56 



PHYSIOLOGY OF NUTRITION 



Bacteria are present in all foods, milk furnishing especially favorable condi- 

 tions for their development. When fresh this liquid generally contains no bac- 

 teria, but they develop very quickly from spores that fall from the air. Thus 

 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. Sterlization 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 



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



as knives, scissors, glass rods, forceps, slides and cover glasses, platinum needles, 

 etc., may be sterilized by heating in a gas or alcohol flame. Platinum instru- 

 ments may be brought to a red heat but for other objects a few moments in the 

 flame suffices, so that germs clinging to the surface may be destroyed. A dry- 

 ing 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 



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

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

 Anleitung zur Kultur der Mikroorganismen für den Gebrauch in zoologischen, botanischen, medizinischen 

 und landwirtschaftlichen Laboratorien. Leipzig and Berlin, 1907. 



