Antifungal and Antibacterial Activity of Extracts and Alkaloids of Selected Amaryllidaceae Species.

Alkaloidal extracts of six selected species of Amaryllidaceae were studied with respect to their antibacterial and anti-yeast activity and their alkaloidal fingerprint. Twenty-five alkaloids were determined by GC/MS, and sixteen of them identified from their mass spectra, retention times and retention indexes. In the antimicrobial assay, Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus were used, along with isolates of the human pathogenic yeasts Candida albicans, C. glabrata, C. dubliniensis and Lodderomyces elongiosporus. The six extracts, together with 19 Amaryllidaceae alkaloids isolated in our laboratory, showed almost no inhibitory activity against the bacteria tested. However, promising anti-yeast properties were detected; the most potent activity was shown by lycorine, which inhibited C. dubliniensis with a MIC of 32 µg/mL, C. albicans and L. elongiosporus, both with MICs of 64 µg/mL, followed by caranine inhibiting C. dubliniensis with a MIC of 128 µg/mL. Among the alkaloidal extracts, Narcissus jonquilla cv. Baby Moon showed the most potent anti-yeast activity, with minimal and average MIC values of 128 and 192 µg/mL, respectively, followed by Leucojum aestivum, Narcissus poeticus var. recurvus and N. canaliculatus (average MICs 256, 267 and 299 µg/mL, respectively). The lowest MIC value among extracts was obtained for N. canaliculatus against L. elongiosporus (MIC 64 µg/mL).

Selective growth-inhibitory effect of 8-hydroxyquinoline towards Clostridium difficile and Bifidobacterium longum subsp. longum in co-culture analysed by flow cytometry.

The major risk factor for Clostridium difficile infection (CDI) is the use of antibiotics owing to the disruption of the equilibrium of the host gut microbiota. To preserve the beneficial resident probiotic bacteria during infection treatment, the use of molecules with selective antibacterial activity enhances the efficacy by selectively removing C. difficile. One of them is the plant alkaloid 8-hydroxyquinoline (8HQ), which has been shown to selectively inhibit clostridia without repressing bifidobacteria. Selective antimicrobial activity is generally tested by culture techniques of individual bacterial strains. However, the main limitation of these techniques is the inability to describe differential growth dynamics of more bacterial strains in co-culture within the same experiment. In the present study, we combined fluorescent in situ hybridization and flow cytometry to describe the changes in active and non-active cells of a mixed culture formed by the opportunistic pathogen C. difficile CECT 531 and the beneficial Bifidobacterium longum subsp. longum CCMDMND BL1 after exposure to 8HQ. It was observed that without 8HQ, the proportion of both strains was almost equal, oscillating between 22.7 and 77.9 % during a time lapse of 12 h, whereas with 8HQ the proportion of active C. difficile decreased after 4 h, and persisted only between 8.8 and 17.5 %. In contrast, bifidobacterial growth was not disturbed by 8HQ. The results of this study showed the selective inhibitory effect of 8HQ on clostridial and bifidobacterial growth dynamics, and the potential of this compound for the development of selective agents to control CDIs.

In vitro selective inhibitory effect of 8-hydroxyquinoline against bifidobacteria and clostridia.

8-Hydroxyquinoline (8HQ) inhibited Clostridium tertium, Clostridium clostridioforme, Clostridium difficile and Clostridium perfringens in vitro with MICs of 8, 16, 32 and 32 µg/mL, respectively. In contrast, MICs of most bifidobacteria (84%) were 512 µg/mL or higher. Thus, 8HQ could be used as anti-clostridial agent or in selective media for bifidobacteria isolation.