Osmitopsis asteriscoides (Asteraceae)-the
antimicrobial activity and essential oil composition of a Cape-Dutch remedy
Abstract
The essential oil composition and antimicrobial activity
of Osmitopsis asteriscoides, a medicinal plant used in traditional
herbal preparations in South Africa has been investigated. Three different
antimicrobial methods (disc diffusion, minimum inhibitory concentration by
micro-titer plate and time-kill studies) were comparatively evaluated
against Candida albicans, Staphylococcus aureus and Pseudomonas
aeruginosa. A preliminary screening was done using the disc diffusion
method on nine bacterial and four fungal isolates. Minimum inhibitory
concentrations showed some correlation with the disc diffusion method. However,
time-kill studies appear to be a more superior method for determining
antimicrobial activity of volatile compounds such as essential oils. Two
moderately susceptible and one resistant organism were selected to further demonstrate
the variability between the three methods. The antimicrobial activity of the
essential oil, tested by means of time-kill methodology at concentrations
ranging from 0.5 to 2% (v/v) indicate a strong fungicidal activity
against Candida albicans and the oil was also found to be
bacteriostatic against Staphylococcus aureus in a
concentration-dependent manner. The essential oil rapidly reduced viable counts
of Pseudomonas aeruginosa, but regrowth was noted after 240 min.
The results have been generated in duplicate in separate microbiology
laboratories using different time-kill methods and the results are congruent.
The two major essential oil components camphor and 1,8-cineole were
investigated indicating the positive antimicrobial efficacy of 1,8-cineole independently
and in combination with camphor. In addition to (−)-camphor and 1,8-cineole, 40
compounds were identified by GC-MS in the hydro-distilled essential oil. The
high concentration of cineole and camphor and their synergistic effect is
presented as a possible explanation for the traditional use of Osmitopsis
asteriscoides for treating microbe-related illnesses.
This attractive white daisy has been in use for various medicinal
purposes for hundreds of years. During spring and early summer many of the
wetland areas of the Western Cape are arrayed with dense groups of this
aromatic plant in full flower.
Description
Fast
growing, Osmitopsis asteriscoides is a leggy, erect,
herbaceous perennial which can grow to 2 m. It is sparsely branched at the base
while the upper stems are well branched. The lower parts of the stems are
leafless, showing the cracked grayish bark. The aromatic leaves are densely
arranged towards the branch ends. The leaves are lance shaped and may be smooth
or velvety, 10 – 18 mm wide and 60 – 80 mm long and have a distinctive
camphor-like smell. The typical daisy flower heads are grouped on the short
upper branches. Each flower head is about 35 mm across, the outer ray florets
are white while the central disc florets are yellow. Flowers may be found
throughout the year but the main flowering season is in spring and early
summer.
The small 3 – 4 ribbed or angled fruit, known
as an achene, is dry and brownish and does not have a pappus (a tuft of hairs
or bristles) which many of the daisy family have. Each fruit contains a single
seed but does not split open to release it.
Conservation status
In the Red List of South African Plants 2009 Osmitopsis
asteriscoides is listed as Least Concern.
Distribution and habitat
The swamp daisy is only found growing in the Western Cape. It
grows in marshes and seeps on sandstone soils, in an area extending from the
Cape Peninsula to the Riviersonderend Mountains. It would be suitable for
growing in temperate winter rainfall areas with soils that are damp to wet all
year round.
Derivation of name and historical aspects
Osmitopsis is the Latin derivation of
“like Osmites” which is a genus which no longer exists. Osmites is
derived from the Greek “osme” which means “scent”. The plants
have aromatic leaves.
“Asteriscoides ” is derived from “oides ”
meaning “similar to” and a daisy genus “Asteriscus”, therefore
“similar to Asteriscus”.
The antimicrobial activity and essential oil
composition of medicinal aromatic plants used in African traditional healing
Van Vuuren, Sandra Freda
Date: 2008-02-29
Abstract:
Abstract A study on the essential oil chemistry and
microbiological activity of South African indigenous medicinal plants with the
aim of establishing a scientific rationale for their anti-infective properties
was undertaken. For the purpose of this study, nine medicinal aromatic plants
were selected. The hydrodistilled essential oil was analyzed by gas
chromatography combined with mass spectrometry (GC-MS) and various techniques
were used to document the antimicrobial activity. Disc diffusion studies on Myrothamnus
flabellifolius indicated highest activities against the fungal test organisms
Cryptococcus neoformans and Aspergillus niger with radial inhibition zones of 8
mm and 10 mm respectively. The highest antimicrobial activity noted in the
minimum inhibitory concentration (MIC) assay was for C. neoformans (2 mg/mL).
Time-kill studies demonstrated the death kinetic progression on M.
flabellifolius essential oils where the killing rate was greatest for Candida
albicans. Osmitopsis asteriscoides, a plant used traditionally for cuts and
swellings showed highest disc diffusion antimicrobial efficacy against
Staphylococcus aureus (3 mm) and C. neoformans (3 mm). The MIC study indicated
highest susceptibilities (4 mg/mL) for Escherichia coli and Staphylococcus epidermidis.
Death kinetics for the three organisms studied demonstrated that the killing
rate was greatest for C. albicans. The role of the two major constituents
(1,8-cineole and (-)-camphor) act synergistically to enhance antimicrobial
activity. Disc diffusion assays undertaken on Artemisia afra showed highest
activity against Candida tropicalis (5 mm). In the MIC assay the highest
susceptibility was against Serratia odorifera (4 mg/mL). Time-kill assays on
Artemisia afra showed a concentration dependent bactericidal activity, with
evidence that the major constituents independently and in combination were not
responsible for the overall activity of the plant. Lippia javanica, a plant
used to treat coughs, colds and bronchitis, indicated highest susceptibility
against the respiratory pathogen Klebsiella pneumoniae (5 mm) with the disc
diffusion assay. The MIC assay indicated highest susceptibilities (4 mg/mL)
against C. neoformans and E. coli. Death kinetic assays for three test
organisms showed that the killing rate was the greatest for K. pneumoniae. The
time-kill study for L. javanica in combination with A. afra demonstrated that
the oils in combination act synergistically against K. pneumoniae. The
antimicrobial activity of the essential oils and extracts were determined for
Helichrysum cymosum subsp. cymosum where the extracts demonstrated at least a
six times greater MIC efficacy than the essential oils. Using column
chromatography, the antimicrobially active compound was isolated from H.
cymosum subsp. cymosum and identified as helihumulone. The traditional use of
plants as a treatment for infectious diseases is not always restricted to a
single part of the plant as was noted in the study on Croton gratissimus var.
subgratissimus, where the leaf, bark and root extracts were investigated
singularly and combined in various ratios to establish possible interaction.
The MIC and fractional inhibitory concentration (FIC) results indicated
variable efficacies for the plant combinations. The greatest synergistic profile
was noted for C. neoformans in the leaf and root combination (MIC 0.4 mg/mL and
FIC of 0.4). Further isobologram combination studies were thereafter conducted
on varying ratios of leaf and root extracts, indicating greatest synergy for
Bacillus cereus, Enterococcus faecalis, C. albicans and C. neoformans. While
seasonal variation had very little impact on the MIC results obtained from
Heteropyxis natalensis, the ratio of the two major compounds (1,8-cineol and
limonene) fluctuated on a monthly basis. Moderate antimicrobial activity
(3.0-16.0 mg/mL) was found for most pathogens with higher sensitivities for C.
neoformans. The geographical variation of H. natalensis essential oil indicated
similar profiles for Gauteng, Nelspruit and Waterberg samples. The Lagalametse
sample, however, showed distinct variation both chemically and
microbiologically where efficacy was higher than in all other samples. The
impact of the enantiomeric configuration was investigated for limonene in
combination with 1,8- cineole with (+/-)-limonene in combination with
1,8-cineole having the most significant synergistic ratios against Pseudomonas
aeruginosa. The antimicrobial activities of the non-volatile and volatile
fractions of Tarchonanthus camphoratus and Plectranthus grandidentatus,
singularly and in combination demonstrated that the volatile constituents
contribute to the total efficacy of the plant. Isobologram representation of
the combination of various ratios of T. camphoratus and P. grandidentatus
essential oil and non-volatile extracts devoid of essential oils present a
predominant synergistic profile for all pathogens studied. A comparative study
on five indigenous oils (M. flabellifolius, O. asteriscoides, H. natalensis, A.
afra and L. javanica) was undertaken with five popular commercial oils
(Lavendula angustifolia, Thymus vulgaris, Melaleuca alternifolia, Mentha
piperita and Rosmarinus officinalis). The highest antimicrobial activity was
noted for Thymus vulgaris in the MIC assay, followed by M. flabellifolius, O.
asteriscoides and M. alternifolia. With the time-kill assay, M. flabellifolius
showed the most rapid cidal effect against all three pathogens tested. The
comparative evaluation of commercial essential oils with indigenous oils
validated the use of South African aromatic plants for their anti-infective
properties.
