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 intra-specific 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.

Keywords: Bacillus subtilis; GC/EI-MS; GC/MS; VOCs; identification; secondary metabolites; volatiles.

FIGURE 1

Detected compounds per B. subtilis isolate. Volatile secondary metabolites emitted by 26 B. subtilis isolates were summarized from 20 original scientific publication (see Table 1). n.d., isolate was not defined; red line is the arithmetic average of 14; B. subtilis 168 was analyzed twice; the results of 8133, 8114, and 2274 are aggregated in the original study.

FIGURE 2

Classification of detected VOCs according to chemical classes. Two hundred and thirty-one volatile secondary metabolites emitted from 26 B. subtilis isolates were summarized from 20 original scientific publication (see Table 1) and grouped in chemical classes.

FIGURE 3

Specificity of volatile secondary metabolites. Volatile secondary metabolites emitted by 26 B. subtilis isolates were summarized from 20 original scientific publication (see Table 1). Compounds were related to the number of isolates responsible for emission. (A) Production frequency of all 231 compounds (entire data set of the survey). (B) Production frequency of compound emitted upon cultivation on lysogeny broth (LB). (C) Production frequency of compounds emitted upon cultivation on nutrient broth (NB).

FIGURE 4

Specificity of biological-active volatile secondary metabolites. Volatile secondary metabolites emitted by 26 B. subtilis isolates were summarized from 20 original scientific publication (see Table 1). Biological-active compounds were related to the total number of volatiles and the number of emitting B. subtilis isolates.

FIGURE 5

Best practice procedures and documentation. Workflow to obtain consistent, reliable and re-usable bacterial profiles in future studies.