Identification of a Czc-like operon of the periodontal pathobiont Porphyromonas gingivalis involved in metal ion efflux

The study aimed to investigate the role of the PGN2012 gene of the periodontitis contributing pathobiont Porphyromonas gingivalis. PGN2012 is a homolgue of TolC and is a gene our group previously showed was overexpressed in hyperinvasive cells. The study used a combination of bioinformatics, knockout mutagenesis, growth experiments, biofilm assays and human cell invation assays to investigate PGN2012 function. Bioinformatics identified that PGN2012 is part of one of four TolC containing gene loci in P. gingivalis that we predicted may encode a metal resistance RND family tripartite pump, similar to those present in other Gram-negative bacteria, but which are not well understood in anaerobic bacteria. A ΔPGN2012 deletion displayed slightly reduced growth in liquid culture but did not effect biofilm formation or human cell invasion. When metal ions were included in the medium the mutant displayed significantly increased sensitivity to the divalent metal ions Zn2+ (500 μM), Co2+ (2 mM), and Cd2+(0.1 mM) but not Cu2+. We propose to rename the PGN2012-2014 genes czcCBA, which we suggest plays a role in intracellular stress resistance where zinc is often employed by host cells in antibacterial defence with implications for chronic infection in humans.