Role of plasmid plasticity and mobile genetic elements in the entomopathogen Bacillus thuringiensis serovar israelensis

Abstract

Bacillus thuringiensis is a well-known biopesticide that has been used for more than 80 years. This spore-forming bacterium belongs to the group of Bacillus cereus that also includes, among others, emetic and diarrheic pathotypes of B. cereus, the animal pathogen Bacillus anthracis and the psychrotolerant Bacillus weihenstephanensis. Bacillus thuringiensis is rather unique since it has adapted its lifestyle as an efficient pathogen of specific insect larvae. One of the peculiarities of B. thuringiensis strains is the extent of their extrachromosomal pool, with strains harbouring more than 10 distinct plasmid molecules. Among the numerous serovars of B. thuringiensis, ‘israelensis’ is certainly emblematic since its host spectrum is apparently restricted to dipteran insects like mosquitoes and black flies, vectors of human and animal diseases such as malaria, yellow fever, or river blindness. In this review, the putative role of the mobile gene pool of B. thuringiensis serovar israelensis in its pathogenicity and dedicated lifestyle is reviewed, with specific emphasis on the nature, diversity, and potential mobility of its constituents. Variations among the few related strains of B. thuringiensis serovar israelensis will also be reported and discussed in the scope of this specialised insect pathogen, whose lifestyle in the environment remains largely unknown.

Diversity of Bacillus cereus sensu lato mobilome

“…), is a group of Gram-positive spore-forming bacteria that comprises nine ecologically diverse species with a wide pathogenicity spectrum [1], despite being phylogenetically close. The most studied species of this group are B. anthracis, the anthrax causative pathogen [2, 3], B. cereus sensu stricto (s.s.) , to which belong foodborne pathogenic strains, and B. thuringiensis, an entomopathogenic bacterium widely used as biopesticide [4, 5]. B. thuringiensis insecticidal toxins, also known as δ-endotoxins, form a parasporal crystal, released in the environment at the end of the sporulation phase.…”

“…(serovar) israelensis [24, 25], and some are conjugative (e.g. pXO16, [26] and pAW63, [27]), mobilizable or of prophage-like nature [5, 28].…”

Completed Genomic Sequence ofBacillus thuringiensisHER1410 Reveals aCry-Containing Chromosome, Two Megaplasmids, and an Integrative Plasmidial Prophage

“…Alternatively, this region might also contain a conjugation origin, as several conjugal transfer proteins were annotated in this location. Moreover, we could predict an S-layer protein, involved in plasmid mobilization (Gillis et al, 2018). Overall, these results suggest that pLUSID1 could be transferred by conjugation.…”

“…To establish the position of B. thuringiensis HER1410 within the population structure of B. cereus-thuringiensis , two taxon sets were created. The first set (Supplemental Table 5) comprised up to five strains of B. cereus or B. thuringiensis selected from differentiated clades in BCSL_114 and B. thuringiensis phylogenetic trees from previous population structure analyses ( Bazinet, 2017 , Gillis et al , 2018 ). The second set comprised 1) the closest genomes to our strain identified using the phylogeny reconstruction of the first dataset (underlined in Figure 5), 2) 121 strains closely related to those according to phylogeny analysis of B. cereus group strains from Carroll et al (2020) , and 3) B. thuringiensis YBT-1518 type strain as outgroup (Supplemental Table 6).…”

“…Cry and cyt genes are located in plasmids such as the well-studied B. thuringiensis sv. israelensis pBtoxis which harbors up to five different cry genes and three cyt genes (Gillis et al, 2018). These genes are usually flanked by mobile elements (insertion sequences and transposons) and can grouped with other insecticidal proteins to form pathogenicity islands (PAI).…”