2 Dr. G. R. Damodaran College of Science, Department of Microbiology, Avinashi road, Coimbatore 641 014, Tamilnadu, India
3 Aravind Eye Hospital and Postgraduate Institute of Ophthalmology, Department of Microbiology, Avinashi road, Coimbatore 641 014, Tamilnadu, India
Fusarium spp. are the most important fungal pathogens causing keratomycosis in South India. Without an appropriate treatment this corneal infection can easily become more serious and finally ends in blindness. A great number of Fusarium strains, especially in the Fusarium solani species complex, are resistant or show low susceptibility to conventional antifungal agents. This fact makes it hard to find an effective therapy for this disease. The alternative cures, such as essential oils, might be the potential solution for this problem.
The aims of this study were (I) to examine the in vitro inhibitory effect of 9 different essential oils on Fusarium strains isolated from human keratomycosis, (II) to investigate the in vitro interaction between the main component of the most effective oil and natamycin, which previously proved to be the most effective antifungal drug in our studies and (III) to determine the effect of the most efficient oil and its main component by microscopic observations.
The in vitro effect of the essential oils on 19 Fusarium isolates belonging to 5 different species complexes were determined by the broth microdilution method. For these tests commercially available oils were used, extracted from the following plants: Cinnamomum zeylanicum, Citrus limon, Eucalyptus citriodora, Gaultheria procumbens, Juniperus communis, Melaleuca alternifolia, Origanum majoranna, Salvia sclarea and Thymus vulgaris. For the interaction tests the checkerboard microdilution method was used. The effect of the most active oil and its component was investigated by light and fluorescent microscopy. FUN-1 staining was used to determine whether the presence of oil causes a reduced fungal metabolic activity.
The lowest minimal inhibitory concentration values were observed in the case of C. zeylanicum oil (0.5-0.125 microL/mL); and its component, trans-cinnamaldehyde (tCA) was also tested and showed the same activity against the investigated isolates. Whereas, the C. limon oil proved to be the less effective in the inhibition of the growth of Fusaria. The interaction tests revealed mostly no interaction between tCA and natamycin. Based on the microscopic observations cinnamon oil caused delayed or inhibited conidia germination. FUN-1 staining revealed that the C. zeylanicum oil causes reduced cellular metabolism.
According to our results cinnamon oil and its component, tCA can be potentially used in the treatment of Fusarium keratitis. However, the preliminary in vitro studies suggest that the simultaneous application with antifungal drugs, as natamycin, will not increase the efficacy of the therapy.
The research of M.H., Cs.V. and L.G. was supported by the European Union and the State of Hungary, co-financed by the European Social Fund in the framework of TÁMOP 4.2.4.A/2-11-1-2012-0001 'National Excellence Program'. The relating research groups were also supported by the INSA-HAS interacademic bilateral project (SNK-49/2013) providing infrastructure and research equipment.