| Summary: | 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.
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