70 BACTERIOLOGICAL METHODS 



interfere with the growth and development of certain bacteria. 

 Investigators have also noted great variations in results with 

 different brands or makes of peptones used, the quantitative 

 differences amounting to 50 per cent, in some cases. Equally 

 remarkable are the differences due to the kinds of water used in 

 the preparation of the culture media. For instance it is known 

 that agar made up with sewage encourages the development of 

 sewage organisms while the same medium made up with tap 

 water encourages the growth of bacteria predominating in such 

 tap water. 



The gelatine used is yet another important factor in cultural 

 results, depending upon the age of the gelatine, its purity, the de- 

 gree of heating to which it has been exposed, its origin, possible 

 contamination with arsenic, with bacteria and mold. Other 

 ingredients used in the preparations of culture media cause more 

 or less marked variation in comparable results. The above 

 statements make it evident that it is absolutely necessary to 

 adopt and to adhere to um'form methods in order that the com- 

 parable results may be approximately uniform. 



2. Glassware. — Different investigators have found that the 

 number of bacteria in and upon culture media varied with the 

 composition of the glass containers used. The comparatively 

 soluble glass, for example, yielded enough free alkali to inhibit the 

 development of the more sensitive bacteria. The size of the 

 containers and the thickness of the glass yielded differences in 

 the results. It is therefore very desirable to adopt Petri dishes 

 and test-tubes of standard form and thickness of standard cubic 

 contents. 



3. Other Factors. — The form and size of the incubating 

 chamber, the degree of ventilation, degree of darkness, amount of 

 oxygen present, etc., cause variations in the results. 



Of even greater importance than any of the factors so far 

 mentioned, is the personal equation in the laboratory technique. 

 No two workers follow out the same details in the different steps 



