Abstract
Besides sporulation, Bacillus cereus can undergo a differentiation process in which short swimmer cells become elongated and hyperflagellated swarmer cells that favor migration of the bacterial community on a surface. The functionally enigmatic flagellar protein FlhF, which is the third paralog of the signal recognition particle (SRP) GTPases Ffh and FtsY, is required for swarming in many bacteria. Previous data showed that FlhF is involved in the control of the number and positioning of flagella in B. cereus. In this study, in silico analysis of B. cereus FlhF revealed that this protein presents conserved domains that are typical of SRPs in many organisms and a peculiar N-terminal basic domain. By proteomic analysis, a significant effect of FlhF depletion on the amount of secreted proteins was found with some proteins increased (e.g., B component of the non-hemolytic enterotoxin, cereolysin O, enolase) and others reduced (e.g., flagellin, L2 component of hemolysin BL, bacillolysin, sphingomyelinase, PC-PLC, PI-PLC, cytotoxin K) in the extracellular proteome of a ΔflhF mutant. Deprivation of FlhF also resulted in significant attenuation in the pathogenicity of this strain in an experimental model of infection in Galleria mellonella larvae. Our work highlights the multifunctional role of FlhF in B. cereus, being this protein involved in bacterial flagellation, swarming, protein secretion, and pathogenicity.
Highlights
Bacillus cereus is a Gram-positive, motile, spore-bearing rod, frequently isolated from the soil, where the spore ensures its persistence under adverse conditions
In a previous work (Salvetti et al, 2007), we showed that B. cereus FlhF possesses a C-terminal G domain, that is strongly conserved among signal recognition particle (SRP)-GTPases (Zanen et al, 2004; Bange et al, 2007; Salvetti et al, 2007; Balaban et al, 2009; Green et al, 2009; Schniederberend et al, 2013; Schuhmacher et al, 2015), and a less conserved N-terminal B domain
The B domain was predicted by the Raptor X Structure Prediction Server only, with a P-value of 2.5 × 10−2, while the NG domain was predicted by both programs with a good P-value (7.43 × 10−7 for Raptor X Structure Prediction Server and 100.0% confidence by the single highest scoring template with Phyre2) and it is 100% similar in structure to B. subtilis FlhF
Summary
Bacillus cereus is a Gram-positive, motile, spore-bearing rod, frequently isolated from the soil, where the spore ensures its persistence under adverse conditions. The pathogenic potential of this bacterium is related to the secretion of several virulence proteins, e.g., hemolysins, phospholipases, trimeric toxins (hemolysin BL, HBL; non-hemolytic enterotoxin, NHE), cytotoxin K (CytK), proteases (Senesi and Ghelardi, 2010; Ramarao and Sanchis, 2013; Jeßberger et al, 2015), and to motility modes, such as swimming and swarming (Senesi et al, 2010; Celandroni et al, 2016). Bacterial swarming is a flagellum-driven social form of locomotion in which cells undergo a periodical differentiation process leading to the production of long and hyperflagellated elements, the swarmer cells, which coordinately migrate across surfaces Swarming increases HBL secretion by B. cereus (Ghelardi et al, 2007) and enhances the pathogenicity of this bacterium in an experimental endophthalmitis model (Callegan et al, 2006)
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