Antibacterial activity and antibiotic-potentiating effects of methanol extracts from Ocimum basilicum and Sarcocephalus latifolius against multidrug-resistant Gram-negative bacteria overexpressing efflux pumps

Background: The inappropriate use of antibiotics against bacterial infections leads to increased bacterial drug resistance. In the present study, the effectiveness of methanol extracts from the leaves and flowers of Ocimum basilicum and the roots of Sarcocephalus latifolius against multidrug-resistant (MDR) Gram-negative bacteria overexpressing efflux pumps was assessed. Methods: The antibacterial properties of crude extracts (botanical, with and without phenylalanine arginine β-naphthylamide (PAβN), an inhibitor of the efflux pumps were performed using the liquid microdilution method. The effects of O. basilicum leaf extract on proton pumps H+/ATPases were analyzed using a spectrophotometric method. The study also involved screening the different extracts for phytochemicals using standard methods. Results: The extracts of O. basilicum leaves and flowers and S. latifolius roots were active against at least 80% of the tested bacteria, with excellent activity against E. coli (AG100 and ATCC10536), E. aerogenes (EA3 and EA27), P. stuartii (PS2636 and ATCC29916) and K. pneumoniae KP55 (MICs ranging from 16 µg/mL to 64 µg/mL). At 32 µg/mL, the extract of O. basilicum leaves exhibited an inhibitory effect on the H+-proton pumps/ATPases of E. coli AG100. In the presence of PAβN, an improvement in the activity of the extracts against 100% of the tested bacteria was observed. All extracts enhanced the activity of tetracycline (TET), ciprofloxacin (CIP), cefixime (CFX), imipenem (IMI), levofloxacin (LEV), ceftriaxone (CTX), penicillin (PEN), and ampicillin (AMP) at MIC/2 and MIC/4. The activity improvement factors (AIF) ranged from 2 to 256. Terpenoids, saponins, and phenols were found in all the different extracts, meanwhile, flavonoids and alkaloids were specifically detected in the extract of the leaves of O. basilicum and the roots of S. latifolius. Conclusion: The tested plants, O. basilicum and S. latifolius are important sources of antibacterial substances to combat bacterial infections involving MDR Gram-negative bacteria overexpressing efflux pumps.