**Abstract**

The World Health Organization highlighted the increase in the resistance to conventional antibiotics for most pathogens and observed also a decrease of the threshold for all mechanisms of cell-cell microbial communication, leading to the formation of biofilms and to the increase of microbial pathogenicity. Scientific community is therefore oriented to the identification and study of alternative substances to antibiotics. In such context, substances of vegetal source, such as essential oils (EOs), always used in traditional medicine, stimulated—particularly in recent decades—the scientific world to discover and identify substances, intended as a mixture or single components capable to fight pathogenic microorganisms. From this point of view, the study of plants is very interesting and offers many interesting ideas and results. This brief chapter describes the basis of the microbial communication, until the formation of biofilm, and some mechanisms through which essential oils, or some of their main components, may decrease or inactivate the complex mechanisms that lead to pathogenicity, both of prokaryotes and eukaryotes.

**Keywords:** bacterial resistance, quorum sensing, biofilm, essential oils

#### **1. Bacterial resistance**

In recent years, the World Health Organization repeatedly highlighted with alarm the problem of an increase in the resistance of most pathogens to conventional antibiotics. Several causes determined such alarming picture, not least the lack of availability on the market of "new" molecules (given the low economic appealing that such a study raises on the pharmaceutical industries). Wrong behaviors on the part of man are also included among the recurring causes, such as an unjustified abuse of antibiotics [1], as well as possible incorrect medical prescriptions of the drug or the duration of antibiotic treatment. An extensive and indiscriminate use of antibiotics also in agriculture and livestock breeding can cause an indirect contribution to antibiotic resistance also in humans indeed. The often inconsiderate use of broad-spectrum antibiotics could indiscriminately reduce also the number of the so-called "commensal" microorganisms, favoring the onset of diseases more serious than those for which the use of the drug was initially required, varying and consequently altering the relationship between

microorganism and host. Microorganisms that generally do not cause diseases in their natural habitats, due to this new environmental situation, can become highly pathogenic. Normal constituents of the intestinal flora, such as *Escherichia coli*, may therefore become harmful in other districts, such as the urinary bladder, spinal cord, lungs, etc. Other microorganisms can become highly pathogenic under certain conditions: for instance, *Streptococcus viridans* physiologically present in the oropharyngeal tract, in some circumstances can invade different organs through the bloodstream, causing serious diseases (e.g., bacterial endocarditis). Today, there is much talk about the so-called "multidrug-resistant" (MDR), "extensively drugresistant" (XDR), and "pan-drug-resistant" (PDR) strains. Such microorganisms can be figuratively enclosed in a cluster comprising pathogens of infections that are today intractable [2]. Unfortunately, it is also difficult to fight those pathogens belonging to the so-called "ESKAPE" group, an acronym comprising the microbial species *Enterococcus faecium*, *Staphylococcus aureus*, *Klebsiella pneumoniae*, *Acinetobacter baumannii*, *Pseudomonas aeruginosa*, and *Enterobacter* spp. It is widely believed by the scientific community that the study of alternative strategies to the use of conventional antibiotics could represent an important way to be taken into consideration, to combat this dangerous situation. The use of bacteriophages in phage therapies, known for their high specificity, the development of new vaccines against *P. aeruginosa* [3] and *A. baumannii* [4], and the use of strategies to inhibit the bacterial virulence factors can be considered some of the solutions. Recently, research also focused on the exploitation and identification of new microorganisms, isolated, for example, from the ground, enabling to block the microbial growth of one or more species belonging to the ESKAPE group [5].
