and Laboratory Methods. 119*^ 



A Ventilated Dish for Bacteria Cultures. 



When Koch's original form of plate for bacteria culture was abandoned for 



the more convenient Petri dish, a step was undoubtedly taken in the right 



direction ; yet in one respect it was a step backward. Koch's plates were placed 



for incubation in a large air-tight receptacle, as a bell-jar, which contained wet 



filter paper. The object of this jar was to prevent the gelatin from drying up 



as it would do if exposed to the atmosphere of the ordinary incubator. In the 



Petri dish drying was prevented by making the cover fit the bottom plate tightly 



— at least such was the intention. As a matter of fact, however, the dishes are 



seldom air-tight because the bottom plates arid covers become mismated in the 



laboratory. Consequently when Petri dishes are used there is almost always a 



slight drying of the gelatin. The loss of water by evaporation in the ordinary 



Petri dish may be as great as 15 per cent, in 72 hours at "20°, but it is ordinarily 



about 5 per cent. Although this evaporation is comparatively small, it is 



sufficient to cause a thickening of the gelatin at the surface, and this thin film 



tends to exclude oxygen from the medium and thus retard or prevent the growth 



of aerobic bacteria. In a former publication* the writer has shown how the 



amount of moisture in the atmosphere of the incubator affects the number of 



bacteria that will develop from a sample of water. The results there set forth 



were summarized in the following table : 



Per Cent, which the Number 



of Bacteria that Developed 



Relative Humidity of the in the Incubator was of the 



Atmosphere of the Incuba- Number that Developed in 



tor in per cent, of Saturation. a Moist Chamber. 



GO 75 



75 82 



95 98 



98 97 



100 100 



It was also shown that air-tight Petri dishes are unfavorable for the growth 

 of aerobic bacteria, because of the partial exhaustion of the oxygen from the 

 somewhat limited air space and the collection there of gaseous products of 

 growth. For example, the air in five sealed Petri dishes was collected and 

 analyzed after bacteria had been allowed to develop in them for 72 hours, and 

 was found to contain only 5 per cent, of oxygen and 5 per cent, of CO.^ ; 

 whereas in ordinary Petri dishes with ill-fitting covers the percentage of oxygen 

 was 15 per cent, and of carbonic acid 2 per cent. 



From these two facts, that an air-tight Petri dish gave too low results on 

 account of exhaustion of oxygen and that an ordinary Petri dish gave too low 

 results on account of evaporation of moisture, it was argued that the best condi- 

 tions would be obtained by cultivating bacteria in a ventilated dish placed in an 



*On the Necessity of Cultivating Water Bacteria in an Atmosphere Saturated with Moisture. 

 Technology Quarterly, Dec. 1899. 



