Anti-Streptococcal Activity of β-Thujaplicin and Its Effect on Natural Low-Level Resistance of Streptococcus Species to Aminoglycoside Antibiotics

β-thujaplicin (BTJ) is an organic compound with medicinal potential. Its activity against pathogenic Streptococcus species has been insufficiently investigated. This study aimed to evaluate the antimicrobial activity of β-thujaplicin against more than 30 reference and clinical isolates of Streptococcus pyogenes, Streptococcus agalactiae, and Streptococcus pneumoniae, and to assess its effect on the intrinsic low-level resistance of these bacteria to aminoglycosides. Susceptibility of β-thujaplicin and selected aminoglycosides was determined using the broth microdilution method. The modulatory effect of β-thujaplicin on aminoglycoside activity was assessed using the checkerboard assay, with interactions evaluated based on the fractional inhibitory concentration index (FICI). β-thujaplicin exhibited antimicrobial activity against S. pyogenes and S. pneumoniae, with minimum inhibitory concentration (MIC) values ranging from 16 to 32 µg/mL, exceeding the activity of amikacin (AN). Synergistic interactions between β-thujaplicin (2–8 µg/mL) and aminoglycosides were observed in 100% of S. pneumoniae strains tested, 50% of S. pyogenes, and 80% of S. agalactiae isolates. In the presence of β-thujaplicin, aminoglycoside MICs decreased by 2- to 64-fold depending on the species. β-thujaplicin demonstrates in vitro bactericidal activity against most tested Streptococcus strains and exhibits synergistic effects when combined with aminoglycosides. These findings suggest that β-thujaplicin may reduce the natural resistance of strepto-cocci to these antibiotics.