Prevalence and Diversity Analysis of Candidate Prophages to Provide An Understanding on Their Roles in Bacillus Thuringiensis.

Yajuan Fu,Meiying Gao,Yihui Yuan,Yan Wu

doi:10.3390/v11040388

Abstract

Bacillus thuringiensis (Bt) is widely used in producing biological insecticides. Phage contaminations during Bt fermentation can cause severe losses of yields. Lots of strategies have been engaged to control extrinsic phage contamination during Bt fermentation, but their effectiveness is low. In this study, the candidate endogenous prophages (prophages) in 61 Bt chromosomes that had been deposited in GenBank database were analyzed. The results revealed that all chromosomes contained prophage regions, and 398 candidate prophage regions were predicted, including 135 putative complete prophages and 263 incomplete prophage regions. These putative complete prophages showed highly diverse genetic backgrounds. The inducibility of the prophages of ten Bt strains (4AJ1, 4BD1, HD-1, HD-29, HD-73, HD-521, BMB171, 4CC1, CT-43, and HD-1011) was tested, and the results showed that seven of the ten strains’ prophages were inducible. These induced phages belonged to the Siphoviridae family and exhibited a broad host spectrum against the non-original strains. The culture supernatants of the two strains (BMB171, 4CC1) could lyse Bt cells, but no virions were observed, which was speculated to be caused by lysin. The functional analysis of the putative complete prophage proteins indicated that some proteins, such as antibiotic resistance-associated proteins and restriction endonucleases, might increase the fitness of the Bt strains to different environments. The findings of this study provided understanding on the high prevalence and diversity of Bt prophages, as well as pointed out the role of prophages in the life cycle of Bt.

Highlights

Bacillus thuringiensis (Bt) is a ubiquitous Gram-positive, spore-forming bacterium that produces parasporal crystals at the end of its life cycle
The chromosome sequences of 61 Bt strains were used for prophage prediction, and all strains were found to contain candidate prophage regions (Table S1)
The putative complete prophages, which contain the essential genes of phages and most likely can be induced from bacteria, were found in 50 (81.9%) Bt strains

Summary

Introduction

Bacillus thuringiensis (Bt) is a ubiquitous Gram-positive, spore-forming bacterium that produces parasporal crystals at the end of its life cycle. Due to the insecticidal ability of the parasporal crystal proteins, Bt is widely used in producing bio-pesticides [1,2]. During Bt fermentation, the outbreak of phage contamination always causes severe losses on the yields of crystal and spore, which are the main active components against insect pests. The phage contamination usually results in losses of 15 to 30% of the total batches, sometimes up to 50 to 80%. The phage contamination may cause a failure in fermentation [3]. Though many strategies—such as screening phage-resistant Bt strains, treating the seed broth with a high temperature (≥85 ◦ C), and strengthening the air filtration system—have been employed

Methods

Results

Conclusion