The oral pathogen Porphyromonas gingivalis resists the antimicrobial peptide LGL13K and evades the D-enantiomer by synonymous mutations in hagA

Porphyromonas gingivalis is a keystone pathogen for periodontal disease. The bacteria are black-pigmented and require heme for growth. P. gingivalis exhibit resistance to many antimicrobial peptides, including the L-enantiomer of the antimicrobial peptide GL13K, which contributes to their success in the oral cavity. P. gingivalis W50 was resistant to LGL13K but susceptible to the stereo-isomer DGL13K. Upon prolonged exposure to DGL13K, a novel non-pigmented mutant was isolated that showed a low minimum inhibitory concentration and two-fold extended minimum duration for killing by DGL13K, consistent with tolerance to this peptide. The DGL13K tolerant bacteria exhibited synonymous mutations in the hagA gene. The mutations did not prevent mRNA expression but were predicted to alter mRNA structure. The non-pigmented bacteria were deficient in hemagglutination and hemoglobin binding, suggesting that the HagA protein was not expressed. This was supported by whole cell ELISA and gingipain activity assays, which suggested the absence of HagA but not two closely related gingipains. In vivo virulence was similar for wild-type and non-pigmented bacteria in the Galleria mellonella model. Loss of the hemagglutinin HagA may allow bacteria to escape from a biofilm that is under attack by antimicrobial peptides.