Ribosome reconstruction during recovery from high hydrostatic pressure-induced injury in Bacillus subtilis

Vegetative cells of Bacillus subtilis can recover from injury after high hydrostatic pressure (HHP) treatment at 250 MPa. DNA microarray analysis revealed that substantial number of ribosomal genes and translation-related genes (e.g., translation initiation factors) were up-regulated during growth arrest phase after HHP treatment. The transcript levels of cold shock responsive genes, whose products play key roles in efficient translation, and heat shock responsive genes, whose products mediate correct protein folding or degrade misfolded proteins, were also up-regulated. In contrast, the transcript level of hpf, whose product (Hpf) is involved in ribosome inactivation through the dimerization of 70S ribosomes, was down-regulated during the growth arrest phase. Sucrose density gradient sedimentation analysis revealed that ribosomes were dissociated in a pressure-dependent manner and then reconstructed. We also found that cell growth after HHP-induced injury was apparently inhibited by the addition of Mn2+ or Zn2+ to the recovery medium. Ribosome reconstruction in the HHP-injured cells was also significantly delayed in the presence of Mn2+ or Zn2+. Moreover, Zn2+, but not Mn2+, promoted dimer formation of 70S ribosomes in the HHP-injured cells. Disruption of the hpf gene suppressed the Zn2+-dependent accumulation of ribosome dimers, partially relieving the inhibitory effect of Zn2+ on the growth recovery of HHP-treated cells. In contrast, it was likely that Mn2+ prevented ribosome reconstruction without stimulating ribosome dimerization. Our results suggested that both Mn2+ and Zn2+ can prevent ribosome reconstruction, thereby delaying the growth recovery of HHP-injured B. subtilis cells. IMPORTANCE HHP treatment is used as a nonthermal processing technology in the food industry to inactivate bacteria, while retaining high quality of foods under suppressed chemical reactions. However, some populations of bacterial cells may survive the inactivation. Although the survivors are in a transient non-growing state due to HHP-induced injury, they can recover from the injury and then start growing, depending on the post-processing conditions. The recovery process in terms of cellular components after the injury remains unclear. Transcriptome analysis using vegetative cells of Bacillus subtilis revealed that the translational machinery can preferentially be reconstructed after HHP treatment. We found that both Mn2+ and Zn2+ prolonged the growth arrested stage of HHP-injured cells by delaying ribosome reconstruction. It is likely that ribosome reconstruction is crucial for the recovery of growth ability in HHP-injured cells. This study provides further understanding of the recovery process in HHP-injured B. subtilis cells.