Magoosh GRE

The genome sequence of Bacillus

| March 14, 2015

Timothy D. Read*†, Scott N. Peterson*‡, Nicolas Tourasse§#, Les W. Baillie*†k, Ian T. Paulsen*{, Karen E. Nelson*, Herve ́ Tettelin*, Derrick E. Fouts*, Jonathan A. Eisen*{, Steven R. Gill*, Erik K. Holtzapple*, Ole Andreas Økstad§#, Erlendur Helgason§#, Jennifer Rilstone*, Martin Wu*, James F. Kolonay*, Maureen J. Beanan*, Robert J. Dodson*, Lauren M. Brinkac*, Michelle Gwinn*, Robert T. DeBoy*, Ramana Madpu*, Sean C. Daugherty*, A. Scott Durkin*, Daniel H. Haft*, William C. Nelson*, Jeremy D. Peterson*, Mihai Pop*, Hoda M. Khouri*, Diana Radune*, Jonathan L. Benton*, Yasmin Mahamoud*, Lingxia Jiang*, Ioana R. Hance*, Janice F. Weidman*, Kristi J. Berry*, Roger D. Plaut*, Alex M. Wolf*, Kisha L. Watkins*, William C. Nierman*, Alyson Hazen*, Robin Cline*, Caroline Redmond†, Joanne E. Thwaite†, Owen White*, Steven L. Salzberg*{, Brendan Thomasonq, Arthur M. Friedlander**, Theresa M. Koehler††, Philip C. Hannaq, Anne-Brit Kolstø§# & Claire M. Fraser*

The authors propose that it is the tough, environmentally resistant endospore and ability to cause lethal inhalational anthrax that is responsible for the notoriety of Bacillicus anthracis as a candidate for biological warfare.

Inhalational anthrax occurs when resilient forms of the Bacillus anthracis bacteria called endospores are ingested by the respiratory system’s defensive alveolar macrophages. The endospores then germinate within the macrophage to form vegetative cells, which eventually escape and infect blood causing septicemia and toxemia.

Previous research has shown that expression of tripartite toxin and poly-D-glutamic acid are necessary for Bacillus anthracis to produce a full disease infection. The aim of this research was to identify any additional genes that may contribute to the virulence of Bacillus anthracis by sequencing the chromosome. It was postulated that the virulence of the pathogen might be influenced by its ability to evade the immune system and thus the extent of damage that it causes to its animal host, or its ability to encode the necessary functions to survive and escape the macrophage.

The Bacillus anthracis chromosome sequenced in this study was derived from an isolate from a Texan cow, and was shown to not differ significantly from that used in the 2001 attacks in Florida (differing in 11 single nucleotide polymorphisms).

METHODS

The chromosome was shown to have features in common with the Gram-positive genera Bacillus and Clostridium, including a bias for genes on the replication leading strand, and the concentration of ribosomal RNA, transfer RNA and ribosomal protein genes around the replication origin. The authors suggest that these arrangements may maximise protein synthesis during early rounds of DNA replication after germinating from endospores. The authors discovered homology between Bacillus anthracis proteins and those of Bacillus cerus, a closely related strain. Out of the 5508 proteins sequenced from the Bacillus anthracis, only 141 did not match proteins in the Bacills cerus ATCC 10987 sequence. This suggests that these potential virulence-enhancing genes are actually part of a common arrangement of the Bacillus cerus bacterium rather than associated with the specific pathogenicity of Bacillus anthracis. Homologues of genes involved with Bacillus cereus and Bacillus thuringiensis, and sequence homology to proteins associated with the virulence of Listeria monocytogenes were also elucidated in Bacillus anthracis. The authors suggest that the homology with listeria monocytogenes may imply similar pathways of the intercellular survival and multiplication of Listeria monocytegones, and the germination, survival, and ability to escape from macrophages of Bacillus anthracis. Additionally, it was found that B. anthracis has seven paralogues of the gerA tricistronic operon, which enables endospores to recognise certain molecules to initiate germination, a vital instigator in the infectious cycle of B. anthracis.

Bacillus anthracis was also shown to contain a gene that encodes for a homologue of the enhancin protein, a protein found in baculoviruses, which boosts virulence by degrading the mucin lining of insect guts. Other homologues virulent proteins that survive in mammals and insects were found in Bacillus anthracis, postulated to be evidence of an insect infecting ancestry.

Several similarities between Bacillus anthracis and Bacillus subtilis, a bacterium commonly found in soil, were discovered. These include homologues of DNA protection proteins (which are important for protecting the bacteria during dormancy) and the full array of DNA repair proteins, with additional repair capabilities that protect b. anthracis from UV-induced damage. The bacteria also share common machinery for sporulation (reproduction through producing and releasing spores).

The authors suggest that it is the differences and idiosyncrasies of B. anthracis that will elucidate the ecology of the bacteria and its particular virulence. There were several proteins found to be encoded by B. anthracis which mitigate damage from radicals and detoxify the bacterium, one of which (superoxide dismutase, SodC) has been shown to counteract nitric oxide-mediated killing in the macrophage, which therefore increases virulence. Differences occur in the constituent sequences for the composition of the endospores’ outer surface. B anthracis was found to have a greater number of peptide binding proteins, protrases and peptidases relative to B. subtilis, and a greater number of amino acid transporters as well as some amino acid utilisation genes yet undiscovered in other Bacillus genomes. The authors propose that this is suggestive of an increased capacity for amino acid and peptide utilisation, and may imply that B. anthracis is adapted to protein rich environments. It was discovered that B. anthracis has a greater array of iron-acquisition genes that may support scavenging in an animal host. It was demonstrated that B. anthracis has less of a capacity for sugar utilisation than B. subtilis, however it contains a complete operon for polyester biosynthesis, indicative of an alternative energy store.

Read et al (2003) also designed a DNA microarray for B. anthracis, which allowed comparative genome hybridization with 19 members of the B. cereus group. B anthracis was shown to have between 66-92% common chromosomal genes. PXO1 homologues were identified in half of the strains, however, the toxin genes were only found in B anthracis, and these are central to the anthrax infection.

From their investigations, the authors conclude that B anthracis is a soil-dwelling organism with numerous genes with virulent potential. They propose that these factors combined with the bacterium’s preference for protein rich environments suggest that it has evolved from B cereus ancestor, acquiring plasmid-encoding toxin, capsule and regulatory loci. It is proposed that the major differences between B cereus and B anthracis may be due to altered gene expression, as the function of the shared genes associated with proteases, motility, tyrosine degredation, penicillin resistance, haemolysins and extracellular chitinases differ between the bacterium. It is concluded that this adaptation is likely to be due to the acquisition of the lethal toxin on PXO1.

These findings present further questions as to how the virulence genes in B anthracis interact to produce anthrax, and additionally how proteins in B anthracis mediate interactions between the bacterium and its environment. Further understanding of these aspects could provide information as to targets for drug or vaccine design.

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