Enumeration of thermophilic Bacillus species in composts and identification with a Random Amplification Polymorphic...

The thermophilic microbial flora of general garden and domestic wastes composts, derived from thermogenic, post-thermogenic and maturation phases, was analysed using spore and total plate counts in combination with an optimised RAPD protocol. A total of 459 isolates were recovered obtained at 55 degrees C, and another 56 at 70 degrees C using tryptic soy-starch agar plates, with near-equal numbers being derived from total plate counts or spore preparations. The isolates were obtained from 11 compost samples and were assigned to eighteen different RAPD fingerprint types, with 76.1% of these ultimately being positively assigned by their RAPD profiles to just 2 species including Bacillus thermodenitrificans and B. licheniformis. Viable cell numbers ranged from 1.4 to 150 x 10(6) colony forming units per gram compost (wet weight), with the highest two counts being from 2 week and 4 week old compost samples with temperatures of 70 degrees C and 55 degrees C, respectively. B. thermodenitrificans was a dominant isolate (representing more than 50% of isolates from total plate counts) in 7 of the 11 individual compost total plate count samples between 30 degrees C to 73 degrees C, and accounted for 68.9% of all isolates overall. Another relatively common Bacillus species that was identified with RAPDs in significant numbers included B. licheniformis (7.2% of all isolates and dominant isolate in 1 sample). Three other relatively common RAPD profiles could not be identified by comparison with known species in a RAPD profile database but were tentatively identified using 16S rDNA sequence comparisons. These were B. sporothermodurans (4.9% of all isolates and dominant in 1 sample), B. thermosphaericus (7.4% and dominant in 1 sample) and Terrabacter tumescens (5.0%). Overall, based on the vegetative and spore count results and the subsequent RAPD-based identification, the data strongly support a significant role for B. thermodenitrificans in the composting process, and casts doubt on the notion that B. stearothermophilus sensu strictu (DSMZ 22) is a prominent member within compost ecology.