The Bacillus subtilis (B. subtilis) group is a ubiquitous Gram-positive bacteria with a remarkable adaptability potential that enables it to survive in highly diverse environments.
The B. subtilis group consists of four species, including Bacillus subtilis, Bacillus pumilus, Bacillus licheniformis, and Bacillus amyloliquefaciens, discovered several decades ago. Over time, numerous new species and subspecies have been described based on molecular evolution, physiology, and chemotaxonomic characterization [3][1]. B. subtilis is a model organism used to investigate cell motility, biofilm formation, protein secretion, cell division, secondary metabolites biosynthesis, adherence to plant root and fungal hyphae, cytoplasm altercation via intracellular nanotubes, and kin-recognition [4][2]. In biotechnological industries, B. subtilis is a popular workhorse used for the biosynthesis of a broad range natural products, from enzymes to purified bioactive compounds [5,6][3][4]. Its natural competence to genetic engineering and well-described gene expression system makes it attractive on many occasions [6][4]. Moreover, recently, it also earned attention as a biocontrol agent in agronomy by antagonizing phytopathogens and promoting plant growth [7][5].
The impressive skill set of B. subtilis for producing diverse bioactive metabolites was recognized in the last decade. It has been demonstrated that ~5% of a wild-type B. subtilis genome is exclusively devoted to the synthesis of bioactive compounds [8][6]. For a long time, it was considered for the production only of cyclic peptides such as iturins, surfactins, and fengycins [9,10][7][8]. Nonetheless, due to the discovery of numerous antimicrobial exhibiting linear lipopeptides, PKs, and volatile metabolites, it has gained a high commercial interest. The versatile bioactive metabolites produced by the B. subtilis group may be classified based on several criteria, including their biosynthetic pathways, function, structure, source, physicochemical properties, molecular targets, or bonding patterns [11][9].
Gageopeptides
Recently, several linear lipopeptides have been reported and characterized from B. Subtilis. For instance, Gageostatin was reported from a marine-derived B. subtilis [39][30]. Gageostatin consists of 3-beta hydroxyl fatty acid attached to heptapeptide. It is composed of the same residues as reported for surfactin. However, differences were found in their structures and molecular masses. Gageostatins were found in linear form with exclusively L-leucine, while surfactins are cyclic lipopeptides with L and D-leucine.
Siderophores
Siderophores are small molecules having a high affinity toward ferric iron. Besides iron scavenging, they are also used to form stable complexes with environmentally important metals. Based on their chemical moiety, siderophore can be categorized into three types, i.e., hydroxamate, catecholate, and carboxylate siderophores. Most of the siderophores produced by bacteria are catecholate, and few are carboxylate and hydroxamate [41][31]. Bacillibactin is a well-known catecholate siderophore produced by different B. subtilis strains that exhibits strong antibacterial properties and moderate cytotoxicity [42][32]. Bacillus spp. are widely studied for the synthesis of bioactive metabolites. However, their siderophore-producing capabilities have not yet been much explored. Previously, a siderophore-producing Bacillus spp. was reported that enhanced the bioremediation of metals and increased plant growth. B. subtilis strain CAS15 was isolated from the rhizosphere and identified as producing siderophore, and it also inhibits the growth of phytopathogens [43][33].
Difficidins
Difficidins are unsaturated macrocyclic polyene synthesized by the type 1 PKS. Oxydifficidin is an oxidative form of difficidin having an additional hydroxyl group at position 5 [67]. It is encoded by the
operon that has 14 open reading frames. Several KR, DH, and ER domains are missing within the
operon and deviate from the colinearity rule. Moreover, the function of
and
are unknown, and their activities do not appear in the final product [68]. Difficidin has broad-spectrum antibacterial activity and inhibits the biosynthesis of protein in
Difficidins
Hybrid metabolites are the products of biosynthetic pathways that comprise both (NRPS/PKS) types of modular enzymes. Questions relating to the synthesis of hybrid products are of great present-day interest, as their answer concerns genetic engineering efforts. Both depend on thio-template for acyl chain elongation and monomers triggering.
Macrolactins
Bacillaene has a linear structure and was first reported from B. subtilis strains 55,422 and 3610 [128][77]. It is encoded by a hybrid PKS-NRPS biosynthetic gene cluster known as bacillaene PksX synthase (Figure 64). The pksX mega gene cluster in B. subtilis 168 genome consisted of 5 open reading frames named pksJ, pksL, pksM, pksN, and pksR [104,129][68][78]. The first two adenylation domains of pksJ incorporate glycin and α-hydroxy-isocaproic acid. The third adenylation domain (pksN) is responsible for the incorporation of alanine.