Abstract
Piscirickettsia salmonis is the causative bacterial agent of piscirickettsiosis, a systemic fish disease that significantly impacts the Chilean salmon industry. This bacterium possesses a type IV secretion system (T4SS), several proteins of the type III secretion system (T3SS), and a single heat shock protein 60 (Hsp60/GroEL). It has been suggested that due to its high antigenicity, the P. salmonis Hsp60 could be surface-exposed, translocated across the membrane, and (or) secreted into the extracellular matrix. This study tests the hypothesis that P. salmonis Hsp60 could be located on the bacterial surface. Immunogold electron microscopy and proteomic analyses suggested that although P. salmonis Hsp60 was predominantly associated with the bacterial cell cytoplasm, Hsp60-positive spots also exist on the bacterial cell envelope. IgY antibodies against P. salmonis Hsp60 protected SHK-1 cells against infection. Several bioinformatics approaches were used to assess Hsp60 translocation by the T4SS, T3SS, and T6SS, with negative results. These data support the hypothesis that small amounts of Hsp60 must reach the bacterial cell surface in a manner probably not mediated by currently characterized secretion systems, and that they remain biologically active during P. salmonis infection, possibly mediating adherence and (or) invasion.
Highlights
Piscirickettsiosis is caused by the facultative intracellular Gram-negative bacterium Piscirickettsia salmonis [1,2,3]
Hsp60-positive spots were identified in the cell envelope, but only a few were found on the extracellular surface
Positive spots were identified on P. salmonis outer membrane vesicles (OMVs) (Supplementary Figure S2)
Summary
Piscirickettsiosis is caused by the facultative intracellular Gram-negative bacterium Piscirickettsia salmonis [1,2,3]. The bacterial 60-kDa HSP (Hsp, known as GroEL), a highly conserved protein and dominant antigen of most pathogenic bacteria, is involved in the pathogenesis of several infectious diseases. Hsp is secreted into the extracellular space or pathogen-containing host-cell vacuoles during infection by Legionella pneumophila [14], and Helicobacter pylori [17], among others. A vaccine based on a mixture of the recombinant P. salmonis Hsp and Hsp, as well as the flagellar protein FlgG, elicits a strong protective humoral response in challenged fish [22]. Besides their potential vaccination benefits, the high antigenicity of P. salmonis Hsp suggests exposure on the bacterial cell surface. This study was designed to test the hypothesis that Hsp is a putative virulence effector protein secreted by P. salmonis, its value as a vaccine target must be considered
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