In total, 26 natural compounds of different chemical
classes and 19 crude extracts were investigated. Determination
of the MICs by means of the broth microdilution
method showed that 13 compounds (Table 1) and 16
extracts tested (Table 2) exhibited an antimicrobial effect
against P. larvae with MICs values ranging from 2 to
256 mg/ml. Out of all plant-derived products tested, the
greatest antibacterial action against P. larvae was observed
for sanguinarine with MIC values 4mg/ml, followed by
thymoquinone with MIC values ranging from 8 to 16mg/
ml (depending upon the strain tested), capsaicin (MIC
32mg/ml), nordihydroguaiaretic acid (MIC 32mg/ml),
trans-2-hexenal (32mg/ml) and extracts of H. lupulus
(MIC 2–4mg/ml) and M. communis (MIC 2–8 mg/ml). MIC
values of reference antibiotics (Table 1) ranged from 0.016
to 0.031mg/ml. Both isolated strains were more susceptible
to natural products than the type strain but not to
antibiotics.