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Antibacterial and antibiofilm activities of a traditional herbal formula against respiratory infection causing bacteria

The plants, Althaea officinalis, Tilia cordata and Psidium guaja have been used traditionally to treat respiratory infection symptoms. Flowers of A. officinalis and leaves of T. cordata and P. guaja have been used to treat cough, sore throat, catarrh, oral and pharyngeal mucosa irritation. Therefore, this study was designed to examine the antibacterial and antibiofilm effects of these plants individually as well as in combination, as a formula against respiratory infections causing pathogens. The tested pathogens were Extended Spectrum Beta-Lactamase producing Escherichia coli (ESBL), Beta-Lactamase producing Escherichia coli (BL), Beta-Lactamase producing Klebsiella pneumoniae (BL), Beta-Lactamase producing Pseudomonas aeruginosa (BL), Enterobacter cloacae, and Beta-Lactamase producing Staphylococcus aureus (BL). The tested plants were extracted using ethanol and then fractionated using different polarity solvents (hexane, ethyl acetate and water). Disc diffusion and microdilution (Minimum Inhibitory Concentration) methods were used to evaluate the antibacterial activity while the antibiofilm activity was tested using crystal violet assay. The results showed that A. officinalis and T. cordata extracts and fractions exhibited weak antibacterial activity having MIC values ranged from 6.25 to 12.5 mg/mL. P. guaja exhibited moderate antibacterial activity with MIC values ranged from 6.25 to 1.56 mg/mL. Combination between these plants extracts and fractions in equal proportion provides stronger antibacterial (with MIC values ranged from 6.25 to 0.8 mg/mL) and antibiofilm activities (MBIC50 was 0.2 mg/mL). Therefore, this study provides a valuable scientific knowledge to support the use of plants in combination rather than individually.