Role of ppk in Response of Porphyromonas gingivalis to Stress

Objectives: The present study was performed to investigate the role of polyphosphate kinase gene (ppk) in response of Porphyromonas gingivalis to stress by comparing gene expression patterns between ppk-defective mutant CW120 and its parental strain P. gingivalis 381 in the presence of polyphosphate with a chain length of 75 (polyP75), an antibacterial agent, as a stressor.

Methods: P. gingivalis 381 and CW120 in the exponential phase of growth were further incubated with or without 0.03% polyP75 and the pattern of their growth was observed by measuring optical density of the broth culture. Microarray was employed to identify differentially expressed genes by polyP75 and the results were confirmed by real-time PCR. The cells were observed under the transmission electron microscope (TEM).

Results: The growth curve of CW120 was lower than that of 381 in the absence of polyP75. The addition of polyP75 retarded the growth of the both strains. Their growth rate and pattern were almost same, but CW120 entered the death phase immediately after the peak of the growth. In microarray analysis, the two strains showed similar expression patterns: i) the down-regulated genes included genes related to biosynthesis of purine, pyrimidine, nucleoside and nucleotide, and genes involved in DNA replication, cell envelope synthesis, cell division and energy metabolism; ii) genes related to biosynthesis of ribosomal proteins were dominant among the up-regulated genes. However, the repression of energy metabolism-related genes and the induction of genes involved in ribosomal protein synthesis were more prominent in CW120. Real-time PCR confirmed the up- and down-regulation of some selected genes. In TEM, no detectable difference in morphology between the strains was observed.

Conclusion: ppk may be required for the normal growth, stress resistance and survival of P. gingivalis.