Extracts from Callistemon rigidus (Myrtaceae) and Gnetum africanum (Gnetaceae) Inhibit the Growth of Selected Bacteria-causing Pneumonia

Bacterial pneumonia involves acute lung parenchyma and alveolar inflammation triggered by various bacteria, leading to fever, cough, dyspnea, and potential complications like sepsis or respiratory failure. The treatment of this infection is increasingly hindered by antibiotic resistance, thus justifying the pressing need to search for effective treatments against bacterial pneumonia. This study aims to evaluate the antimicrobial activity of Gnetum africanum (G. africanum) and Callistemon rigidus (C. rigidus) extracts on selected pneumonia-causing bacteria. Extracts from the bark and flowers of C. rigidus and from the leaves of G. africanum, were obtained by successive maceration using solvents of increasing polarity, including hexane, dichloromethane, ethyl acetate, methanol and water. The inhibitory effects of the as-prepared extracts (20) were assayed against five pneumonia-causing bacteria, including Staphylococcus aureus (two strains), Klebsiella pneumoniae, Pseudomonas aeruginosa and Acinetobacter baumannii. The bacterial (S. aureus HM-468) growth kinetics were evaluated with the most active extract (CrMeOHE, methanol extract of C. rigidus bark). The yields of plant extraction ranged from 0.507 to 18.12 %. The CrMeOHE extract was the most active extract with minimum inhibitory concentrations (MICs) of 62.5, 125 and 500 µg/mL on S. aureus NR-46003, S. aureus HM-468 and A. baumannii NR-17784, respectively. The hexane extract of G. africanum inhibited the growth of S. aureus HM-468 with MIC value of 500 µg/mL. The time-kill kinetics of CrMeOHE extract on S. aureus HM-468 revealed a bactericidal effect at MIC, 2 MIC and 4 MIC. These results suggest that extracts of G. africanum and C. rigidus possess antibacterial activity and could serve as starting points for the discovery of drugs against pneumonia.