Bacillus anthracis is the etiologic agent of anthrax — a common disease of livestock
and, occasionally, of humans — and the only obligate pathogen within the genus Bacillus.
B. anthracis is a Gram-positive, endospore-forming, rod-shaped bacterium, with a width of 1.0–1.2
µm and a length of 3–5 µm.
It can be grown in an ordinary nutrient medium under aerobic or anaerobic conditions.
It is one of few bacteria known to synthesize a protein capsule.
Like Bordetella pertussis, it forms a calmodulin-dependent adenylate cyclase exotoxin known as, along
with lethal factor.
It bears close genotypical and phenotypical resemblance to Bacillus cereus and Bacillus
thuringiensis.
All three species share cellular dimensions and morphology.
All form oval spores located centrally in an unswollen sporangium.
B. anthracis spores, in particular, are highly resilient, surviving extremes of temperature,
low-nutrient environments, and harsh chemical treatment over decades or centuries.
The spore is a dehydrated cell with thick walls and additional layers that form inside
the cell membrane.
It can remain inactive for many years, but if it comes into a favorable environment,
it begins to grow again.
It is sometimes called an endospore because it initially develops inside the rod-shaped
form.
Features such as the location within the rod, the size and shape of the endospore, and whether
or not it causes the wall of the rod to bulge out are characteristic of particular species
of Bacillus.
Depending upon the species, the endospores are round, oval, or occasionally cylindrical.
They are highly refractile and contain dipicolinic acid.
Electron micrograph sections show they have a thin outer spore coat, a thick spore cortex,
and an inner spore membrane surrounding the spore contents.
The spores resist heat, drying, and many disinfectants.
Because of these attributes, B. anthracis spores are extraordinarily well-suited to
use as biological weapons.
Such weaponization has been accomplished in the past by at least five state bioweapons
programs — those of the United Kingdom, Japan, the United States, Russia, and Iraq
— and has been attempted by several others.
Historical background
French physician Casimir Davaine demonstrated the symptoms of anthrax were invariably accompanied
by the microbe B. anthracis.
German physician Aloys Pollender is also credited for this discovery.
B. anthracis was the first bacterium conclusively demonstrated to cause disease, by Robert Koch
in 1876.
The species name anthracis is from the Greek anthrax, meaning "coal" and referring to the
most common form of the disease, cutaneous anthrax, in which large, black skin lesions
are formed.
Genome structure B. anthracis has a single chromosome which
is a circular, 5,227,293-bp DNA molecule.
It also has two circular, extrachromosomal, double-stranded DNA plasmids, pXO1 and pXO2.
Both the pXO1 and pXO2 plasmids are required for full virulence and represent two distinct
plasmid families.
pXO1 plasmid The pXO1 plasmid contains the genes that encode
for the anthrax toxin components: pag, lef, and cya.
These factors are contained within a 44.8-kb pathogenicity island.
The lethal toxin is a combination of PA with LF and the edema toxin is a combination of
PA with EF.
The PAI also contains genes which encode a transcriptional activator AtxA and the repressor
PagR, both of which regulate the expression of the anthrax toxin genes.
pXO2 plasmid pXO2 encodes a five-gene operon which synthesizes
a poly-γ-D-glutamic acid capsule.
This capsule allows B. anthracis to evade the host immune system by protecting itself
from phagocytosis.
Expression of the capsule operon is activated by the transcriptional regulators AcpA and
AcpB, located in the pXO2 pathogenicity island.
Interestingly, AcpA and AcpB expression are under the control of AtxA from pXO1.
Strains The 89 known strains of B. anthracis include:
Sterne strain, used by Max Sterne in his 1930s vaccines
Vollum strain, formerly weaponized by the US, UK, and Iraq; isolated from cow in Oxfordshire,
UK, in 1935 Vollum M-36, virulent British research strain;
passed through macaques 36 times Vollum 1B, weaponized by the US and UK in
the 1940s-60s Vollum-14578, UK biotesting contaminated Gruinard
Island, Scotland, in 1940s V770-NP1-R, the avirulent, nonencapsulated
strain used in the BioThrax vaccine
Anthrax 836, highly virulent strain weaponized by the USSR; discovered in Kirov in 1953
Ames strain, isolated from a cow in Texas in 1981; famously used in AMERITHRAX letter
attacks Ames Ancestor
Ames Florida
H9401, isolated from human patient in Korea; used in investigational anthrax vaccines
Evolution Whole genome sequencing has made reconstruction
of the B. anthracis phylogeny extremely accurate.
A contributing factor to the reconstruction is B. anthracis being monomorphic, meaning
it has low genetic diversity, including the absence of any measurable lateral DNA transfer
since its derivation as a species.
The lack of diversity is due to a short evolutionary history that has precluded mutational saturation
in single nucleotide polymorphisms.
A short evolutionary time does not necessarily mean a short chronological time.
When DNA is replicated, mistakes occur which become genetic mutations.
The buildup of these mutations over time leads to the evolution of a species.
During the B. anthracis lifecycle, it spends a significant amount of time in the soil spore
reservoir stage, a stage in which DNA replication does not occur.
These prolonged periods of dormancy have greatly reduced the evolutionary rate of the organism.
Nearest neighbors B. anthracis belongs to the B. cereus group
consisting of the strains: B. cereus, B. anthracis, B. thuringiensis, B. weihenstephanensis, B.
mycoides, and B. pseudomycoides.
The first three strains are pathogenic or opportunistic to insects or mammals, while
the last three are not considered pathogenic.
The strains of this group are genetically and phenotypically heterogeneous overall,
but some of the strains are more closely related and phylogenetically intermixed at the chromosome
level.
The B. cereus group generally exhibits complex genomes and most carry varying numbers of
plasmids.
B. cereus is a soil-dwelling bacterium which can colonize the gut of invertebrates as a
symbiont and is a frequent cause of food poisoning It produces an emetic toxin, enterotoxins,
and other virulence factors.
The enterotoxins and virulence factors are encoded on the chromosome, while the emetic
toxin is encoded on a 270-kb plasmid, pCER270.
B. thuringiensis is an insect pathogen and is characterized by production of parasporal
crystals of insecticidal toxins Cry and Cyt.
The genes encoding these proteins are commonly located on plasmids which can be lost from
the organism, making it indistinguishable from B. cereus.
Pseudogene PlcR is a global transcriptional regulator
which controls most of the secreted virulence factors in B. cereus and B. thuringiensis.
It is chromosomally encoded and is ubiquitous throughout the cell.
In B. anthracis, however, the plcR gene contains a single base change at position 640, a nonsense
mutation, which creates a dysfunctional protein.
While 1% of the B. cereus group carries an inactivated plcR gene, none of them carries
the specific mutation found only in B. anthracis.
The plcR gene is part of a two-gene operon with papR.
The papR gene encodes a small protein which is secreted from the cell and the reimported
as a processed heptapeptide forming a quorum-sensing system.
The lack of PlcR in B. anthracis is a principle characteristic differentiating it from other
members of the B. cereus group.
While B. cereus and B. thuringiensis depend on the plcR gene for expression of their virulence
factors, B. anthracis relies on the pXO1 and pXO2 plasmids for its virulence.
Clinical aspects
Pathogenesis B. anthracis possesses an antiphagocytic capsule
essential for full virulence.
The organism also produces three plasmid-coded exotoxins: edema factor, a calmodulin-dependent
adenylate cyclase, causes elevation of intracellular cAMP, and is responsible for the severe edema
usually seen in B. anthracis infections; lethal toxin is responsible for tissue necrosis;
protective antigen mediates cell entry of edema factor and lethal toxin.
Manifestations in human disease Three forms of human anthrax disease are recognized
based on their portal of entry.
Cutaneous, the most common form, causes a localized, inflammatory, black, necrotic lesion.
Pulmonary, the highly fatal form, is characterized by sudden, massive chest edema followed by
cardiovascular shock.
Gastrointestinal, a rare but also fatal type, results from ingestion of spores.
Prevention and treatment A number of anthrax vaccines have been developed
for preventive use in livestock and humans.
Infections with B. anthracis can be treated with β-lactam antibiotics such as penicillin,
and others which are active against Gram-positive bacteria.
Penicillin-resistant B. anthracis can be treated with fluoroquinolones such as ciprofloxacin
or tetracycline antibiotics such as doxycycline.
Laboratory research Components of tea, such as polyphenols, have
the ability to inhibit the activity both of B. anthracis and its toxin considerably; spores,
however, are not affected.
The addition of milk to the tea completely inhibits its antibacterial activity against
anthrax.
Activity against the B. athracis in the laboratory does not prove that drinking tea affects the
course of an infection, since it is unknown how these polyphenols are absorbed and distributed
within the body.
Recent research Advances in genotyping methods have led to
improved genetic analysis for variation and relatedness.
These methods include multiple-locus variable-number tandem repeat analysis and typing systems
using canonical single-nucleotide polymorphisms.
The Ames ancestor chromosome was sequenced in 2003 and contributes to the identification
of genes involved in the virulence of B. anthracis.
Recently, B. anthracis isolate H9401 was isolated from a Korean patient suffering from gastrointestinal
anthrax.
The goal of the Republic of Korea is to use this strain as a challenge strain to develop
a recombinant vaccine against anthrax.
The H9401 strain isolated in the Republic of Korea was sequenced using 454 GS-FLX technology
and analyzed using several bioinformatics tools to align, annotate, and compare H9401
to other B. anthracis strains.
The sequencing coverage level suggests a molecular ratio of pXO1:pXO2:chromosome as 3:2:1 which
is identical to the Ames Florida and Ames Ancestor strains.
H9401 has 99.679% sequence homology with Ames Ancestor with an amino acid sequence homology
of 99.870%.
H9401 has a circular chromosome, the pXO1 plasmid, and the pXO2 plasmid.
As compared to the Ames Ancestor chromosome above, the H9401 chromosome is about 8.5 kb
smaller.
Due to the high pathogenecity and sequence similarity to the Ames Ancestor, H9401 will
be used as a reference for testing the efficacy of candidate anthrax vaccines by the Repbulic
of Korea.
Host interactions As with most other pathogenic bacteria, B.
anthracis must acquire iron to grow and proliferate in its host environment.
The most readily available iron sources for pathogenic bacteria are the heme groups used
by the host in the transport of oxygen.
To scavenge heme from host hemoglobin and myoglobin, B. anthracis uses two secretory
siderophore proteins, IsdX1 and IsdX2.
These proteins can separate heme from hemoglobin, allowing surface proteins of B. anthracis
to transport it into the cell.
References
External links Bacillus anthracis genomes and related information
at PATRIC, a Bioinformatics Resource Center funded by NIAID
Hazards in Animal Research Database - Bacillus anthracis
Pathema-Bacillus Resource