43rd NATIONAL CONGRESS OF THE ITALIAN SOCIETY OF MICROBIOLOGY

Introduction: Capsaicin (8-methyl-N-vanillyl- 6-nonenamide) is the active component responsible for the fruit pungency of Capsicum plants, cultivated for food and also for medicinal uses since ancient times. Besides its multiple pharmacological and physiological properties (pain relief, cancer prevention, weight reduction, cardiovascular, and gastrointestinal benefits), capsaicin has recently received attention because of its antimicrobial activity and anti-virulence properties. The aim of the present study was to investigate the effects of capsaicin on Streptococcus pyogenes, the most common cause of acute bacterial pharyngotonsillitis. Materials and Methods: The erythromycin-resistant [erm(B)/cMLS], high cell-invasive, and strong biofilm producer S. pyogenes pharyngeal isolate SP1070 was used throughout the study. Capsaicin was purchased from Sigma-Aldrich and stored (10 mg/mL stock solution) in absolute ethanol at -20°C. The MIC and MBC were determined according to the CLSI guidelines. Survival in presence of capsaicin was studied by the live/dead assay. Biofilm formation was tested by a microtiter assay and quantified by measuring the absorbance at 690 nm. Cell experiments were performed using the human alveolar carcinoma A549 cell line. Results: The MIC and the MBC of capsaicin were both 128 μg/mL. In the live/dead assay, several red cells were detected as early as 15 min after incubation with capsaicin at MIC; all cells were red after 60 min of incubation. At capsaicin sub-MICs (1/2– 1/16 × MIC), a significant increase in biofilm production and in the number of streptococci adherent to A549 cells was observed; whereas a strong reduction in the number of intracellular bacteria was detected. Discussion and Conclusions: Our findings reveal that capsaicin has a dual effect on S. pyogenes. High-level capsaicin exerts a bactericidal effect, probably due to the disruption of the cell membrane, this result being in agreement with previous studies on Gram-positive and Gram-negative pathogens; while sub-lethal capsaicin modifies virulence properties in vitro, such as the ability to form biofilm and to adhere/invade epithelial cells. Capsaicin-induced effects on biofilm formation seem to be similar to those observed for a variety of antibiotics that at sub-lethal concentrations can act as agonists of bacterial biofilm production in vitro. Overall, capsaicin-induced effects on S. pyogenes deserve further studies.