On the origin and evolution of Bacillus anthracis

Abstract

The paper presents data on the monitoring of the origin and evolution of B. anthracis, which show that the microbe became virulent for humans and animals through the initial transformation into B. cereus group containing a number of closely related species of many spore forming microorganisms inhabiting soil. This was followed by the divergence of B. anthracis from the rest of B. cereus group as a result of obtaining virulence factors such as plasmids рХО1 and рХО2, which determine synthesis of the main virulence factors — the toxin and the capsule. The evolution of Bac. anthracis and the disease caused by the pathogen occurs at the present time as well, possibly even in a reversed direction, as suggested by multiple observations on the circulation of capsule devoid, avirulent forms in animals and in the environment. The new stage in the evolution of Bac. anthracis has started in conjunction with the mass vaccination of animals against anthrax with spore vaccines. In these conditions of vaccination and the presence of active immunity the anthrax bacillus is incapable of infecting an animal, subsequent multiplication, passage to the environment and conversion to the spore form. According to several authors, vegetation of the anthrax microbe in the environmental conditions different from a living organism where the reproduction occurs leads to the loss of virulent properties because they are not required to live in the soil. Other mechanisms of Bac. anthracis evolution cannot be excluded when it resides in the soil, especially at old burial sites where the anthrax bacillus can vegetate during the warm season and to be influenced by action of bacteriophages in the form of genetic transduction, transformation and conjugation. At present, the «anthrax-like» diseases of animals and humans caused by virulent strains of Bac. cereus and Bac. thuringiensis are being registered at increased rate. Diagnosing infectious diseases with clinical and gross-pathological findings of anthrax it is necessary to account the possibility of detection of unusual strains of Bac. anthracis or other bacilli. The correct diagnosis of anthrax can be made only with a complex approach including bacteriological and serological examination, biological assays in laboratory animals, and, essentially, molecular-genetic methods