Effect of cinnamon (Cinnamomum verum) bark essential oil on the halitosis-associated bacterium Solobacterium moorei and in vitro cytotoxicity

Abstract

Objectives: Halitosis, also known as bad breath or oral malodour, is a condition affecting a large proportion of the population. Solobacterium moorei is a Gram-positive anaerobic bacterium that has been specifically associated with halitosis. In this study, we investigated the effects of essential oils, more particularly cinnamon bark oil, on growth, biofilm formation, eradication and killing, as well as hydrogen sulfide (H₂S) production by S. moorei.

Methods: A broth microdilution assay was used to determine the antibacterial activity of essential oils. Biofilm formation was assessed by a crystal violet staining assay and scanning electron microscopy. The biofilm of S. moorei was characterized by enzymatic treatments. Biofilm killing was determined by a luminescence assay monitoring ATP production. H₂S production was quantified with a colorimetric assay. The biocompatibility of cinnamon oil was investigated using a gingival keratinocyte cell line.

Results: Among the ten essential oils tested, cinnamon oil was found to be the most powerful against S. moorei with MIC and MBC values of 0.039% and 0.156%, respectively. The biofilm formed by S. moorei was then characterized. The fact that DNase I and to a lesser extent proteinase K significantly reduced biofilm formation by S. moorei and induced its eradication suggests that the extracellular matrix of S. moorei biofilm may be mainly containing a DNA backbone associated with proteins. At concentrations below the MIC, cinnamon oil reduced S. moorei biofilm formation that resulted from an attenuation of bacterial growth. It was also found that treatment of a pre-formed biofilm of S. moorei with cinnamon oil significantly decreased its viability although it did not cause its eradication. Cinnamon oil had an inhibitory effect on the production of H₂S by S. moorei. Lastly, it was found that at concentrations effective against S. moorei, no significant loss of viability in gingival keratinocytes occurred after a 1-h exposure.

Conclusions: Our study brought evidence that cinnamon oil may be a promising substance to incorporate into oral hygiene products for controlling bad breath by inhibiting growth, killing biofilm, and reducing H₂S production by S. moorei. Moreover, at the effective concentrations, cinnamon oil was found to have no toxic effects on oral keratinocytes.

Keywords: Biofilm; Cinnamon; Essential oil; Halitosis; Hydrogen sulfide; Solobacterium moorei; Volatile sulfur compounds.