In Vitro Antimicrobial Potential of Different Platelet Concentrates Against Eight Clinically Relevant Oral Pathobionts

<b>Background/Objectives:</b> Oral infections are caused by a wide spectrum of bacterial and fungal species and remain clinically challenging, particularly against the background of increasing antimicrobial resistance and efforts to reduce antibiotic use in dentistry. Platelet concentrates are widely applied in periodontal and oral surgery due to their regenerative and immunomodulatory properties, and accumulating evidence suggests additional antimicrobial effects. This study evaluated the antimicrobial activity of platelet-rich plasma (PRP), platelet-rich fibrin (PRF), and injectable PRF (i-PRF) against clinically relevant oral microorganisms. <b>Methods:</b> PRP, PRF, and i-PRF were prepared from venous blood of five healthy donors and evaluated using diffusion-dependent, qualitative-semiquantitative agar diffusion assays against <i>Aggregatibacter actinomycetemcomitans</i>, <i>Porphyromonas gingivalis</i>, <i>Prevotella intermedia</i>, <i>Staphylococcus aureus</i>, <i>Streptococcus mutans</i>, <i>Streptococcus mitis</i>, <i>Enterococcus faecalis</i>, and <i>Candida albicans</i>, with inhibition zones assessed after species-specific incubation times. Chlorhexidine (2%) and amoxicillin served as positive controls and NaCl (0.9%) as negative control. Inhibition zones were digitally quantified and analyzed using non-parametric statistics (Kruskal-Wallis, Friedmann) due to skewed distributions and frequent zero values. <b>Results:</b> All platelet concentrates demonstrated microorganism-dependent inhibition zones in vitro. Overall, i-PRF demonstrated the strongest inhibitory effect across all pathogens (<i>p</i> < 0.001). Significant differences were detected for <i>E. faecalis</i> and <i>C. albicans</i>, where i-PRF produced markedly larger inhibition zones compared to PRP and PRF. Descriptively, anaerobic periodontal pathogens and <i>S. aureus</i> tended to be more susceptible, while streptococci and <i>C. albicans</i> demonstrated lower inhibition. <b>Conclusions:</b> These findings support a potential adjunctive antimicrobial role of platelet-derived preparations in dental infection management but should be interpreted with caution, as agar diffusion results do not necessarily reflect clinical performance.