In microbiological work it is often desired to determine changes in the total cellular content of protein, carbohydrate, and lipid in relation to changes in light intensity, temperature, or mineral element deficiencies. The methods most generally used for estimating total lipid are (a) gravimetric determination of extracted lipid, (5) calculations involving the caloric content of cells (R value) as est.imated by carbon, hydrogen, nitrogen elementary analyses (1)) and (c) subtraction of the weight of protein and carbohydrate from the total dry weight. These procedures all require milligram quantities of cellular material and hence are not useful in experiments in which the number of samples limits the dry weight of cells per sample to less than one milligram.
Gas chromatography and specific calorimetric analyses (2) enable one to determine specific lipid components, e.g., fatty acids, with high sensitivity, but these are not necessarily suitable for rapid and simple determinat,ions of t.he total amount of lipid in a large number of samples.
This paper describes a method for the quantitative determination of lipid in microorganisms in samples containing as little as 20 pg of lipid. The procedure is based upon oxidation of the solvent-extracted lipids to carbon dioxide and measurement of the liberated carbon dioxide by infrared analysis. The results obtained by this method for various microorganisms and spinach chloroplasts are compared with those obtained by conventional gravimetric and calorimetric methods.