Bacitracin

Bacitracin and colistin are polypeptide antibacterial agents that are used as feedstuff additives to prevent and treat infections and to promote growth in modern animal husbandry. Both compounds consist mainly of amino acids and several different components, depending on the type and combination of amino acids. Bacitracin A (Fig. 1a) is the major component of bacitracin and has potent therapeutic activity towards Grams-positive cocci and bacilli. Colistin was often used for treating diseases caused by Gram-negative bacteria in the 1960s but it was then found to exhibit high toxicity and hence had limited applications in human. Some recent medical studies, on the other hand, reported that colistin was effective against multi-resistant Gram-negative bacteria, in particular Pseudomonas aeruginosa and was proven to be a potential antibiotic for treating multi-resistant strains in clinical settings. Colistin A and colistin B (Fig. 1b) are the major components among the 30 elucidated congeners; they account for more than 85% of the total colistins used in pharmaceutical products. Like other antibiotics, residual bacitracin and colistin in milk and edible animal tissues might lead to the allergies and emergence of resistant microbes. As a consequence, national health authorities have established legal guidance to ensure proper use of bactracin and colistin and other controlled veterinary antibiotics.

Bacitracin, a metal-dependent dodecapeptide produced by Bacillus subtilis and Bacillus licheniformis (Toscano and Storm 1982), was first discovered in 1943 from a bacterial culture isolated from a wound of a 7-year-old American girl (Ming and Epperson 2002). Commercial bacitracin preparations are comprised of a mixture of many closely related analogues, named bacitracin A–I, of which bacitracin A represents the major component with the highest activity (Konigsberg and Craig 1962; Hirotsu et al. 1978). It is also known that bacitracins A and B account for 95% of the biological activity of the mixtures (Tsuji and Robertson 1975). Bacitracin A, a dodecapeptide, contains a heptapeptide cyclic structure and a thiazoline ring formed between the L-cysteine and L-isoleucine located at the N-terminal end of the acylic peptide side chain (Konz et al. 1997). Bacitracin has activity primarily against grampositive cocci and bacilli, including Staphylococcus, Streptococcus and Clostridium difficile, as well as some archaea such as Methanobacterium, Mathanococcus and Halococcus (Storm 1974). It has a medical application in the treatment of gastrointestinal infections, such as antibiotic-associated colitis and diarrhea caused by C.difficile (Chang et al. 1980). It is widely utilized as an animal feed supplement to improve animal body weight and to prevent diseases in agriculture (Nagaraja and Chengappa 1998; Abdulrahim et al. 1999). Consequently, bacitracin is important to the pharmaceutical and livestock industries throughout the world.

Bacillus licheniformis strains produce various peptide antibiotics, e.g., bacitracins, licheniformin, lichenysins, and amoebicins. Bacitracin consists of a group of closely related peptides and bacitracin A is a main component of the peptides. Bacitracins are produced by certain strains of B. licheniformis and B. subtilis, and are synthesized by a multifunctional enzyme system composed of bacitracin synthetases 1, 2, and 3 which are encoded by bacitracin synthetase genes bacA, bacB, and bacC, respectively. Almost all biochemical and molecular biological studies on bacitracin production in B. licheniformis have been carried out with strain ATCC 10716. Cloning and DNA sequencing of the entire bacitracin synthetase gene cluster from this strain have been carried out. We found no reports on distribution of bacitracin synthetase gene sequences among strains of B. licheniformis. Therefore, as the first step in studies on strain specificity of peptide antibiotic production in Bacillus strains, we investigated distribution of bacitracin syn- thetase gene sequences in laboratory stock strains of B. licheniformis by genomic Southern blot hybridization.