Enhancement of bacitracin biosynthesis by branched-chain amino acids in a regulatory mutant of Bacillus licheniformis

Optimization of enhanced microbial production of zinc bacitracin by submerged fermentation technology

Bacitracin is one of the most important antibiotics used in different biomedical fields. It helps to achieve sizeable amount of foreign exchange due to its use in the poultry feed. The cheap agricultural wastes are readily available for the preparation of fermentation media used for bacitracin production. The microorganisms could be mutated with different chemicals and UV radiation to improve bacitracin production. Thus, the current study was focused on the synthesis of low-cost and effective bacitracin by mutant strains of Bacillus licheniformis, employing the submerged fermentation technique. The bacteria were exposed to the UV irradiation for various time periods ranging from 5 to 40 min. These mutants were named as BLAA-5-BLAA-40. Mutant strain BLAA-25 produced maximum bacitracin, with significantly high activity (142.81 IU/mg) against Klebsiella pneumoniae but less activity against Escherichia coli (115.19 IU/mg). Several fermentation conditions were investigated to optimize bacitracin production. The highest bacitracin yield was obtained by an inoculum size of 10%, fermentation period 48 hr, pH 7, T = 37°C, using soybean meal as a substrate. Among all substrates, cucumber peel was the substrate showing the highest minimum inhibitory concentration (2.3 mg/ml and 2.7 mg/ml against K. pneumoniae and E. coli respectively). A comparison between commercial and experimentally produced Zn bacitracin showed that commercial bacitracin has a low activity (63.2 IU/mg) as compared with experimental bacitracin. Hence, the agro wastes and mutation could be used to increase the synthesis of Zn bacitracin in B. licheniformis.

Increased flux through the TCA cycle enhances bacitracin production by Bacillus licheniformis DW2

The dodecapeptide antibiotic bacitracin, produced by several strains of Bacillus licheniformis and Bacillus subtilis, is widely used as an antibacterial animal feed additive. Several genetic strategies were explored to enhance its production. The availability of building block amino acids for bacitracin production was found to play an important role in its synthesis. In this study, the TCA cycle in the industrial strain B. licheniformis DW2 was strengthened by overexpression of the key enzymes citrate synthase and isocitrate dehydrogenase (ICDH). As the central metabolic pathway, the TCA cycle is a major source for energy supply and intermediates for anabolism. By enhancing flux through the TCA cycle, more energy and precursors were generated for amino acid biosynthesis and uptake, resulting in enlarged intracellular pool of bacitracin-containing amino acids for bacitracin production. This study unveiled the metabolic responses of the increased TCA cycle flux in B. licheniformis and provided a novel strategy for enhancing bacitracin production.

Optimization of Inexpensive Agricultural By-Products as Raw Materials for Bacitracin Production in Bacillus licheniformis DW2

Bacitracin is a broad-spectrum antibiotic used extensively as a feed additive. In this study, inexpensive agricultural by-products were used as nitrogen sources for bacitracin production. Based on both the orthogonal tests, a combination of 7% soybean meal (SBM) +2% low protein rapeseed cake (LPRC) was optimal for bacitracin production. Compared to the original formula, the titer of bacitracin increased by 20.5% reaching 910.4 U/ml in flasks. The titer of bacitracin and the ratio of bacitracin A increased by 12.4 and 6.8% in a 50-l fermentor. Furthermore, this study also explored the effects of exogenously adding different amino acids on the yield of bacitracin. The addition of Cys and Glu enhanced bacitracin production by 5.7 and 5.0%, respectively. This study provided the inexpensive nutrient inputs into efficient bacitracin production and also the insight to further research enabling better utilization of oil cakes for economic viability of the bioprocess industry.

Systematic mutagenesis method for enhanced production of bacitracin by Bacillus licheniformis Mutant Strain UV-MN-HN-6

The purpose of the current study was intended to obtain the enhanced production of bacitracin by Bacillus licheniformis through random mutagenesis and optimization of various parameters. Several isolates of Bacillus licheniformis were isolated from local habitat and isolate designated as GP-35 produced maximum bacitracin production (14±0.72 IU ml^-1). Bacitracin production of Bacillus licheniformis GP-35 was increased to 23±0.69 IU ml^-1 after treatment with ultraviolet (UV) radiations. Similarly, treatment of vegetative cells of GP-35 with chemicals like N-methyl N’-nitro N-nitroso guanidine (MNNG) and Nitrous acid (HNO₂) increased the bacitracin production to a level of 31±1.35 IU ml^-1 and 27±0.89 IU ml^-1respectively. Treatment of isolate GP-35 with combined effect of UV and chemical treatment yield significantly higher titers of bacitracin with maximum bacitracin production of 41.6±0.92 IU ml^-1. Production of bacitracin was further enhanced (59.1±1.35 IU ml^-1) by optimization of different parameters like phosphate sources, organic acids as well as temperature and pH. An increase of 4.22 fold in the production of bacitracin after mutagenesis and optimization of various parameters was achieved in comparison to wild type. Mutant strain was highly stable and produced consistent yield of bacitracin even after 15 generations. On the basis of kinetic variables, notably Y_p/s (IU/g substrate), Y_p/x (IU/g cells), Y_x/s(g/g), Y_p/s, mutant strain B. licheniformis UV-MN-HN-6 was found to be a hyperproducer of bacitracin.

From natural products discovery to commercialization: a success story

In order for a natural product to become a commercial reality, laboratory improvement of its production process is a necessity since titers produced by wild strains could never compete with the power of synthetic chemistry. Strain improvement by mutagenesis has been a major success. It has mainly been carried out by "brute force" screening or selection, but modern genetic technologies have entered the scene in recent years. For every new strain developed genetically, there is further opportunity to raise titers by medium modifications. Of major interest has been the nutritional control by induction, as well as inhibition and repression by sources of carbon, nitrogen, phosphate and end products. Both strain improvement and nutritional modification contribute to the new process, which is then scaled up by biochemical engineers into pilot scale and later into factory size fermentors.

Achievements in microbial technology

In 1973, recombinant DNA technology was born and the age of the "new biotechnology" came upon us. Today we are seeing the amazing results of recombinant DNA technology, hybridoma technology, enzyme engineering and protein engineering. These techniques are exerting major effects on basic research and on health care, diagnostics and agriculture and soon will bring about changes in other industries such as petroleum, mining, foods and chemicals. Entire pathways of primary and secondary metabolism have been cloned and expressed in foreign microorganisms. The development of recombinant DNA technology is having its major impact on the production of rare polypeptides such as mammalian enzymes, hormones, antibodies and biological response modifiers. In addition to natural polypeptides, analogs are being produced by recombinant DNA technology and this has added an extra dimension of excitement to the field. The future is thus insured for the expanded use of microorganisms in the biotechnological world and the continued improvement in microbial processes to reduce the cost of drugs, enzymes and specialty chemicals.

Structure and function of branched chain aminotransferases

Branched chain aminotransferases (BCATs) catalyze transamination of the branched chain amino acids (BCAAs) leucine, isoleucine, and valine. Except for the Escherichia coli and Salmonella proteins, which are homohexamers arranged as a double trimer, the BCATs are homodimers. Structurally, the BCATs belong to the fold type IV class of pyridoxal phosphate (PLP) enzymes. Other members are D-alanine aminotransferase and 4-amino-4-deoxychorismate lyase. Catalysis is on the re face of the PLP cofactor, whereas in other classes, catalysis occurs from the si face of PLP. Crystal structures of the fold type IV proteins show that they are distinct from the fold type I aspartate aminotransferase family and represent a new protein fold. Because the fold type IV enzymes catalyze diverse reactions, it is not surprising that the greatest structural similarities involve residues that participate in PLP binding rather than residues involved in substrate binding. The BCATs are widely distributed in the bacterial kingdom, where they are involved in the synthesis/degradation of the BCAAs. Bacteria contain a single BCAT. In eukaryotes there are two isozymes, one is mitochondrial (BCATm) and the other is cytosolic (BCATc). In mammals, BCATm is in most tissues, and BCATm is thought to be important in body nitrogen metabolism. BCATc is largely restricted to the central nervous system (CNS). Recently, BCATc has been recognized as a target of the neuroactive drug gabapentin. BCATc is involved in excitatory neurotransmitter glutamate synthesis in the CNS. Ongoing structural studies of the BCATs may facilitate the design of therapeutic compounds to treat neurodegenerative disorders involving disturbances of the glutamatergic system.

Peptides

If we include beta-lactam antibiotics on the grounds that they have the same biosynthetic origin, peptides remain commercially the most important group of pharmaceuticals. However, our increasing knowledge of the genetic and enzymic background to biosynthesis, and of the regulation of metabolite production, will eventually bring a more unified approach to bioactive compounds. Mixing of structural types will become important, and we will be able to use our knowledge of biosynthetic genes and their regulatory networks. We will also benefit from an appreciation of the modular organization of catalytic functions, substrate transfer mechanisms and signalling between interacting enzymes. Since all of this is, in fact, the basis for enzymic synthesis of complex natural products in vivo, the exploitation of living cells requires mastery of a formidable network of cellular controls and compartments. For the present we are able to see fascinating connections emerging between genes in a variety of reaction sequences, not only in biosynthetic but also in degradative pathways. Peptide synthetases show surprising similarities to acylcoenzyme A synthetases, which are key enzymes in forming polyketides as well as in generating the CoA-derivatives that serve as substrates in degradative pathways. 4'-Phosphopantetheine, the functional half of CoA, plays a key role as the intrinsic transfer cofactor in various multienzyme systems. The comparatively small catalogue of reactions modifying natural products, notably epimerization, methylation, hydroxylation, decarboxylation (of peptides) and reduction/dehydration (of polyketides) can be found within or amongst biosynthetic proteins, generally as modules and organized in a specified order. The biochemist is coming close to the synthetic chemist's recipes, and may soon be recruiting proteins to carry them out.