Thymoquinone Antifungal Activity against Candida glabrata Oral Isolates from Patients in Intensive Care Units—An In Vitro Study

Abstract

The number of <i>Candida</i> spp. infections and drug resistance are dramatically increasing worldwide, particularly among immunosuppressed patients, and it is urgent to find novel compounds with antifungal activity. In this work, the antifungal and antibiofilm activity of thymoquinone (TQ), a key bioactive constituent of black cumin seed <i>Nigella sativa</i> L., was evaluated against <i>Candida glabrata,</i> a WHO 'high-priority' pathogen. Then, its effect on the expression of <i>C. glabrata EPA6</i> and <i>EPA7</i> genes (related to biofilm adhesion and development, respectively) were analyzed. Swab samples were taken from the oral cavity of 90 hospitalized patients in ICU wards, transferred to sterile falcon tubes, and cultured on Sabouraud Dextrose Agar (SDA) and Chromagar <i>Candida</i> for presumptive identification. Next, a 21-plex PCR was carried out for the confirmation of species level. <i>C. glabrata</i> isolates underwent antifungal drug susceptibility testing against fluconazole (FLZ), itraconazole (ITZ), amphotericin B (AMB), and TQ according to the CLSI microdilution method (M27, A3/S4). Biofilm formation was measured by an MTT assay. <i>EPA6</i> and <i>EPA7</i> gene expression was assessed by real-time PCR. From the 90 swab samples, 40 isolates were identified as <i>C. glabrata</i> with the 21-plex PCR. Most isolates were resistant to FLZ (<i>n</i> = 29, 72.5%), whereas 12.5% and 5% were ITZ and AMB resistant, respectively. The minimum inhibitory concentration (MIC₅₀) of TQ against <i>C. glabrata</i> was 50 µg/mL. Importantly, TQ significantly inhibited the biofilm formation of <i>C. glabrata</i> isolates, and <i>EPA6</i> gene expression was reduced significantly at MIC₅₀ concentration of TQ. TQ seems to have some antifungal, antibiofilm (adhesion) effect on <i>C. glabrata</i> isolates, showing that this plant secondary metabolite is a promising agent to overcome <i>Candida</i> infections, especially oral candidiasis.