Development of Selective Growth Media for Isolation and Enumeration of Biodegraders From Soil

Cellulose and lignin are the main components of crop residues that must be decomposed by heterotrophic microbes either on the surface (zero-tillage) or in the soil (conventional tillage) to release plant nutrients and sustain humus formation. This study was conducted to develop and evaluate improved agar media for the selective enumeration and isolation of a wide range of cellulolytic and ligninolytic microbes from field samples. An existing cellulose-Congo Red agar was modified by replacing the cellulose powder with carboxymethylcellulose (CMC) and by doubling the concentration of soil extract. The new medium (CMCCRA) for cellulolytic bacteria was evaluated, in comparison with the traditional minimal salts-CMC agar (MSCMCA), by spreadplating of dilutions from samples of three, surface-placed crop residues (wheat, pea and flax) and the underlying soil, collected in spring 1996 from a field study a t Melfort. Cellulolytic activity was easy to detect on CMCCRA plates due to the distinct zones of clearing around all active colonies, that are clearly visible against a deep red background without having to flood the agar with a liquid reagent as is necessary when using the MSCMC agar. Thus, counts of cellulose degrading bacteria on crop residues and in soil were consistently higher on CMCCRA and also less variable than on MSCMCA. By adding streptomycin we made the CMCCR agar more selective for fungal growth and used it to reliably enumerate cellulolytic fungi, which - as a group - accounted for about 30% of total number of viable fungi in the surface soil. We also explored the feasibility of using a polymeric dye agar (with Poly R-478) as selective medium for the primary detection of ligninolytic organisms and their subsequent isolation from enrichment cultures.