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The survival of Bacillus anthracis: How does this bacterium survive for decades in extreme environments?
Bacillus anthracis, a gram-positive bacillus, is the cause of anthrax, a deadly disease that is fatal to livestock and occasionally infects humans. This is the only absolutely pathogenic bacterium in the genus Bacillus, and its infection is zoonotic, transmitted from animals to humans. The bacterium was first discovered by German physician Robert Koch in 1876 and became the first bacterium to be experimentally verified as a pathogen, and his research provided important scientific evidence for the pathogen theory.
This bacterium has demonstrated an amazing ability to survive in extreme environments, even staying put for decades under adverse conditions. What exactly makes it so resilient?
B. anthracis is about 3 to 5 microns long and 1 to 1.2 microns wide, and often forms long chains after cultivation. On the culture medium, white or cream-colored colonies several millimeters wide will form. Most strains of B. anthracis produce a protective coating called a capsule that further enhances their ability to evade immune responses. The capsule is made of poly-D-gamma-glutamic acid, which allows the bacteria to hide from the host's immune system.
B. anthracis feeds on hemoglobin in the blood and uses two secreted glycoproteins, IsdX1 and IsdX2, to acquire iron. Not only can they strip the blood matrix from hemoglobin, but they can also bring iron into the cell through cell surface proteins. Once in a suitable environment, the endospores of B. anthracis will immediately activate and begin to grow, which makes this bacterium capable of persisting in nature.
The key to survival: endospores
The endospores of B. anthracis are key to its survival and have a special structure, including a thick cell wall and multiple layers of membranes. These structures allow endospores to resist heat, desiccation, and many disinfectants, and to remain viable even after decades or centuries of extreme environments.
Studies have shown that endospores of B. anthracis can survive extreme temperatures and low-nutrient environments, making them potential bioweapons.
Gene structure and pathogenicity
B. anthracis has a circular chromosome of approximately 5,227,293 bp in length and two exogenous double-stranded DNA plasmids, pXO1 and pXO2, which are key factors in its pathogenicity. In particular, the pXO1 plasmid contains genes related to anthrax toxin, and the expression of these genes is regulated by the carrier protein.
Infection and clinical manifestations
Untreated B. anthracis infection is often fatal, and symptoms of infection vary depending on the route of entry. Cutaneous anthrax is the most common, accounting for about 95% of cases, and eventually forms localized black necrotic lesions at the site of infection. Inhalation anthrax is extremely deadly and often causes cold-like symptoms followed by serious respiratory problems.
Although the anthrax vaccine was developed as early as 1881 by French chemist Louis Pasteur, there are still several vaccines available today. In the treatment of infection, commonly used antibiotics such as penicillin and fluoroquinolones can show good efficacy.
Evolving Survival Techniques
Through whole genome sequencing, the evolutionary history of B. anthracis and its relationship with other bacterial species have been more clearly revealed. The genome of B. anthracis is very consistent, with relatively few mutations, which also makes it evolve relatively slowly. Such characteristics enable B. anthracis to effectively adjust its survival strategy when facing environmental challenges.
The interactions between cells of B. anthracis and the immune system are extremely complex and demonstrate the ability of the bacterium to modulate the host immune response. These survival strategies undoubtedly enable it to show strong resilience and adaptability in biology.
With the advancement of science, our understanding of B. anthracis has continued to deepen, but this bacterium still poses a major threat. From ancient times to the present, do we truly understand its way of survival?
