Comparative chemical profiling and antimicrobial activity of two interchangeably used ‘Imphepho’ species (Helichrysum odoratissimum and Helichrysum petiolare)

Abstract Helichrysum petiolare and Helichrysum odoratissimum, collectively known as ‘Imphepho’, are popular medicinal herbs that are indigenous to South Africa. The species are used interchangeably to relieve gastrointestinal and respiratory conditions, fever and urinary tract infections, implying that they produce antibacterial compounds. Although these herbs are sold in informal markets, they have not been commercialized. The aim of this study was to compare the chemical profiles and the antimicrobial activities of solvent extracts of the two species. The profiles of the smoke condensates were also compared, since the smoke from both species is inhaled during cultural rituals to induce central nervous system effects. After optimizing the extractant, the chemical profiles of aqueous methanol extracts of the aerial parts of twelve H. odoratissimum and fifteen H. petiolare samples, collected from several different localities, were obtained using ultra performance liquid chromatography-mass spectrometry (UPLC-MS). Three isomeric compounds were found to be present in both species. The construction of chemometric models from the data indicated a large degree of inter-population variation within H. odoratissimum, with the samples loosely clustering into two main groups. In contrast, the chemistry of H. petiolare was highly conserved. Discriminant analysis identified four distinguishing marker compounds for each species. One of the isomers common to both species, was isolated from H. odoratissimum and identified as 4,5-dicaffeoylquinic acid using nuclear magnetic resonance (NMR) spectroscopy, and is herein reported for the first time from the two species. Following validation of a developed UPLC-photodiode array (PDA) detection method, it was determined that the concentrations of the compound ranged from 3.89 - 31.1 µg/g in H. odoratissimum and from 2.3 to 13.4 µg/g in H. petiolare. High performance thin layer chromatography (HPTLC) fingerprints also indicated a greater degree of chemical variation within H. odoratissimum than in H. petiolare. This was confirmed through multivariate analysis of densitograms obtained from images of the fingerprints through the application of rTLC software, effectively mirroring the UPLC-MS results. The antibacterial activity of extracts of all the samples was determined using the minimum inhibitory concentration (MIC) serial dilution assay against selected Gram-positive (Staphylococcus saprophyticus, Enterococcus faecalis, Streptococcus agalactiae) and Gram-negative (Proteus mirabilis, Escherichia coli, Moraxella catarrhalis) pathogens that affect the urinary, respiratory and gastro-intestinal tracts. The MICs obtained for H. odoratissimum extracts indicated that these were more effective towards a larger range of bacteria than extracts of H. petiolare. The methanol extracts yielded average MICs denoting noteworthy or moderate activity towards three pathogens (E. coli, M. catarrhalis and S. agalactiae), while the chloroform:methanol extracts displayed noteworthy or moderate activity towards the same three pathogens, but also towards E. faecalis. These results justify to some degree the traditional use of H. odoratissimum to treat conditions of the gastro-intestinal, respiratory and urinary tracts. The UPLC-PDA profiles of smoke condensates of the two species, obtained under controlled conditions, revealed five major compounds that were common to both species. Although the similarity of the smoke profiles rationalises their interchangeable use for inhalation, the solvent extracts displayed little congruence regarding their chemical profiles or antibacterial activities.