Bacillus sporothermodurans, a New Species Producing Highly Heat-Resistant Endospores

Researchers identified a previously unknown species of Bacillus bacterium, Bacillus sporothermodurans, isolated from ultrahigh-temperature (UHT)-treated milk. These bacteria produce highly heat-resistant endospores that can survive industrial sterilization processes at 135-142°C and subsequently contaminate milk products during storage. The problem was first detected in southern Europe in 1985 and has since spread to multiple European countries and some non-European dairies. Using 16S rRNA gene sequencing, DNA reassociation studies, ribotyping, and pyrolysis mass spectrometry, the authors established that HHRS (highly heat-resistant spore) strains form a homogeneous taxonomic group distinct from related Bacillus species. The bacteria are strictly aerobic, do not produce acid from sugars, and grow poorly on standard nutrient agar but well on brain-heart infusion medium. While the contaminating spores germinate and multiply in stored milk, reaching approximately 10^5 cells per milliliter, they cause no noticeable spoilage and are nonpathogenic. The type strain M215 (DSMZ 10599) has a G+C content of 36 mol% and produces characteristic long, filamentous rods with unusual granular Gram-stain appearance.

Key findings

• Bacillus sporothermodurans is a newly described species that produces exceptionally heat-resistant spores surviving UHT milk processing temperatures of 135-142°C
• The bacterium is phylogenetically distinct from closely related Bacillus species (B. firmus, B. benzoevorans, B. circulans) based on 16S rRNA analysis and DNA-DNA reassociation studies showing <30% homology
• HHRS strains are homogeneous at molecular, phenotypic, and physical levels, confirmed by ribotyping, pyrolysis mass spectrometry, and standardized phenotypic tests
• The bacteria are strictly aerobic obligates that do not ferment sugars and grow preferentially on brain-heart infusion medium rather than standard nutrient agar
• Strain M215 contains unusual rRNA gene heterogeneity with three distinct 16S rRNA types differing in the variable V1 and V2 regions, a finding rarely documented in bacterial species