Climatic influence on mesophilic Bacillus cereus and psychrotolerant Bacillus weihenstephanensis populations in tropical, temperate and alpine soil

Bacillus weihenstephanensis strains are psychrotolerant and grow from below 7 degrees C to 38 degrees C. Closely related mesophilic Bacillus cereus st rains can grow from above 7 degrees C to 46 degrees C. We classified 1060 B , cereus group isolates from different soil samples with respect to their p sychrotolerant and mesophilic genotypes by polymerase chain reaction (PCR) targeting of specific 16S rDNA and cold shock protein A gene signatures. In parallel, growth tests at 7 degrees C were carried out to determine the th ermal phenotype, The geographic distribution of psychrotolerant and mesophi lic isolates was found to depend significantly on the prevalent annual aver age temperature. In one tropical, one temperate and two alpine habitats, th e proportion of psychrotolerant cspA genotypes was found to be 0%, 45% and 86% and 98%, respectively, with the corresponding annual average temperatur es being 28 degrees C, 7 degrees C, 4 degrees C and 1 degrees C. In the tro pical habitat, only the mesophilic B. cereus was found, characterized by co rrespondence of thermal genotype and phenotype, In the alpine habitat, almo st only the psychrotolerant B. weihenstephanensis was isolated. In the temp erate habitat, mesophilic B. cereus and psychrotolerant B. weihenstephanens is as well as 'intermediate thermal types' occurred, the latter having oppo site thermal genotypes and phenotypes or opposing sets of thermal DNA signa tures, characterized by the coexistence of mesophilic and psychrotolerant 1 6S rDNA operon copies within a single isolate. Both sugar utilization and D NA fingerprinting patterns revealed a high, probably non-clonal microsite d iversity within the population of the temperate habitat. We interpret our o bservations in terms of a temperature-dependent selection regime, acting on recombining B. cereus/B. weihenstephanensis populations in soil.