RESUMEN
Phytopathogenic nematodes (PPNs) are responsible for substantial damages within agricultural crops worldwide which can be controlled employing beneficial microorganisms and/or their metabolites in an ecofriendly way. Nevertheless, the success of the control regards not only on the virulence of the strains or the toxicity of their metabolites but also on their ability to colonize and remain in the rhizospheric environment, particularly in those crops affected by abiotic stresses promoted by the climate change. Consequently, the bioprospection of beneficial microorganisms able to control PPNs and to thrive in adverse conditions has attracted attention. On this way, deserts are perfect ecological niches to isolate microorganisms adapted to harsh enviroments. The purpose of this research was to isolate and characterize bacteria from rhizospheric soil samples collected in the Northwestern Desert of Mexico with potential for PPNs control. As first screening, secretomes of each isolate were tested in vitro for nematicidal activity (NA). Then, activities from secretomes and endospores from the selected isolate were confirmed in vivo assays. From 100 thermotolerant isolates, the secretome of the isolate identified as Bacillus paralicheniformis TB197 showed the highest NA (>95%) against Meloidogyne incognita, both in vitro and in vivo tests, suppressing infections caused by M. enterolobii in tomato crops, too. In open field tests, the endospores of TB197 strain showed a reduction of 81% in the infection severity caused by M. enterolobii (p ≤ 0.05), while the galling index (GI) was reduced 84% (p ≤ 0.05) in tomato greenhouse-tests. Also, a reduction of the root necrosis (81%) caused by Radopholus similis in banana plantations (p ≤ 0.05), compared to the control was observed. Owing to their efficacy in controlling PPNs infections, the endospores and secondary metabolites of B. paralicheniformis TB197 strain could be used in bionematicidal formulations.
RESUMEN
The aim of this work was to develop a comparative study between Bacillus paralicheniformis TB197 and B. subtilis ATCC 21332 strains in terms of growth, cyclic lipopeptide production, nematicidal activity, and active lipopeptide characteristics. Crude lipopeptide extracts (CLEs) from their fermentation broths were obtained, and their nematicidal activity (NA) was estimated as the mean lethal dose (LD50), employing Caenorhabditis elegans. Using a bioguided approach, CLE components were fractionated by semipreparative thin layer chromatography, and active lipopeptides were characterized by mass spectrometry. Both strains produced similar concentrations of CLEs (p ≥ 0.05) (0.99 ± 0.11 and 1.14 ± 0.15 mg/mL by TB197 and ATCC 21332, respectively). The estimated LD50 values of CLEs from the TB197 and ATCC 21332 strains were 3.88 and 8.15 mg/mL, respectively, showing that the NA of the TB197 strain CLE was 2.1-fold higher (p ≤ 0.05). Mass spectrometry revealed that strain TB197 synthesizes several families of lipopeptides, namely, fengycin A (C14-C17), fengycin B (C16-C17), surfactin (C15-C17), and lichenysin (C12, C13, C14, and C16), from which fengycins and lichenysins possess the highest NA (100 and 60% mortality in C. elegans larvae, respectively), while the ATCC 21332 strain produces mainly surfactin (C13-C17) (NA 63% mortality). The main differences found in this study were that the TB197 strain has a higher tolerance to inhibition by the product, and the lipopeptides they synthesize have a higher nematicidal activity due to the diversity of families compared to ATCC 21332. Likewise, it was shown that more polar lipopeptides (fengycins) are more effective at causing mortality in C. elegans larvae. KEY POINTS: ⢠The nematicidal activity of lipopeptides from TB197 is higher than from ATCC 21332 ⢠TB197 produces surfactin, lichenysin, and fengycin, while ATCC 21332 mainly produces surfactin ⢠The most polar lipopeptides (fengycins) cause more mortality in C. elegans L2.