Proteomic approaches were applied to investigate whether Photobacterium damselae subsp. piscicida (Phdp) can directly sense and respond to growth conditions under different salinities, 0.85% and 3.5% NaCl concentrations, mimicking the osmotic conditions in host and marine water bodies, respectively. Proteins significantly altered were analyzed by two-dimensional gel electrophoresis (2-DE), liquid chromatography-electrospray ionization-quadrupole-time-of-flight tandem mass spectrometry (LC-ESI-Q-TOF MS/MS) and bioinformatics analysis, thus resulting in 16 outer membrane proteins (OMPs), 12 inner membrane proteins (IMPs), and 20 cytoplasmic proteins (CPs). Quantitative real-time PCR was also applied to monitor the mRNA expression level of these target proteins. Cluster of orthologous groups of protein (COG) analysis revealed that when shifting from 3.5% to 0.85% salinity, the majority of the up-regulated proteins were involved in posttranslational modification, protein turnover, and chaperones, while the down-regulated proteins were mainly related to energy production and conversion, compatible solutes (carbohydrates, amino acids and their derivatives) biogenesis and transport. Differentially expressed proteins identified in the current study could be used to elucidate the salt adaptation mechanisms of Phdp during their transition between host cells and the marine habitats.
