Abstract
Background Bacillus species are spore-forming bacteria that are ubiquitous in the environment and display a range of virulent and avirulent phenotypes. This range is particularly evident in the Bacillus cereus sensu lato group; where closely related strains cause anthrax, food-borne illnesses, and pneumonia, but can also be non-pathogenic. Although much of this phenotypic range can be attributed to the presence or absence of a few key virulence factors, there are other virulence-associated loci that are conserved throughout the B. cereus group, and we hypothesized that these genes may be regulated differently in pathogenic and non-pathogenic strains.Methodology/Principal FindingsHere we report transcriptional profiles of three closely related but phenotypically unique members of the Bacillus cereus group—a pneumonia-causing B. cereus strain (G9241), an attenuated strain of B. anthracis (Sterne 34F2), and an avirulent B. cereus strain (10987)—during exponential growth in two distinct atmospheric environments: 14% CO2/bicarbonate and ambient air. We show that the disease-causing Bacillus strains undergo more distinctive transcriptional changes between the two environments, and that the expression of plasmid-encoded virulence genes was increased exclusively in the CO2 environment. We observed a core of conserved metabolic genes that were differentially expressed in all three strains in both conditions. Additionally, the expression profiles of putative virulence genes in G9241 suggest that this strain, unlike Bacillus anthracis, may regulate gene expression with both PlcR and AtxA transcriptional regulators, each acting in a different environment.Conclusions/SignificanceWe have shown that homologous and even identical genes within the genomes of three closely related members of the B. cereus sensu lato group are in some instances regulated very differently, and that these differences can have important implications for virulence. This study provides insights into the evolution of the B. cereus group, and highlights the importance of looking beyond differences in gene content in comparative genomics studies.
Highlights
Bacillus species are a highly diverse, spore-forming group of bacteria that can be found ubiquitously in the environment
In 1994, a B. cereus strain (G9241) was isolated from a welder suffering from severe pneumonia that was similar in many ways to inhalational anthrax, and other similar cases of anthrax-like disease caused by B. cereus have since been observed [4,5]
The medium used in this study, Modified G Medium (MGM), is typically used to produce high levels of B. anthracis spores
Summary
Bacillus species are a highly diverse, spore-forming group of bacteria that can be found ubiquitously in the environment. Bacillus species are spore-forming bacteria that are ubiquitous in the environment and display a range of virulent and avirulent phenotypes This range is evident in the Bacillus cereus sensu lato group; where closely related strains cause anthrax, food-borne illnesses, and pneumonia, but can be non-pathogenic. Much of this phenotypic range can be attributed to the presence or absence of a few key virulence factors, there are other virulenceassociated loci that are conserved throughout the B. cereus group, and we hypothesized that these genes may be regulated differently in pathogenic and non-pathogenic strains
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