Abstract
Bacillus velezensis ML122-2 is an antimicrobial-producing strain isolated from the leaf of Assam tea or Miang [Camellia sinensis var. assamica (J.W.Mast.) Kitam.]. The cell-free supernatant (CFS) of strain ML122-2 exhibits a broad-spectrum antimicrobial activity against various Gram-positive and Gram-negative bacteria as well as the mold Penicillium expansum. The genome of B. velezensis ML122-2 was sequenced and in silico analysis identified three potential bacteriocin-associated gene clusters, that is, those involved in the production of mersacidin, amylocyclicin, and LCI. Furthermore, six gene clusters exhibiting homology (75–100% DNA sequence identity) to those associated with the secondary metabolites bacilysin, bacillibactin, surfactin, macrolactin H, bacillaene, and plipastatin were identified. Individual antimicrobial activities produced by B. velezensis ML122-2 were purified and characterized by Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry analysis, revealing three antimicrobial peptides with molecular masses corresponding to surfactin, plipastatin, and amylocyclicin. Transcriptional analysis of specific genes associated with mersacidin (mrsA), amylocyclicin (acnA), plipastatin (ppsA), and surfactin (srfAA) production by B. velezensis ML122-2 showed that the first was not transcribed under the conditions tested, while the latter three were consistent with the presence of the associated peptides as determined by mass spectrometry analysis. These findings demonstrate that B. velezensis ML122-2 has the genetic capacity to produce a wide range of antimicrobial activities that may support a specific community structure and highlight the biotechnological properties of Assam tea.
Highlights
Members of the Bacillus genus, which represent Gram-positive and endospore-forming bacteria, are widespread in a variety of environments including air, soil, aquatic ecosystems, foods, skin, and the gastrointestinal tract of animals (Abriouel et al, 2011)
B. velezensis ML122-2 cell-free supernatant (CFS) was demonstrated to inhibit 19 out of 23 assessed bacterial strains, and one out of the two molds tested in agar well diffusion assays with inhibitory/clearing zones ranging between 9.0 and 16.5 mm
Analysis of the genome sequence of B. velezensis ML122-2 using BAGEL4 identified two distinct bacteriocin gene clusters including those associated with the production of mersacidin and amylocyclicin genes, while six gene clusters potentially involved in the synthesis of secondary metabolites consisting of bacilysin, bacillibactin, surfactin, macrolactin H, bacillaene, and plipastatin identified using antiSMASH
Summary
Members of the Bacillus genus, which represent Gram-positive and endospore-forming bacteria, are widespread in a variety of environments including air, soil, aquatic ecosystems, foods, skin, and the gastrointestinal tract of animals (Abriouel et al, 2011). Some species of Bacillus are believed to play a key role in biological control through the production of antimicrobial compounds (e.g., bacteriocins, non-ribosomal polypeptides, and polyketides) and in plant growth promotion, such as Bacillus amyloliquefaciens, Bacillus subtilis, and Bacillus tequilensis (Chen et al, 2007; Gao et al, 2017; Li et al, 2018). Several species of Bacillus have been described to produce non-ribosomally synthesized peptides (NRPs) and polyketides (PKs) with antimicrobial properties (Patel et al, 1995; Pathak and Keharia, 2013). AntiSMASH is a genome database for gene cluster analysis responsible for the synthesis of secondary metabolite compounds, such as NRPs, PKs, and other antimicrobials (Medema et al, 2011; Weber et al, 2015)
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