Growth of Porphyromonas gingivalis on Human Serum Albumin

Porphyromonas gingivalis is an oral anaerobe belonging to the phylum Bacteroidetes that contributes to the onset and progression of periodontal disease. Various clinical isolates display genotypic and phenotypic variations which impact host-pathogen interactions and virulence. This bacterium inhabits the subgingival crevice which is bathed in a serum exudate (gingival crevicular fluid; GCF). Hence, GCF constitutes the primary growth substrate for P. gingivalis in vivo. This work aimed to evaluate how well P. gingivalis utilizes human serum albumin (HSA), a major constituent of GCF, as a growth substrate. We show that although bovine serum albumin supports growth, P. gingivalis grows to a higher density and at a faster rate on HSA. Surprisingly, the study discovered that two highly related type strains (W50 and W83) differ dramatically in their utilization, growth rate, and survival when grown on HSA. Strain W83 lysed soon after entering stationary phase (within 21-22 hours post inoculation), while strain W50 persisted in stationary phase for an additional 20 hours. Transcriptomic analysis revealed that differential expression of cis-encoded antisense RNA molecules is a critical determinant in the onset of autolysis, and that a locus containing a soluble lytic transglycosylase was expressed at higher levels in strain W83 when compared to strain W50 before the onset of lysis. Deletion of this gene (PG0139) in strain W83 resulted in a strain that did not lyse and restoring the coding region of PG0139 on the chromosome restored the HSA lysis phenotype, indicating that this enzyme plays a central role in autolysis. Lastly, hemin availability was found to be a determinant in the onset of autolysis. Overall, the findings provide further confirmation of striking differences between P. gingivalis strains and new insights into the basic physiology of P. gingivalis by using an HSA-based medium that more closely simulates nutritional conditions of the subgingival environment.