Bacteria expression are the most widely used protein expression systems for their rapid growth, high yield, ease of manipulation and scale-up. The most widely used bacteria host is of course Escherichia coli. E. coli is the most established tool and is almost always the first choice if applicable.

Another bacterium, Bacillus subtilis, is an increasingly popular protein expression host. It is a FDA designated Generally Regarded As Safe (GRAS) organism. B.subtilis is a Gram-positive bacterium, which makes this system free of endotoxin concern. It also has much less codon bias than E. coli. Moreover, it secretes protein directly into the culture media, which highly simplifies subsequent protein purification steps. All the above features make it an attractive system for the production of soluble and properly folded recombinant proteins at both laboratory and industrial levels.