ASSIMILATION OF CARBON 



57 



a cubic centimeter of milk that had stood since milking at a temperature of 

 i5.5°C, contained the following numbers of bacteria per cubic centimeter. 



Hours after Milking 



4 



9 



24 



Bacteria per cc. 

 34,000 

 100,000 

 4,000,000 



The intestinal tract of man is densely populated with bacteria, which fre- 

 quently cause decomposition of foods in the intestine. We are thus not only 

 externally surrounded by bacteria, but are even internally infested with them. 

 This seems to explain why these organisms appear so promptly in all kinds of 

 organic material that they decompose. 



§5. Sterilization and Disinfection. 1 — In view of the fact that microorganisms 

 are so universally present, all objects used in handling them must be absolutely 

 free from spores or germs of any 

 kind, especially if pure cultures of 

 a certain species are desired. This 

 is accomplished by sterilization. 

 Such small objects as knives, scis- 

 sors, glass rods, forceps, slides and 

 cover glasses, platinum needles, 

 etc., may be sterilized by heating 

 in a gas or alcohol flame. Plat- 

 inum instruments may be brought 

 to a red heat but for other objects 

 a few moments in the flame suffices, 

 so that germs clinging to the sur- 

 face may be destroyed. A drying 

 oven, or dry-air sterilizer, is used 

 for the sterilization of larger objects 

 (Fig. 33). This is usually equipped 

 with double walls, the products of combustion from the gas flame below passing 

 between the two walls and thus rendering the heating uniform. 



Objects that cannot endure dry heat are sterilized in a steam sterilizer, 

 such as Koch's apparatus. This is a cylinder of tinned sheet iron or copper 

 with a cover above. The lower part is filled with water and the objects to be 

 sterilized are placed upon a perforated rack in the upper part. A burner below 

 the cylinder heats the water to boiling and the contained objects are sterilized 



1 Abel, Rudolf V. L., Taschenbuch fur den bakteriologischen Praktikanten. [Abel's Laboratory hand- 

 book of bacteriology. Tr. from 10th German ed. by M. H. Gordon. London, 1907.] Kiister, Ernst, Anlei- 

 tung zur Kultur der Mikroorganismen fur den Gebrauch in zoologischen, botanischen, medizinischen und 

 landwirtschaftlichen Laboratorien. Leipzig and Berlin, 1007. 



c For most satisfactory work the oven should have an automatic temperature-regulator, 

 various forms of which are available for gas. Electrically heated, automatically regulated 

 ovens are also obtainable, some of which are so well insulated that but little heat escapes to the 

 exterior. — Ed. 



Fig. 33. — Dry-air sterilizer heated by gas. 



