Abstract
Plants in the genus Kalanchoe (Family: Crassulaceae) are used in traditional medicine throughout the tropics for treating a variety of conditions. Two species, Kalanchoe mortagei and K. fedtschenkoi, have established ethnobotanical usage but have been neglected in previous research concerning their potential bioactivity. Here, we provide a thorough review of the reported antimicrobial activities of Kalanchoe genus and evaluate the in vitro antibacterial effects of two previously unexplored species against a panel of multidrug-resistant bacteria, the ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter cloacae). Plant specimens were collected and voucher specimens deposited in the Emory University Herbarium. Dried plant material was ground into a powder and extracted as ethanolic macerations or as aqueous decoctions. Extracts were tested against the ESKAPE pathogens for growth inhibitory activity. Cytotoxicity to human cells was assessed via a lactate dehydrogenase assay of treated human keratinocytes (HaCaTs). K. fedtschenkoi extracts demonstrated growth inhibitory effects against two Gram-negative species, A. baumannii (strain CDC-33) and P. aeruginosa (AH-71), as well as S. aureus (UAMS-1). In these cases, growth inhibition greater than 50% (IC50) was generally observed at concentrations of 256 μg mL-1, though one K. fedtschenkoi extract (1465, prepared from stems) exhibited an IC50 against A. baumannii at 128 μg mL-1. All extracts were well tolerated by HaCaTs (LD50 ≥ 256 μg mL-1). Chemical characterization using HPLC and chemical standards established the presence of caffeic acid and quercetin in both plant species, as well as kaempferol in K. fedtschenkoi. These results reveal K. fedtschenkoi to be a plant of medicinal interest, and future research should aim to characterize the bioactivity of this species and its active constituents through bioassay-guide fractionation. Effects on bacterial biofilm formation and quorum-sensing are also research topics of interest for this genus.
Highlights
Ethnopharmacological Relevance of Kalanchoe SpeciesPlants in the genus Kalanchoe (Crassulaceae), though originating mostly in Madagascar and Southeast Africa, have a global distribution in warm climates
Initial screening of extracts at 256 μg mL−1 demonstrated an IC50 of K. fedtschenkoi extracts (1421, 1465, and 1469) against three of the ESKAPE pathogens: S. aureus, A. baumannii, and P. aeruginosa
Further testing by serial dilution assays revealed that K. fedtschenkoi extracts had IC50 values ranging from 128 to 256 μg mL−1 for these pathogens (Table 6)
Summary
Ethnopharmacological Relevance of Kalanchoe SpeciesPlants in the genus Kalanchoe (Crassulaceae), though originating mostly in Madagascar and Southeast Africa, have a global distribution in warm climates. The presence of toxic cardiac glycosides make some Kalanchoe spp. a grazing hazard for animals in agriculture, with documented issues in Brazil, South Africa, and Australia (Botha C., 2013; Botha C.J., 2013; Mendonça et al, 2018) These plants display a diverse array of stunning forms and are often grown as ornamentals for their strange beauty. Because of its widespread distribution and ubiquitous ethnobotanical use, much research has been focused on K. pinnata, a species native to Madagascar but cultivated and distributed throughout the tropics (Descoings, 2003; Biswas et al, 2011a; Quazi Majaz et al, 2011; Pattewar, 2012; Rajsekhar et al, 2016) This species has even been the subject of bioengineering – a transgenic K. pinnata that produces an antimicrobial peptide (AMP cecropin P1) has recently been developed (Zakharchenko et al, 2016; Lebedeva et al, 2017)
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