| Summary: | The emergence of multidrug-resistant microorganisms represents a global challenge that has led to a search for new antimicrobial compounds. Essential oils (EOs) from medicinal aromatic plants are a potential alternative for conventional antibiotics. In this study, the antimicrobial and anti-biofilm potential of 15 EOs was evaluated on planktonic and biofilm-associated cells of <i>Salmonella enterica</i> serovar Enteritidis ATCC 13076 (<i>S. enteritidis</i>) and <i>Salmonella enterica</i> serovar Typhimurium ATCC 14028 (<i>S. typhimurium</i>). In total, 4 out of 15 EOs showed antimicrobial activity and 6 EOs showed anti-biofilm activity against both strains. The EO from the <i>Lippia origanoides</i> chemotype thymol-carvacrol II (LTC II) presented the lowest minimum inhibitory concentration (MIC<sub>50</sub> = 0.37 mg mL<sup>−1</sup>) and minimum bactericidal concentration (MBC = 0.75 mg mL<sup>−1</sup>) values. This EO also presented the highest percentage of biofilm inhibition (>65%) on both microorganisms, which could be confirmed by scanning electron microscopy (SEM) images. Transcriptional analysis showed significant changes in the expression of the genes related to quorum sensing and the formation of the biofilm. EOs could inhibit the expression of genes involved in the quorum sensing mechanism (<i>luxR, luxS</i>, <i>qseB</i>, <i>sdiA</i>) and biofilm formation (<i>csgA</i>, <i>csgB,</i> <i>csgD</i>, <i>flhD</i>, <i>fliZ</i>, and <i>motB</i>), indicating their potential use as anti-biofilm antimicrobial agents. However, further studies are needed to elucidate the action mechanisms of essential oils on the bacterial cells under study.
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