Diversity and Distribution of Volatile Secondary Metabolites Throughout Bacillus subtilis Isolates

Abstract

Bacillus subtilis releases a broad range of volatile secondary metabolites, which are considered as long-and short distance infochemical signals mediating inter-and intraspecific processes. In addition, they often show antimicrobial or antifungal activities. This review attempts to summarize yet known volatile secondary metabolites produced and emitted by Bacillus subtilis isolates focusing on the structural diversity and distribution patterns. Using in vitro volatile-collection systems, 26 strains of B. subtilis isolated from different habitats were found to produce in total 231 volatile secondary metabolites. These volatile secondary metabolites comprised mainly hydrocarbons, ketones, alcohols, aldehydes, ester, acids, aromatics, sulfur-and nitrogen-containing compounds. Reviewed data revealed to a great extent isolate-specific emission patterns. The production and release of several volatile bioactive compounds was retained in isolates of the species B. subtilis, while volatiles without a described function seemed to be isolate-specifically produced. Detailed analysis, however, also indicated that the original data were strongly influenced by insufficient descriptions of the bacterial isolates, heterogeneous and poorly documented culture conditions as well as sampling techniques and inadequate compound identification. In order to get deeper insight into the nature, diversity, and ecological function of volatile secondary metabolites produced by B. subtilis, it will be necessary to follow well-documented workflows and fulfill state-of-the-art standards to unambiguously identify the volatile metabolites. Future research should consider the dynamic of a bacterial culture leading to differences in cell morphology and cell development. Single cell investigations could help to attribute certain volatile metabolites to defined cell forms and developmental stages.

Bacillus Responses to Plant-Associated Fungal and Bacterial Communities

“…The efficacy of such bacilli in plant protection, as well as their constant presence in the strongly competitive rhizosphere niche, are due to their high potential to synthesize a wide range of volatile organic compounds (VOCs) and soluble bioactive secondary metabolites (BSMs). High structural diversity is observed in the patterns of VOCs formed by Bacillus (Caulier et al, 2019;Kai, 2020) but also in BSMs which can be either ribosomally synthesized and post-translationally modified like bacteriocins and lantibiotics or enzymatically formed via multi-modular mega-enzymes as in the case of polyketides (PKs), di-peptides or cyclic lipopeptides (CLPs) (Harwood et al, 2018;Kaspar et al, 2019;Rabbee et al, 2019). A prime role of some soluble BSMs and volatiles in plant protection is related to their strong antimicrobial activity leading to direct antagonism against plant pathogens (Raaijmakers and Mazzola, 2012;Borriss, 2015;Chowdhury et al, 2015a;Fan et al, 2018;Caulier et al, 2019;Rabbee et al, 2019;Kai, 2020).…”

“…High structural diversity is observed in the patterns of VOCs formed by Bacillus (Caulier et al, 2019;Kai, 2020) but also in BSMs which can be either ribosomally synthesized and post-translationally modified like bacteriocins and lantibiotics or enzymatically formed via multi-modular mega-enzymes as in the case of polyketides (PKs), di-peptides or cyclic lipopeptides (CLPs) (Harwood et al, 2018;Kaspar et al, 2019;Rabbee et al, 2019). A prime role of some soluble BSMs and volatiles in plant protection is related to their strong antimicrobial activity leading to direct antagonism against plant pathogens (Raaijmakers and Mazzola, 2012;Borriss, 2015;Chowdhury et al, 2015a;Fan et al, 2018;Caulier et al, 2019;Rabbee et al, 2019;Kai, 2020). A second important biocontrolrelated trait of those compounds is their ability to trigger an immune reaction in the host plants which leads to systemic resistance (Induced SR) rendering the plant less susceptible to pathogen infection (Pieterse et al, 2014;Chowdhury et al, 2015a;Fan et al, 2018;Caulier et al, 2019;Rabbee et al, 2019).…”

Weaponizing volatiles to inhibit competitor biofilms from a distance

“…Self-produced organic volatiles, 3-methyl-1-butanol and 2nonanon, confirm specific inhibition of biofilm development B. subtilis produces a broad range of volatile compounds, with volatiles profiles differing dramatically between strains and growth conditions 52 . We first directly tested the effect of a central bacterial inorganic volatile-ammonia.…”

“…The volatile repertoire produced by bacteria varies significantly depending on conditions 52 . To directly test which volatiles are produced by B. subtilis and E. coli biofilm colonies, we performed GC-MS of organic volatiles (VOCs).…”

“…4d). We used this targeted approach to evaluate the presence of several commercially available bioactive VOCs that are known to be produced by B. subtilis 35,52 and were previously reported to modulate bacterial development. Those included propionic acid 39 ; glyoxylic acid 41 ; 2-nonanone 40 ; 2-undercanone 40 ; 1-butanol 37 ; ethanol 37 ; and 1pentanol 41 .…”