Identification and evaluation of Anisochilus carnosus (L. fil.) Wall. as a novel candidate with therapeutic potential against multi-drug resistant Staphylococcus aureus

Background: Staphylococcus aureus has been constantly emerging with newer drug resistance patterns; vancomycin intermediate S. aureus (VISA) and vancomycin resistant S. aureus (VRSA) being recent entries. Hence the growing quest for newer antibiotics has entered the realm of the plants as they are less explored and usually have negligible side effects. Although a number of medicinal plants are being used traditionally, their spectrum and mechanism of action are not completely deciphered. Objectives: To evaluate the aqueous (the traditionally administered extract) and acetone/ethanol (organic solvents with the highest capacity to extract phytoconstituents) extracts of ten medicinal plants traditionally used to treat respiratory infections in south India for antibacterial activity against S. aureus. Methods: The antibacterial activity against S. aureus standard strain was evaluated by the broth dilution method. The extracts exhibiting antibacterial activity were further validated using 50 S. aureus isolates by the broth dilution and agar diffusion methods. Results: Only Anisochilus carnosus acetone extract (ACace; MIC-0.33 +/- 0.14 mg/mL) exhibited significant antibacterial activity, and its zone diameter break points were determined. The sensitivity of the 50 S. aureus clinical isolates to ACace (72 %) was comparable to methicillin (68 %), linezolid (94 %) and clindamycin (82 %). Additionally, ACace, inhibited majority of the MRSA (87.5 %), VISA (70 %) and VRSA (83 %) isolates. Conclusion: This is the first study to report the antibacterial potential of a novel candidate, ACace against S. aureus, evaluate its inhibitory effect on clinical isolates including drug-resistant strains and highlight the importance of zone diameter break points for the evaluation of medicinal plant extracts.