Abstract
Every year, the pharmaceutical and food industries produce over 1000 tons of essential oils (EOs) exploitable in different fields as the development of eco-friendly and safe antimicrobial inhibitors. In this work we investigated the potential of some EOs, namely Cinnamomum verum, Cymbopogon martini, Cymbopogon citratus and Cymbopogon flexuosus, on the growth, biofilm formation and gene expression in four strains of enterohemorrhagic Escherichia coli O157:H7. All EOs were analyzed by gas chromatography-mass spectrometry (GC-MS). The antimicrobial activity was performed by using dilutions of EOs ranging from 0.001 to 1.2% (v/v). Subinhibitory doses were used for biofilm inhibition assay. The expression profiles were obtained by RT-PCR. E. coli O157:H7 virulence was evaluated in vivo in the nematode Caenorhabditis elegans. All EOs showed minimal inhibitory concentrations (MICs) ranging from 0.0075 to 0.3% (v/v). Cinnamomum verum bark EO had the best activity (MIC of 0.0075% (v/v) in all strains) while the C. verum leaf EO had an intermediate efficacy with MIC of 0.175% (v/v) in almost all strains. The Cymbopogon spp. showed the more variable MICs (ranging from 0.075 to 0.3% (v/v)) depending on the strain used. Transcriptional analysis showed that C. martini EO repressed several genes involved in biofilm formation, virulence, zinc homeostasis and encoding some membrane proteins. All EOs affected zinc homeostasis, reducing ykgM and zinT expression, and reduced the ability of E. coli O157:H7 to infect the nematode C. elegans. In conclusion, we demonstrated that these EOs, affecting E. coli O157:H7 infectivity, have a great potential to be used against infections caused by microorganisms.
Highlights
The results emerged from this study demonstrated that essential oils (EOs) can inhibit bacterial growth and reduce the ability to form a biofilm
The EOs from C. verum and Cymbopogon species demonstrated to be effective against several pathogen E. coli O157:H7 strains
C. martini EO interferes with the expression of genes that are directly or indirectly responsible for the formation of biofilm and genes implicated in the pathogenesis
Summary
Bacterial infections are becoming a serious healthcare challenge because of the increased dissemination of multi-drug resistant bacteria. In the European Union multidrug resistant infections are responsible for approximately 25,000 patient deaths per year [1]. This increasing resistance of microorganisms to conventional drugs has induced scientists to search for novel substances with antimicrobial activity and with possible minor sideeffects. The persistence and the resistance of bacteria to disinfection are often associated with bacterial ability of aggregating to form a biofilm, a complex multicellular community of microorganisms. Biofilm is the predominant lifestyle of bacteria in all environments [2], and is more resistant to antibiotics and disinfectants [3,4]
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