D-histidine exhibited anti-biofilm activity against <i>Aggregatibacter actinomycetemcomitans</i>

<i>Aggregatibacter actinomycetemcomitans</i> is a key pathogen implicated in periodontitis. The bacterium in biofilms exhibits significant resistance to antimicrobial agents and host immune responses compared to its planktonic form, posing a major challenge for periodontal therapy. Recently, D-histidine has emerged as a promising anti-biofilm agent against <i>Pseudomonas aeruginosa</i> infections. However, its potential application in the oral field remains unexplored. This study investigated the anti-biofilm effect of D-histidine on <i>A. actinomycetemcomitans</i> and examined its influence on the expression of virulence factor genes to elucidate possible underlying mechanisms. Our results demonstrated that D-histidine inhibited biofilm formation and disrupted established biofilms in a concentration-dependent manner, without affecting bacterial growth. Furthermore, D-histidine downregulated the expression of virulence factors by inhibiting quorum sensing (QS)-related genes. Notably, combining D-histidine with antibiotics, such as amoxicillin, minocycline, and metronidazole, synergistically enhanced biofilm eradication and enabled the use of lower antibiotic dosages. These findings support the further evaluation of D-histidine as a potential anti-biofilm agent in the treatment of periodontitis. IMPORTANCE The increasing prevalence of antibiotic-resistant <i>A. actinomycetemcomitans</i> biofilms posed a significant challenge in periodontitis management. This study demonstrated that D-histidine effectively targeted <i>A. actinomycetemcomitans</i> biofilms by disrupting structural integrity and suppressing virulence gene expression, without exerting bactericidal effects that could promote resistance development. Notably, D-histidine showed potent synergy with minocycline, significantly enhancing biofilm eradication while potentially enabling reduced antibiotic dosages. These findings established D-histidine as a promising adjunctive therapeutic agent, addressing the urgent need for novel approaches to overcome biofilm-associated antibiotic tolerance in periodontal treatment.