 large size (see Plates 5 and 6), but in ordinary practice it will be 

 found that the 9 cm. Petri dish is sufficient, and by means of these 

 an accurate idea may be obtained of the relative richness of organ- 

 isms of the air in different localities or situations. A series showing 

 such comparative results will be found on Plates 10, 11, and 12. 



As will be seen, however, the above method is of no use when 

 any approach to exactness as to the number of bacteria in a given 

 volume of air is desired, although by a simple calculation an idea 

 can be gained thereby of the number of organisms falling on a 

 given area in a given space of time.^ 



' To calculate the area of a circular dish, square the diameter, and multiply 

 by 785. According to Petri, the number of bacteria present in 10 litres of air 

 are deposited upon 100 square centimetres in five minutes. 



