Plant Polysaccharides Modulate Biofilm Formation and Insecticidal Activities of Bacillus thuringiensis Strains.

Mengmeng Li,Tianpei Huang,Wang Ke,Xiaoxiao Li,Xiong Guan,Changlong Shu,Yiyan Yu

doi:10.3389/fmicb.2021.676146

Abstract

After the biological pesticide Bacillus thuringiensis (Bt) is applied to the field, it has to remain on the surface of plants to have the insecticidal activities against insect pests. Bt can form biofilms on the surface of vegetable leaves, which were rich in polysaccharides. However, the relationship between polysaccharides of the leaves and the biofilm formation as well as the insecticidal activities of Bt is still unknown. Herein, this study focused on the effects of plant polysaccharides pectin and xylan on biofilm formation and the insecticidal activities of Bt strains. By adding pectin, there were 88 Bt strains with strong biofilm formation, 69 strains with weak biofilm formation, and 13 strains without biofilm formation. When xylan was added, 13 Bt strains formed strong biofilms, 98 strains formed weak biofilms, and 59 strains did not form biofilms. This indicated that two plant polysaccharides, especially pectin, modulate the biofilm formation of Bt strains. The ability of pectin to induce biofilm formation was not related to Bt serotypes. Pectin promoted the biofilms formed by Bt cells in the logarithmic growth phase and lysis phase at the air–liquid interface, while it inhibited the biofilms formed by Bt cells in the sporangial phase at the air–liquid interface. The dosage of pectin was positively correlated with the yield of biofilms formed by Bt cells in the logarithmic growth phase or lysis phase at the solid–liquid interfaces. Pectin did not change the free-living growth and the cell motility of Bt strains. Pectin can improve the biocontrol activities of the spore–insecticidal crystal protein mixture of Bt and BtK commercial insecticides, as well as the biofilms formed by the logarithmic growth phase or lysis phase of Bt cells. Our findings confirmed that plant polysaccharides modulate biofilm formation and insecticidal activities of Bt strains and built a foundation for the construction of biofilm-type Bt biopesticides.

Highlights

Biofilms are abundantly present in insect intestines, phyllospheres, rhizospheres, and soils
13 Bacillus thuringiensis (Bt) strains formed strong biofilms, 98 strains formed weak biofilms, and 59 strains did not form biofilms (Figure 1). This indicated that plant polysaccharides pectin and xylan can induce Bt strains to produce biofilms, and pectin can induce 92.4% of the tested Bt strains to form biofilms
The results showed that 25 mg/mL pectin can inhibit the toxicity of 33.33 μg/mL Cry1Ac against P. xylostella, but it had no effect on the toxicities of Cry1Ac at 100, 11.11, 3.07, and 1.234 μg/mL (Figure 7C), indicating that pectin did not promote the insecticidal activity of the Cry1Ac protein alone, and it may promote the production of other virulence factors in Bt

Summary

Introduction

Biofilms are abundantly present in insect intestines, phyllospheres, rhizospheres, and soils. Bacterial biofilms are aggregates of bacteria embedded in extracellular polymeric substances (EPSs) produced by the bacteria themselves. Their EPSs mainly contain extracellular polysaccharides, proteins, lipids, and extracellular DNA, which can adhere to interfaces of solid–liquid, solid–gas, liquid–liquid, and liquid–gas (Flemming et al, 2016; Flemming and Wuertz, 2019). Bacteria can colonize almost all kinds of natural or synthetic surfaces and form biofilms (Hall-Stoodley et al, 2004; Sweet et al, 2011; Flemming and Wuertz, 2019). The average numbers of bacteria in the biofilms locating at termite intestines, phyllospheres, and soil on the land surface are 6 × 1023, 2 × 1026, and 3 × 1029, and the average number of bacteria in the rhizosphere biofilms may be comparable to that of the rhizospheres (Flemming and Wuertz, 2019)

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Results

Conclusion