Anti-virulence activities of selected culinary herbs against Pseudomonas aeruginosa

Abstract

Pseudomonas aeruginosa is a Gram-negative opportunistic bacterium that causes severe infections such as cystic fibrosis (CF), skin infections, and urinary tract infections (UTI), especially in immunocompromised patients. The pathogen relies on its virulence factors that get activated following a successful entry inside the host, resulting in diseases causing infections. Conventional techniques to battle this pathogen include the use of antibiotics. However, these antibiotics have proven to cause an increased resistance of the pathogen, which drives the need to come up with other means of fighting emerging multi-resistant pathogenic strains of P. aeruginosa. Alternatively, microbial pathogenicity can be attenuated by disturbing the communication system between bacteria, known as quorum sensing (QS). Therefore, to manage these bacterial infections, a novel strategy of anti-quorum sensing (AQS) could be used, as it exerts no selective pressure on bacterial cells, but only disrupts the bacterial communication. This study aimed to determine antivirulence properties of selected culinary herbs against Pseudomonas aeruginosa. The seven herbs, Apium graveolens, Coriandrum sativum, Rosmarinus officinalis, Salvia africana lutea, Salvia officinalis, Syzygium aromaticum and Thymus vulgaris were selected based on their reported health benefits they possess against diseases and ailments. The seven herbs were extracted with solvents of different polarities (water, hexane, ethyl acetate, methanol and dichloromethane), thereafter and determining minimum inhibitory concentrations (MIC) by microdilution assay against Pseudomonas aeruginosa. The Chromobacterium violaceum (biomonitor strain), which produces the purple violacein, was used for qualitative and quantitative AQS activities using the active extracts. The antivirulence assays (biofilm formation, elastase and pyocyanin productions, and twitching, swimming and swarming motility) of active extracts were evaluated against P. aeruginosa. Additionally, a chemical profiling of the active extracts was performed using gas chromatography coupled with mass spectrometry (GCMS) to identify possible compounds potentially responsible for the desired activities. Eleven extracts of Salvia officinalis (hexane and DCM), Rosmarinus officinalis (hexane and aqueous), Syzygium aromaticum (ethyl acetate, methanol, and DCM), Coriandrum sativum (ethyl acetate, methanol, hexane and DCM) displayed noteworthy MIC values of 1.00 mg/ml against P. aeruginosa. Moreover, the qualitative AQS activity revealed only two hexane extracts of Syzygium aromaticum and Coriandrum sativum potentially possess some AQS potential with inhibition zones of 9.00 mm and 16.00 mm at sub-inhibitory MIC (0.35 mg/ml), respectively. Quantitatively, C. sativum (hexane) extract showed higher percentage inhibition of 69% while and S. aromaticum (hexane) showed least AQS activity of 24% at 0.50 mg/ml. Only AQS active hexane extracts of S. aromaticum and C. sativum were subjected testing against virulence factors of P. aeruginosa. The two extracts of C. sativum and S. aromaticum (hexane) demonstrated percentage high biofilm inhibitions of 70.3% and 80.1% at 1.00 mg/ml, respectively. Furthermore, P. aeruginosa showed susceptibility with reduced twitching motility when treatment with 1.00 mg/ml of the extracts C. sativum hexane (36%) and S. aromaticum (43%). C. sativum hexane extract also reduced the swarming motility of Pseudomonas aeruginosa by 35% at 1.00 mg/ml. Whereas the hexane extract of S. aromaticum displayed 59% inhibition for elastase production. From the hexane extracts of S. aromaticum and C. sativum, 27 compounds were identified using GC-MS. Of the 27 compounds belonging to different classes of chemicals, some are known to possess a potential in inhibiting Pseudomonas sp at the quorum sensing level, such as Tetradecanoic acid, linoleic acid as well as phytol. This study validated and highlighted the herbs of S. aromaticum, and C. sativum possess antibacterial activities and antivirulence properties that could be used in attenuation of resistant P. aeruginosa, thereby used in management of its infections