Label free quantitative analysis of Alicyclobacillus acidoterrestris spore germination subjected to low ambient pH

Inhibition of spore germination or sterilization after induction of spore germination would effectively control low pH food spoilage caused by Alicyclobacillus acidoterrestris spores. However, the characteristics and mechanisms of A. acidoterrestris spore germination in low ambient pH remains poorly understood. In this study, the germination rate of A. acidoterrestris spores at different ambient pH conditions was determined, and subsequently the proteomic profiles of A. acidoterrestris in spore germination were analysed by label-free quantification, in which the specific metabolic pathways involved were identified and key functional proteins were screened and validated using RT-qPCR (real time quantitative PCR). The suitable ambient pH value for the spore germination of A. acidoterrestris ranged from 3.0 to 5.0 with the optimum pH of 4.0. According to the LC-ESI-MS/MS (liquid chromatography electrospray ionization tandem mass spectrometry) analysis, 98 proteins of geminated spores of A. acidoterrestris incubated for 2 h at pH 3.0 were changed significantly in comparison to non-germinated spores, the expression of 20 proteins were up-regulated and that of 78 proteins down-regulated respectively. Those differential expressed proteins were mainly involved in cell wall hydrolysis, cell morphological changes, protein synthesis and folding, perception of external stimuli and signal transduction etc., and we observed that germination receptor D (GerD), cell wall hydrolase, transpeptidase, peptidase S1 and two-component regulatory system phoR were significantly up-regulated, but hydrolase NlpC/P60, peptidoglycan glycosyltransferase, spore coat proteins CotX, CotJB and the Lrp/AsnC (leucine-responsive regulatory protein/asparagine synthase C products) protein were significantly down-regulated in the experiment, which implied the important roles of identified proteins during the spore germination. Furthermore, the pathway analysis showed the possible involvement of differentially expressed proteins in the β-lactam resistance, ribosome, biosynthesis of secondary metabolites, pyruvate metabolism, two-component system and other metabolic pathways, which indicated that synthesis and hydrolysis of cell wall, intracellular substance synthesis, energy generation and signal transduction were likely associated with the initiation of spore germination and restoration of vegetative growth. In conclusion, the quantitative proteomic landscape of A. acidoterrestris spores could provide the theoretic and experimental evidences for the hazard control of A. acidoterrestris spores in the thermal pasteurization process of acidic beverages industry.