Use of sublimation to prepare solid microbial media with water-insoluble substrates

A method was developed to deposit a visible layer of water-insoluble compounds via sublimation onto the surface of solid growth media. The compound is sublimed from a heated aluminum dish onto the surface of an ice-cooled, inoculated basal growth medium in an inverted Petri dish. The method results in the deposition of a thin, even layer of substrate on the agar surface without the use of solvent. After incubation, clearing zones around colonies indicate the presence of compound-degrading microorganisms. Previously published methods for the enumeration of bacteria capable of degrading polycyclic aromatic hydrocarbons were compared with the sublimation method. When the sublimation method was employed, more than twice as many bacteria capable of degrading phenanthrene were detected as the next best method (spray-plate). The results of the method comparisons indicate that solvents present in methods other than the sublimation method are inhibitory to the growth of these bacteria.