MORPHOLOGY AND PHYSIOLOGY OF BACTERIA. 127 



salts, most bacteria flourish best if organic substances contain- 

 ing nitrogen, like peptone and albumen, are furnished them as 

 part of their food. The complicated, unstable, organic mole- 

 cules with high potential energy are converted by them into 

 simple and more stable compounds like carbon dioxide, am- 

 monia and water, with the liberation of energy. These facts 

 become manifest in connection with their important work in 

 decomposition, putrefaction and fermentation. A culture- 

 medium having a slightly alkaline or neutral reaction is favor- 

 able to most bacteria. 



The prolonged artificial cultivation of bacteria may or may 

 not modify their properties. The pathogenic bacteria are 

 likely to undergo considerable modification both in the quality 

 and luxuriance of their growth and the intensity of their path- 

 ogenic characters. 



The growth of bacteria may eventually be hindered by the 

 accumulation of the products of their own metabolism. Many 

 bacteria refuse to grow on. culture-media at all; at least the 

 suitable artificial medium has n6t yet been found for them. 

 Some species are extremely fastidious, and can only be propa- 

 gated on particular nutrient substances, others again will grow 

 in distilled water.* But bacteria show great adaptability, and, 

 once they have been made to start, they can be further culti- 

 vated with less and less difficulty, as a rule. 



Relation to Oxygen. Oxygen is indispensable to the 

 growth of bacteria. Some of them, the aerobes, require 

 oxygen in the free form. Others, the anaerobes require it in 

 some form of compound, and are unable to live in an atmos- 

 phere of free oxygen. Others still are able to flourish either in 

 the presence or absence of free oxygen-facultative aerobes or 

 anaerobes. The first-named varieties are sometimes called 

 strict, or obligate aerobes or anaerobes. 



*Bolton. Zcitschrift fur Hygiene. Ed. I., No. i, 1886. 



