Abstract
The Bacillus cereus group, which includes entomopathogens and etiologic agents of foodborne illness or anthrax, persists in various environments. The basis of their ecological diversification remains largely undescribed. Here we present the genetic structure and phylogeny of 273 soil B. cereus s.l. isolates from diverse habitats in northeastern Poland, with samplings acquired from the last European natural forest (Białowieża National Park), the largest marshes in Europe (Biebrza National Park), and a farm. In multi-locus sequence typing (MLST), despite negative selection in seven housekeeping loci, the isolates exhibited high genetic diversity (325 alleles), mostly resulting from mutation events, and represented 148 sequencing types (131 STs new and 17 STs already described) grouped into 19 complexes corresponding with bacterial clones, and 80 singletons. Phylogenetic analyses showed that 74% of the isolates clustered with B. cereus s.l. environmental references (clade III), while only 11 and 15%, respectively, grouped with isolates of clinical origin (clade I), and B. cereus ATCC 14579 and reference B. thuringiensis (clade II). Predominantly within clade III, we found lineages adapted to low temperature (thermal ecotypes), while putative toxigenic isolates (cytK-positive) were scattered in all clades of the marsh and farm samplings. The occurrence of 92% of STs in bacilli originating from one habitat, and the description of new STs for 78% of the isolates, strongly indicate the existence of specific genotypes within the natural B. cereus s.l. populations. In contrast to the human-associated B. cereus s.l. that exhibit a significant level of similarity, the environmental isolates appear more complex. Thus we propose dividing B. cereus s.l. into two groups, the first including environmental isolates, and the second covering those that are of clinical relevance.
Highlights
Bacillus cereus sensu lato (s.l.) are Gram-positive endospore-forming bacilli that persist in different ecological environments [1,2]. This group of bacteria includes various strains of Bacillus thuringiensis, an enthomopathogen successfully exploited in biocontrol worldwide [3], Bacillus cereus sensu stricto (B. cereus s.s.) which is associated with foodborne illness [4], and Bacillus anthracis, the etiologic agent of anthrax [5]
Occurrence of B. cereus s.l. in different types of soil The estimated density of B. cereus s.l. isolates from the farm, and Białowiez_a and Biebrza national parks (NPs) were, respectively, 0.760.76105, 1.461.76105 and 1.562.26105 (Table 1)
While B. cereus s.s. and B. thuringiensis were isolated from each sample, B. mycoides was present in all marsh and farm samples, but only in 77% of forest samples
Summary
Bacillus cereus sensu lato (s.l.) are Gram-positive endospore-forming bacilli that persist in different ecological environments [1,2]. This group of bacteria includes various strains of Bacillus thuringiensis, an enthomopathogen successfully exploited in biocontrol worldwide [3], Bacillus cereus sensu stricto (B. cereus s.s.) which is associated with foodborne illness [4], and Bacillus anthracis, the etiologic agent of anthrax [5]. Other properties attributed to a particular species may not be intrinsically unique, for example, psychrotolerance, a primary feature of B. weihenstephanensis [6], has recently been reported among B. thuringiensis isolates [19,20]
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