Genomic comparison of invasive and rare non-invasive strains reveals Porphyromonas gingivalis genetic polymorphisms.

Background: Porphyromonas gingivalis strains are shown to invade human cells in vitro with different invasion efficiencies, varying by up to three orders of magnitude.

Objective: We tested the hypothesis that invasion-associated interstrain genomic polymorphisms are present in P. gingivalis and that putative invasion-associated genes can contribute to P. gingivalis invasion.

Design: Using an invasive (W83) and the only available non-invasive P. gingivalis strain (AJW4) and whole genome microarrays followed by two separate software tools, we carried out comparative genomic hybridization (CGH) analysis.

Results: We identified 68 annotated and 51 hypothetical open reading frames (ORFs) that are polymorphic between these strains. Among these are surface proteins, lipoproteins, capsular polysaccharide biosynthesis enzymes, regulatory and immunoreactive proteins, integrases, and transposases often with abnormal GC content and clustered on the chromosome. Amplification of selected ORFs was used to validate the approach and the selection. Eleven clinical strains were investigated for the presence of selected ORFs. The putative invasion-associated ORFs were present in 10 of the isolates. The invasion ability of three isogenic mutants, carrying deletions in PG0185, PG0186, and PG0982 was tested. The PG0185 (ragA) and PG0186 (ragB) mutants had 5.1×10(3)-fold and 3.6×10(3)-fold decreased in vitro invasion ability, respectively.

Conclusion: The annotation of divergent ORFs suggests deficiency in multiple genes as a basis for P. gingivalis non-invasive phenotype.