Herbal plant extracts are potential sources of antibiotic prototypes. Numerous studies have demonstrated the antimicrobial activity of active compounds in herbal plant extracts against commonly found bacteria. Hence, the search for novel plants with medicinal value that are potential alternatives to currently used antibiotics (tetracycline, chloramphenicol etc.) is an important area of research.
In this paper, we investigate the antimicrobial activity of Coleus amboinicus or Plectranthus amboinicus by preparing an aqueous and hexane extract and leaf oil and testing against clinical isolates of Pseudomonas aeruginosa and Proteus mirabilis. Pseudomonas aeruginosa is a medically important, multidrug resistant pathogen causing nosocomial and opportunistic infections whereas Proteus mirabilis causes 90% of Proteus infections.
Both the extracts and components of leaf oil of Coleus amboinicus leaves showed antibacterial activity against these two isolates at all the concentrations tested by agar well diffusion method, using Streptomycin as standard.
The inhibition zone diameters were found to be in the range of 4.7 – 15.3 and 4.7 – 12.7 mm, 11.00 – 21.00 and 10.00 – 19.00 mm , with a standard error on the order of 0.0 to 1.73 as a function of increasing concentration of aqueous extract and hexane extract for Pseudomonas aeruginosa and Proteus mirabilis respectively. On the other hand, the components of leaf oil of Coleus amboinicus showed a maximum zone of inhibition of 17.00 ± 1.73 and 20.33 ± 0.58 mm for Pseudomonas aeruginosa and 20.67 ± 0.58 and 18.33 ± 1.53 mm for Proteus mirabilis with Eluent 1 and Eluent 2 respectively. The minimum inhibitory concentration (MIC) of the extract was found to be 60 mg/ml for Pseudomonas aeruginosa and 120 mg/ml for Proteus mirabilis. These findings indicate that the plant has a potential for the treatment of infections caused by these two bacteria.
Key words- Coleus amboinicus, clinical isolates, Pseudomonas aeruginosa, Proteus mirabilis, leaf oil, antibacterial effect