Essential oils

Plant-based essential oils are primarily a species specific complex mixture of monoterpenoids (such as borneol, camphor, carvacrol, eucalyptol, limonene, pinene, thujone), sesquiterpenoids (such as caryophyllene, humulene) and flavonoids (such as cinnamaldehyde and other phenolic acids). For example, a 24 h old aureusbiofilm on steel was reduced from 10⁷ CFU/mL to 10³ CFU/mL using a Cinnamomum cassia essential oil microemulsion (very rich in cinnamaldehyde) at 2.5% in TSB medium for a 90 min period. A similar CFU reduction was obtained with a 5% essential oil microemulsion of Salvia officinalis, which is rich in thujone, camphor and pinene.

Biofilms from three important Gram-negative pathogens, enterica, E. coli, and P. aeruginosa, were also reduced up to 80% using extracts at 50 μg/mL from the Asian medicinal plants Holarrhena antidysenterica and Andrographis paniculata. In this case, the effect was due to the damage that the essential oil cinnamaldehyde component caused to the bacterial cell membrane.

Cronobacter sakazakiiis also a biofilm producer strain and citral (the main component of lemongrass oil) was proven as an anti-adhesion and anti-biofilm compound. Citral decreased virulence factors and reduced the biosynthesis of flagella in this pathogen. It also interfered with this pathogen’s cell-to-cell signalling, diminishing its virulence and its biofilm formation.

The huge chemical diversity of plant-based essential oils allows for the development of à la carte anti-biofilm preparations against various pathogens. Essential oils from Origanum vulgare show anti-adhesion properties, which are crucial in preventing food spoilage and foodborne pathogens. For example, carvacrol, a monoterpene from oregano essential oil, can be used in vinegars, fresh juices, minced meat and other foods, where it is effective in inhibiting Clostridium perfringens and enterica development. This terpenoid was evenly mixed at 1% in plastic films intended for food applications, inhibiting bacterial growth and biofilm development. In particular, Sicilian oregano (O. vulgare ssp. hirtum) essential oil could potentially prevent or eradicate biofilms in the food industry. Oregano and thyme oils also showed a highly efficient eradication of diverse strains and serotypes of L. monocytogenes biofilms on polystyrene and stainless steel surfaces. Carvacrol is also very effective against L. monocytogenes and S. aureus biofilms. Similarly, thymol, another monoterpenoid from Thymus vulgaris oil, was able to reduce L. monocytogenes biofilms at diverse temperatures (37°C, 25°C, 4°C), as well as S. enterica biofilms. All these examples show the important anti-biofilm activity of essential oil components on diverse pathogens biofilms, although some of these essential oils have been defined by EFSA as potential irritants to skin and other human organs, and therefore are considered as bioactive plant extracts.