40 BIOLOGY OF BACTERIA 



room temperature. Bacteria in cultures may be kept alive much 

 longer if they are tightly sealed up and placed in the refrigerator 

 after the growth has been well developed. This not only keeps them 

 alive much longer, but prevents, to a large extent, the appearance of 

 involution forms. 



Elements Necessary for Growth. Bacteria need for their nutrition, 

 growth, and multiplication substances containing the elements 

 carbon, nitrogen, hydrogen, oxygen, sulphur, and phosphorus, and 

 some salts. They can generally best derive their nutrition from 

 albumins and their derivatives, like 'peptones and gelatins, but 

 even pathogenic bacteria may be grown in albumin-free culture soils 

 composed of very simple compounds. On the other hand, certain 

 strictly parasitic pathogenic bacteria do not thrive well on artificial 

 culture media, particularly during the first generations. The bovine 

 tubercle bacillus is one of this type. Other bacteria have never 

 been successfully cultivated, as, for instance, the leprosy bacillus or 

 the acid-fast bacillus, which is found in such enormous numbers in 

 Johne's disease of cattle. Other pathogenic bacteria grow only after 

 the addition of unaltered hemogoblin to the culture soil, for example, 

 the influenza bacillus. Still others require the addition of natural 

 serous exudates, such as pleuritic or ascitic fluid. Too great a con- 

 centration of the culture soil with a high percentage of solids results 

 in a poor growth or prevents it entirely. 



Reaction of the Medium. All bacteria are rather particular about the 

 reaction of the medium in which they grow. Some favor a neutral, 

 others a slightly alkaline, still others a slightly acid medium. Care 

 must be taken in preparing the medium, as bacteria generally will 

 not thrive if it is more than slightly acid or alkaline, and if there 

 is any marked degree of either the bacteria will die, especially in 

 the presence of mineral acids. 



Plasmolysis and Plasmoptysis. Many bacteria cannot stand the 

 sudden transfer from one fluid to another, differing materially in 

 concentration. Such sudden changes of osmotic pressure may cause 

 a shrinking of the contents of the bacterial cell away from the mem- 

 brane with a loss of fluid to the outside liquid; this is called plasmo- 

 lysis. Or some of the cell contents may become expelled through a 

 rupture in the membrane; this is known as plasmoptysis. These 

 damaged bacteria are, however, not necessarily dead, and they may 

 return to the normal if placed'again under favorable conditions. 



Aerobic and Anaerobic Bacteria. Certain bacteria like the higher 

 plants and animals can only exist and grow in the presence of free 

 oxygen. Such bacteria are called obligate or strict aerobes. To this 

 type belong many saprophytes, and among them particularly the 

 color or pigment-forming so-called chromogenic bacteria, also some 

 microorganisms which form poisonous decomposition products in 

 milk. Among the pathogenic bacteria, the plague bacillus, the 

 influenza bacillus, the pneumococcus, etc., belong to this group. 



