Problems in the Use of Agar for the Enumeration of Soil Microorganisms

PROBLEMS IN THE USE OF AGAR FOR THE ENUMERATION OF SOIL MICROORGANISMS DUKING an investigation of the biological decomposition of herbicides, a study was initiated to ascertain the numbers of microorganisms in soil capable of utilizing these pesticidal chemicals as the sole carbon source. Since the apparent population size observed was far in excess of that expected, many of the colonies were tested in liquid media in order to verify the apparent utilization and decomposition of the herbicidal compounds. These cultures failed to grow in solution culture, however, suggesting that they were using the agar or impurities contained therein as sources of carbon. A number of similar observations have been reported in other studies of herbicide decomposition. As a result of these preliminary observations, an investigation was made of the relative abundance in soil of microorganisms capable of growth in a medium with no source of carbon other than agar. For counting, ten-fold serial dilutions of the soils were plated upon an inorganic salts-agar medium of the following composition: NH4NO3> 0.5 g.; K2HPO4, 0.8 g.; KH2PO4, 0.2 g.; MgSO4'7H2O, 0.2 g.; FeSO4 • 7H2O, 0.03 g.; CaCl2-2H2O, 0.1 g.; agar, 18 g.; demineralized, distilled water, 1,000 ml. The solidifying preparations used