Geobacillus sp., a Thermophilic Soil Bacterium Producing Volatile Antibiotics

Abstract

Geobacillus, a bacterial genus, is represented by over 25 species of Gram-positive isolates from various man-made and natural thermophilic areas around the world. An isolate of this genus (M-7) has been acquired from a thermal area near Yellowstone National Park, MT and partially characterized. The cells of this organism are globose (ca. 0.5 μ diameter), and they are covered in a matrix capsule which gives rise to elongate multicelled bacilliform structures (ranging from 3 to 12 μm) as seen by light and atomic force microscopy, respectively. The organism produces unique petal-shaped colonies (undulating margins) on nutrient agar, and it has an optimum pH of 7.0 and an optimum temperature range of 55–65°C. The partial 16S rRNA sequence of this organism has 97% similarity with Geobacillus stearothermophilus, one of its closest relatives genetically. However, uniquely among all members of this genus, Geobacillus sp. (M-7) produces volatile organic substances (VOCs) that possess potent antibiotic activities. Some of the more notable components of the VOCs are benzaldehyde, acetic acid, butanal, 3-methyl-butanoic acid, 2-methyl-butanoic acid, propanoic acid, 2-methyl-, and benzeneacetaldehyde. An exposure of test organisms such as Aspergillus fumigatus, Botrytis cinerea, Verticillium dahliae, and Geotrichum candidum produced total inhibition of growth on a 48-h exposure to Geobacillus sp.(M-7) cells (ca.10⁷) and killing at a 72-h exposure at higher bacterial cell concentrations. A synthetic mixture of those available volatile compounds, at the ratios occurring in Geobacillus sp. (M-7), mimicked the bioactivity of this organism.