A Soil and Rhizosphere Microorganism Isolation and Enumeration Medium That Inhibits Bacillus mycoides.

A new solid medium has been developed for the enumeration and isolation of soil and rhizosphere microorganisms. This medium, named rhizosphere isolation medium, contains glucose and 15 of the 20 common amino acids. The absence of five other amino acids, namely, aspartic acid, asparagine, cysteine, proline, and threonine, inhibits the growth of Bacillus mycoides, a commonly encountered bacterium that rapidly spreads on agar media and complicates the isolation and enumeration of other microorganisms. Compared with a similar medium containing Casamino Acids, rhizosphere isolation medium had half as many colonies of B. mycoides, with each colony approximately half the diameter. The two media had similar total numbers of bacterial colonies. Isolates were divided into taxononomic groups, roughly corresponding to species and genus, by fatty acid methyl ester analysis and numerical methods. There were 24 genera and 41 species found in the isolates from rhizosphere isolation medium, while 19 genera and 35 species were found in the isolates from the medium prepared with Casamino Acids. No major group of bacteria was found to occur only on one medium or on the other, indicating that the five missing amino acids had no great effect on organisms other than B. mycoides. This medium may prove useful in soil and rhizosphere studies in which the growth of B. mycoides is undesirable.