Antibacterial Activity of Selected Medicinal Plant Extracts and Their Interactions with Antibiotics against Serratia marcescens and Pseudomonas aeruginosa

The emergence of antimicrobial resistance to Gram-negative isolates has rekindled interest in new approaches that would potentiate the activity of traditional antibiotics, such as plant extracts for medicinal purposes. Antibacterial activity of some selected medicinal plant extracts in tandem with standard antibiotics against Pseudomonas aeruginosa and Serratia marcescens was studied. The aqueous crude extracts of the plants (R.aculeata, C.bursa-pastoris, I.coccinea and P.olerecea) were screened individually as well as in combination with ampicillin, ciprofloxacin, erythromycin and amoxicillin in 1:1 proportions by agar-well diffusion technique. On Serratia marcescens, against which all the extracts exhibited moderate activity with inhibition zones of diameter of 12-18 mm, Randia aculeata and Ixora coccinea displayed significant potential among others, Portulaca oleracea was moderately active while Capsella bursa pastoris showed a negative result. Ciprofloxacin exhibited excellent antibacterial activity (28 mm), whereas the other antibiotics showed resistance. Synergistic effects of combinations were not identified and in some cases the inhibitory zones of some antibacterial agents were found to be decrease partners with antagonistic interactions. For P. aeruginosa, none of plant extracts had antibacterial activities and least effective against them were antibiotics (except ciprofloxacin with inhibition zone diameter = 24 mm). Its antibacterial activity decreased even more when combined with plant extracts. Such results indicate that the selected medicinal plant extracts had no synergistic effect on, and even inhibited, antibacterial activity of antibiotics (something like conventional antibiotic-potentiating effects) against Serratia marcescens and Pseudomonas aeruginosa. The study emphasises the need for systematic assessment of plant-antibiotic interaction and caution in concurrent use of plant extracts with antibiotics without prior determination of their interaction.