Effect-directed screening of Bacillus lipopeptide extracts via hyphenated high-performance thin-layer chromatography

Abstract

Bacillus species produce a wide array of biologically active metabolites, including nonribosomaly synthesized lipopeptides (LPs). The high-performance thin-layer chromatography (HPTLC) technique hyphenated with different bioassays and mass spectrometry was demonstrated as a valuable tool for effect-directed analysis of iturins, surfactins, fengycins and kurstakins homologues from complex mixtures of LPs. As proof of this straightforward strategy, the found surfactin and iturin A homologues were characterized and compared with reference substances. This study considered two different extraction methods for LPs produced by five Bacillus strains. The ethyl acetate extraction (Ex-1), and the acidic precipitation followed by methanol extraction (Ex-2) were investigated. Diverse enzyme inhibitions and antimicrobial potentials of LPs were analyzed, and in parallel, high-resolution mass spectra (HRMS) were online recorded from the HPTLC zones of interest. No antimicrobial effect against Gram-positive B. subtilis was evident for iturin, whereas a response was detected for surfactin. The nonpolar kurstakin compounds showed a pronounced B. subtilis antimicrobial activity in Ex-1 of almost all strains, whereas the fengycin homologues were detected in Ex-2 of SS-10.7 and SS-27.2. Iturin had also no activity against Gram-negative Aliivibrio fischeri, while again surfactin showed an enhancing luminescent activity. Contrary, kurstakin compounds caused a decrease in the luminescence in Ex-1 of all strains, particularly for SS-13.1. Both, iturin and surfactin showed a strong acetylcholinesterase (AChE) and α-glucosidase inhibition, but surfactin caused a much stronger inhibition. This was evident in all bacterial strains, except for SS-13.1 in Ex-1 and for SS-38.4 in Ex-2. Although, iturin and surfactin exhibited no DPPH˙ scavenging activity, Ex-1 of all strains contained more intense DPPH˙ scavenging compounds compared to Ex-2, and surfactin methyl esters showed a pronounced DPPH˙ activity, particularly in SS-12.6 in Ex-1. This study pointed to active metabolites of strains that can be used as therapeutics and biocontrol agents with beneficial effects on human health. The straightforward HPTLC profiling served as an excellent bioanalytical tool to control the formed bioactive metabolites. As the fermentation process is very sensitive to external influences, it could be a helpful control tool for standardization of the biotechnological processing.