Abstract
Outer spore envelope proteins of pathogenic bacteria often present specific virulence factors and tools to evade the defence system of their hosts. Brevibacillus laterosporus, a pathogen of invertebrates and an antimicrobial-producing species, is characterised by a unique spore coat and canoe-shaped parasporal body (SC-CSPB) complex surrounding the core spore. In the present study, we identified and characterised major proteins of the SC-CSPB complex of B. laterosporus, and we investigated their entomopathogenic role. Employing a proteomic approach and a B. laterosporus-house fly study model, we found four highly conserved proteins (ExsC, CHRD, CpbA and CpbB) that function as insect virulence factors. CpbA was associated with a significantly higher mortality of flies and greater relative gene expression levels during sporulation, compared to the other SC-CSPB proteins. Taken together, we suggest that spore surface proteins are a part of a complex set of toxins and virulence factors that B. laterosporus employs in its pathogenicity against flies.
Highlights
The primary function of bacterial cell envelopes is to provide structural integrity and protection to the protoplast
Employing a B. laterosporus-house fly study model, we found that some of the proteins associated with the spore coat, exosporium and canoe-shaped parasporal body (CSPB) complex
The majority of studies reporting the insecticidal action of B. laterosporus strains against mosquitoes and flies have been conducted employing sporulated cultures obtained with different preparation methods that may lead to bacterial suspensions consisting of old vegetative cells, unlysed sporangia and free spores mixed in varying proportions[19]
Summary
The primary function of bacterial cell envelopes is to provide structural integrity and protection to the protoplast. Despite significant differences among diverse species, their composition normally includes envelope polymers and surface layer proteins that bacteria typically use to interact with the external environment[1]. The biocontrol potential of this bacterial species in agriculture is not limited to invertebrate pests (insects in different orders, nematodes and mollusks) and includes phytopathogenic bacteria and fungi[11]. This broad-spectrum of activity is associated with a wide variety of molecules, including proteins and antibiotics, it produces. Despite the possible implication of spore surface proteins in the toxic interaction with the midgut of house flies, the virulence factors acting against these insects have not been identified[17,18]. We showed that as these proteins are progressively synthesised during bacterial growth, a proportional increase in the insecticidal activity of this bacterium is observed, being fully toxic when the spore envelopes are completely formed
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