Antibacterial Activity of Phytoalexins from Infected Theobroma cacao L

Aims: To study the antimicrobial activities of the compounds produced due to reactions of cocoa (Theobroma cacao) with Phytophthora palmivora during infection.

Place and Duration of Study: Department of Microbiology, Ekiti State University, Ado Ekiti, Nigeria and the Department of Pharmacognosy, Obafemi Awolowo University, Ile-Ife, Nigeria between September, 2009 and July, 2011.

Methodology: Phytophthora palmivora was used to infect healthy cocoa pods. The phytoalexins were extracted using solvent extraction and purified using standard methods. Agar diffusion and paper disc methods were used to study the antibacterial activities of the compounds extracted.

Results: Two major compounds were subsequently isolated, purified and characterized as FC-3-B21 and FC-4-B22 using the 1HNMR and 13CNMR as well as the 2D cosy data. Compound FC-3-B21 was characterized as 7,8,9,-trihydroxy-2,8-dihydroxy naphtha-10-one while FC-4-B22 was characterized as ester of glycerol: Bacillus subtilis, Escherichia coli, Micrococcus luteus, Pseudomonas aeruginosa and Staphylococcus aureus, were all sensitive to 7,8,9,-trihydroxy-2,8-dihydroxy naphtha-10-one even at 20 mg/ml. Pseudomonas aeruginosa showed the least sensitivity with 2.0 mm zone of inhibition at 20 mg/ml and 12 mm at 100 mg/ml while B. subtilis was the most sensitive with zone of inhibition of 4.0 mm at 20 mg/ml and 21.0 mm at 100 mg/ml. With FC-4-B22 (ester of glycerol) using paper disc method, P. aeruginosa was the most resistant with no zone of inhibition at 20 mg/ml. Meanwhile, S. aureus was the most sensitive with zone of inhibition of 3.0 mm at 20 mg/ml. However, B. subtilis exhibited a zone of 15.0 mm at 100 mg/ml. Using the agar diffusion method, 7,8,9,-trihydroxy-2,8-dihydroxy naphtha-10-one showed an appreciable effect on the tested pathogens. Pseudomonas aeruginosa was the least sensitive with the zone of inhibition of 3.0 mm at 20 mg/ml and 8.0 mm at 100 mg/ml while S. aureus was the most sensitive to the extract at 20 mg/ml with the zone of inhibition of 6.0 mm at 100 mg/ml. Bacillus subtilis was most sensitive to the extract at 100 mg/ml with the zone of inhibition of 19.0 mm. Testing the efficacy of ester of glycerol using agar diffusion method, P. aeruginosa showed the least resistance while B. subtilis was the most sensitive.

Conclusion: From this study, it can be concluded that the two novel compounds exhibited differential antibacterial activities towards the test bacteria.