Pseudomonas aeruginosa is a notorious opportunistic pathogen best known for causing lung infections in patients with cystic fibrosis and compromised immune systems. Interestingly, this pathogen has the capability of communicating with other bacteria via a phenomenon called quorum sensing. P. aeruginosa produces metabolites, often termed autoinducers, that accumulate during growth. When these metabolites reach a certain concentration, they trigger changes in gene expression in surrounding bacteria. These changes can lead to shifts in metabolism as well as changes in virulence potential (1). Inhibiting quorum sensing to reduce virulence and infection has been of interest to many investigators, including a group in India that has shown the inhibitory effects of cinnamon oil on biofilm formation and virulence factors that are regulated by quorum sensing (2).
Like the spice in your kitchen cabinet, cinnamon oil is derived from the bark and leaves of the true cinnamon tree. Cinnamon oil, of which the most abundant ingredient is cinnamaldehyde, has been shown to exhibit antimicrobial properties and is of interest as an alternative to antibiotics in the wake of antibiotic resistance concerns. In this study by Kalia et al., investigators set out to determine the effects of cinnamon oil on the growth, biofilm formation, and production of virulence factors by P. aeruginosa. Concentrations of cinnamon oil greater than 0.2 µl/ml were shown to reduce the growth rate of P. aeruginosa. The minimum inhibitory concentration for cinnamon oil was 1 µl/ml, which can be compared to an MIC of 8 µg/ml of the antibiotic tobramycin (3). In addition to growth, the production of pyocyanin, an important virulence factor that generates reactive oxygen species, was reduced in the presence of ≥0.2 µl/ml cinnamon oil. Cinnamon oil was also shown to inhibit swarming activity of P. aeruginosa, which is important for colonization. The production of alginate, a component of extracellular polysaccharide important for biofilm stability, was also significantly reduced by the addition of cinnamon oil. Overall, biofilm production was shown to be reduced by 31% in the presence of 0.2 µl/ml cinnamon oil.
While this study provides interesting preliminary findings on the effects of cinnamon oil, it is still unclear whether the effects are truly due to the disruption of quorum sensing or other off target effects. The arguments from this study would be more convincing by the addition of transcriptomic and proteomic data. These data would help determine if genes (and the products they encode) that are known to be regulated by quorum sensing are downregulated in the presence of cinnamon oil. In addition, a mammalian infection model in which cinnamon oil is used as a treatment would also add evidence for quorum sensing inhibition. Testing cinnamon oil in an animal model would be an important point for proving its efficacy against P. aeruginosa, assuming the end goal would be to market cinnamon oil as a therapeutic for human infection.
This study is a great example of investigating natural products to be used against pathogens as an alternative to antibiotics to which bacteria can develop resistance. It is evident that cinnamon oil certainly has an inhibitory effect on the growth, biofilm formation, and production of virulence factors by P. aeruginosa, but further investigation is required before it can be denoted as an inhibitor of quorum sensing. And so, it may be better to take a trip to the doctor’s office instead of swallowing a spoonful of cinnamon.
References:
- Rutherford ST, Bassler BL. Bacterial Quorum Sensing: its role in virulence and possibilities of its control. Cold Spring Harbor Prespect Med. 2012; 2(11): a012427.
- Kalia M, Yadav VK, Singh PK, Sharma D, Pandey H, Narvi SS, et al. (2015) Effect of Cinnamon Oil on Quorum Sensing-Controlled Virulence Factors and Biofilm Formation in Pseudomonas aeruginosa. PLoS ONE 10(8): e0135495. doi:10.1371/journal.pone.0135495
- Ratjen F, Brockhaus F, Angyalosi G, 2009. Aminoglycoside therapy against Pseudomonas aeruginosa in cystic fibrosis: a review. J. Cyst. Fibros. 8, 361e369.
