Chemosensory protein 16 has an immune function and participates in host-pathogen interaction in <i>Galleria mellonella</i> infected with <i>Pseudomonas entomophila</i>

Chemosensory protein 16 was identified in the hemolymph of <i>Galleria mellonella</i> as a protein with an amount increasing after oral infection with 10^3 CFU of <i>Pseudomonas entomophila</i>, and decreasing after infection with a higher dose (10^5 CFU) of bacteria. The expression of the CSP16 gene occurred in the fat body and in the gut and correlated with changes in the protein level in the hemolymph. The CSP16 protein inhibited <i>P. entomophila</i> growth in the concentration range from 0.15 to 6 nM. Additionally, the CSP16 protein showed bactericidal activity against <i>P. entomophila, Bacillus thuringiensis</i>, and <i>Escherichia coli</i> in the range of 2–18 μM, but only in the presence of protease inhibitors, otherwise it was degraded by extracellular proteases secreted by <i>P. entomophila</i>. We demonstrated that the bactericidal activity of CSP16 was related to its ability to perforate bacterial cellular membranes in a dose-dependent manner. The antimicrobial properties of this protein were also confirmed with the use of Atomic Force Microscopy, which showed significant changes in the topology of different bacterial cell surfaces. Finally, when CSP16 was injected <i>in vivo</i> into <i>G. mellonella</i> larvae one hour after infection with <i>P. entomophila</i>, more survivors were observed at particular time-points. Taking into account its immune properties and putative ability to bind bacteria-derived compounds, the possible function of CSP16 in the host-pathogen interaction is discussed.