A facile microwave assisted synthesis of thymohydroquinone with antibiofilm properties against Salmonella typhi and Listeria monocytogenes.

Thymoquinone (TQ), a monoterpene isolated from the Nigella sativa L. seeds using the oil expeller method, gave
about 40% of the total oil. Clevenger hydrodistillation of the obtained oil yielded volatile oil (0.55%) with 30%
of TQ. Subsequent purification through silica gel column chromatography resulted in the isolation of pure TQ.
The microwave assisted reduction of TQ with zinc and acetic acid resulted in the formation of thymohydroquinone
within 5 min (THQ, yield 85%). The conventional reduction method gives 42% - 60% yield and is
found to be time-consuming (4–12 h). The prepared THQ was tested at various concentrations for the inhibition
of planktonic as well as biofilm of Listeria monocytogenes Scott A and Salmonella typhi FB231 bacterial cultures.
Minimum inhibitory concentration (MIC) using MTT assay showed 50 μg/mL and 100 μg/mL for
L. monocytogenes and S. typhi, respectively. Further, the biofilm growth inhibition and development were assessed
using a crystal violet (CV) assay, which confirmed that THQ and TQ significantly eradicated biofilm formed after
6 h and 24 h, as evidenced by bright field microscopy. Thus, the results provide scope on the quicker microwave
assisted synthesis, which is considered a greener method and the THQ showed potential antimicrobial and
antibiofilm activities against common food borne pathogens.