Microbes Must Eat 61 



PREREQUISITE II: READILY AVAILABLE FOOD MATERIALS 

 Organisms vary tremendously in their nutritional requirements, 

 varying from simple inorganic salts up to and including living 

 tissues of species of plants or animals. But in general most 

 common bacteria are not too exacting, with the result that many 

 types of food substances can be utilized for food. Minute amounts 

 of accessory growth substances, or vitamins, are required by some 

 organisms, and to that end some species manufacture their own 

 vitamins. This brings in an interesting sidelight in nutritional 

 studies. If an organism requires a given vitamin and this vitamin 

 is not provided by the organism's own chemosynthetic activity, the 

 vitamin content of foods can be determined by attempting to grow 

 these vitamin-dependent microbes on this food. This is called 

 a VITAMIN ASSAY determination, employing biological rather than 

 strictly chemical test tube technics in the analysis. Experience 

 has demonstrated the value and the accuracv of this biolo^^ical 

 method. 



Organisms can be classified on the basis of their nutritional 

 needs into two major groupings, the autotrophs and the hetero- 

 TROPHS. The former bacteria use simple elements or simple com- 

 binations of inorganic material ( iron, manganese, etc. ) as their 

 principal source of food, and carbon dioxide is utilized as a carbon 

 source. Heterotrophs, in general, are unable to assimilate carbon 

 from carbon dioxide and their energy is primarily derived through 

 the chemical breakdown (analysis) of more complex food ma- 

 terials (nitrites, nitrates, etc.), especially organic compounds. An 

 organic compound is a carbon-containing compound derived from 

 plants or from animals. Heterotrophs can be further subdivided 

 into SAPROPHYTES which thrive on dead orsjanic matter, and 

 into PARASITES which depend upon living cells for their survival. 

 There are degrees of parasitism. The term facultative parasites 

 is used to designate those organisms capable of thriving on either 

 living or on inanimate materials, such as common laboratory media. 

 Strict parasitism is not nearly as common as is facultative para- 

 sitism. It is generallv conceded that heterotrophs probably evolved 



