Various expression systems have been developed today and they are very well established commercially, especially for obtaining recombinant proteins. The expression systems utilized include mammalian and insect cultures, Escherichia coli and bacteria. Expression in bacillus is the prominent system that is being used. Ferdinand Cohn was the first to describe the genus Bacillus in 1872 and they include a large number of gram-positive bacterial species like Bacillus subtilis, Bacillus anthracis, Bacillus megaterium and Bacillus.
Bacillus subtilis is a gram-positive bacterium commonly found in soil and it contains only a single membrane making it an ideal framework for secretion of organic molecules. Bacillus subtilis is an attractive host for the production of protein because it has the capability of secreting extracellular enzymes directly into the culture medium. It also has a large excretory capacity. Bacillus subtilis has been used to improve the quality and quantity of the secreted foreign proteins such as interferon, growth hormone, pepsinogen and epidermal growth factor. However, B. subtilis produces and secretes high levels of extracellular proteases which degrade the secreted foreign proteins. The bacillus also lacks well-regulated inducible vectors which limits the wide application of the B. subtilis system.
Bacillus anthracis is a gram-positive spore forming bacteria that dwells in the soil. Upon entry into a human host, it can rapidly proliferate and cause anthrax, a disease involving tovemia and septicemia. An example of Bacillus anthracis’s negative effects was its potential use in biological warfare as demonstrated in the U.S. postal system in 2001. The gene specifying capsule and the toxic factors responsible for anthrax are located on two plasmids, pXO1 and pXO2 and the transcription of these genes is activated by the regulator AtxA during vegetative multiplication. Studies of Bacillus anthracis have centered primarily on the expression of genes associated with most established virulence factor, anthrax toxin composed of protective antigens (PA). The protective antigen of Bacillus anthracis is the major protective immunogen in the current human vaccine against anthrax.
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Bacillus megaterium is one of the largest bacilli found in soil. It is found in many ecological niches because it grows in a wide variety of carbon supplies. The B. megaterium expression system provides a flexible and easy-to-handle tool for stable and high-yield protein production. This is because of several reasons; first, B. megaterium does not possess alkaline proteases enabling it to be a good cloning and expression of foreign proteins without degradation. Secondly, the bacterium readily secretes proteins into the growth medium and thirdly, no endotoxins are found in the cell wall. It produces diverse enzymes, such as amylase used in bread industries and penicillin amidase used for making antibiotics.
Bacillus brevis has been successfully used to produce heterologous proteins (proteins that differ in structure). Not much study has been done about bacillus brevis but it is known to produce soluble proteins, which are insoluble when produced by E. coli system. It is also a safe host that is easy to culture and sterilize. The main disadvantage that limits its use is the low yields of protein.