Antibacterial activity against enterovirulent Escherichia coli strains from Bacillus amyloliquefaciens B31 and Bacillus subtilis subsp. subtilis C4: MALDI‐TOF MS profiling and MALDI TOF/TOF MS structural analysis on lipopeptides mixtures

In this work, two Bacillus strains isolated from honey (Bacillus subtilis subsp. subtilis C4; access code HQ828992) and from a waste of an artisanal tannery (Bacillus amyloliquefaciens B31; access code KP893752) were evaluated in order to determine their antibacterial activity against five enteropathogenic Escherichia coli strains. The number of viable cultivable cells of the different strains of E. coli analyzed was determined by plate count. The crude cell-free supernatants of both Bacillus strains exerted anti-E. coli activities, whereas only the lipopeptide fraction of B31 had significant E. coli inhibition. The lipopeptides produced by the Bacillus were analyzed using matrix-assisted laser desorption-ionization mass spectrometry (MALDI MS). The analysis was conducted combining the profiles (fingerprints) of the lipopeptides mixture and the individual lipopeptide fragmentation (tandem mass spectrometry [MS/MS] mode), both obtained from the same lipopeptides mixture sample, for higher output. Data obtained from C4 and B31 revealed that surfactin homologues were the most abundant lipopeptides produced by both strains studied. Additionally, kurstakin, iturin, and fengycin homologues were detected. Using the MS/MS mode, it was demonstrated that isobar compounds belonging to different families were produced by each Bacillus strain (e.g., C-16 bacillomycin D was detected in B31 samples, meanwhile C-15 iturin C was detected in C4). MS/MS analysis contributed with relevant information about the type of lipopeptides synthesized by Bacillus strains studied in this work.