Phylogenetic Diversity of the <i>Bacillus pumilus</i> Group and the Marine Ecotype Revealed by Multilocus Sequence Analysis

<div><p>Bacteria closely related to <i>Bacillus pumilus</i> cannot be distinguished from such other species as <i>B. safensis</i>, <i>B. stratosphericus</i>, <i>B. altitudinis</i> and <i>B. aerophilus</i> simply by 16S rRNA gene sequence. In this report, 76 marine strains were subjected to phylogenetic analysis based on 7 housekeeping genes to understand the phylogeny and biogeography in comparison with other origins. A phylogenetic tree based on the 7 housekeeping genes concatenated in the order of <i>gyrB</i>-<i>rpoB</i>-<i>pycA</i>-<i>pyrE</i>-<i>mutL</i>-<i>aroE</i>-<i>trpB</i> was constructed and compared with trees based on the single genes. All these trees exhibited a similar topology structure with small variations. Our 79 strains were divided into 6 groups from A to F; Group A was the largest and contained 49 strains close to <i>B. altitudinis</i>. Additional two large groups were presented by <i>B. safensis</i> and <i>B. pumilus</i> respectively. Among the housekeeping genes, <i>gyrB</i> and <i>pyrE</i> showed comparatively better resolution power and may serve as molecular markers to distinguish these closely related strains. Furthermore, a recombinant phylogenetic tree based on the <i>gyrB</i> gene and containing 73 terrestrial and our isolates was constructed to detect the relationship between marine and other sources. The tree clearly showed that the bacteria of marine origin were clustered together in all the large groups. In contrast, the cluster belonging to <i>B. safensis</i> was mainly composed of bacteria of terrestrial origin. Interestingly, nearly all the marine isolates were at the top of the tree, indicating the possibility of the recent divergence of this bacterial group in marine environments. We conclude that <i>B. altitudinis</i> bacteria are the most widely spread of the <i>B. pumilus</i> group in marine environments. In summary, this report provides the first evidence regarding the systematic evolution of this bacterial group, and knowledge of their phylogenetic diversity will help in the understanding of their ecological role and distribution in marine environments. </p>