Quorum sensing (QS) is a bacterial communication mechanism used to express various survival or virulence traits leading to enhanced resistance. Chromobacterium violaceum is a commonly used strain that highlights anti-QS action of bioactive substances. Here, we wanted to see if 12 selected essential oils (EO) could exert anti-QS activity. We measured the sublethal minimal QS inhibitory concentration (MQSIC) by assessing violacein production of C. violaceum along with bacterial growth. To confirm the QS disruption, we also proceed to surface bacterial observations using scanning electron microscopy (SEM). We showed that cis-cis-p-menthenolide extracted and isolated from a plant endemic to occidental Mediterranean Sea islands, Mentha suaveolens ssp. insularis, acts as an inhibitor of violacein production and biofilm formation. Measured MQSIC was much lower than the minimal inhibitory concentration (MIC): 0.10 mg·mL⁻¹ vs. 3.00 mg·mL⁻¹. Moreover, disturbance of QS-related traits was confirmed by the degradation of C. violaceum biofilm matrix. There is a clear structure–activity relationship between cis-cis-p-menthenolide and anti-QS activity. Indeed, its isomer molecule (mintlactone) exerts a poor anti-QS action. These results indicate that inhibition of violacein production and biofilm formation by cis-cis-p-menthenolide might be related to a disruption in the QS mechanism.

In honeys, several molecules have been known for their antibacterial or wound healing properties. Corsican honeys just began to be tested for their antimicrobial activity with promising results on Pseudomonas aeruginosa. So, identification of active molecules and their mode of action was determined. Hydrogen peroxide concentrations were evaluated and, in parallel, the minimal inhibitory concentrations (MIC) values were performed with and without catalase. More, the quantity of phenolic compounds and ORAC assay were measured. Observation of antibacterial action was done using scanning electron microscopy (SEM) followed by plasmidic DNA extraction. MIC values of chestnut grove and honeydew maquis honeys vary between 7 and 8%, showing a strong antimicrobial capacity, associated with a plasmidic DNA degradation. When catalase is added, MIC values significatively increase (25%) without damaging DNA, proving the importance of H₂O₂. This hypothesis is confirmed by SEM micrographies which did not show any morphological damages but a depletion in bacterial population. Although, such low concentrations of H₂O₂ (between 23 μmol l⁻¹ and 54 μmol l⁻¹) cannot explain antimicrobial activity and might be correlated with phenolic compounds concentration. Thus, Corsican honeys seem to induce DNA damage when H₂O₂ and phenolic compounds act in synergy by a putative pro-oxidant effect. Significance and Impact of the Study: We started to determine the antibacterial efficiency of Corsican chestnut grove and honeydew maquis honeys on Pseudomonas aeruginosa. No morphological alteration of the bacterial surface was observed. Antimicrobial action seems to be related to the synergy between hydrogen peroxide and phenolic compounds. The exerted pro-oxidant activity leads to a degradation of P. aeruginosa plasmidic DNA. This is the first study that investigate the primary antibacterial mechanism of Corsican honeys.

Chemoreceptors are usually transmembrane proteins dedicated to the detection of compound gradients or signals in the surroundings of a bacterium. After detection, they modulate the activation of CheA‐CheY, the core of the chemotactic pathway, to allow cells to move upwards or downwards depending on whether the signal is an attractant or a repellent, respectively. Environmental bacteria such as Shewanella oneidensis harbour dozens of chemoreceptors or MCPs (methyl‐accepting chemotaxis proteins). A recent study revealed that MCP SO_1056 of S. oneidensis binds chromate. Here, we show that this MCP also detects an additional attractant (l‐malate) and two repellents (nickel and cobalt). The experiments were performed in vivo by the agarose‐in‐plug technique after overproducing MCP SO_1056 and in vitro, when possible, by submitting the purified ligand‐binding domain (LBD) of SO_1056 to a thermal shift assay (TSA) coupled to isothermal titration calorimetry (ITC). ITC assays revealed a K_D of 3.4 μm for l‐malate and of 47.7 μm for nickel. We conclude that MCP SO_1056 binds attractants and repellents of unrelated composition. The LBD of SO_1056 belongs to the double Cache_1 family and is highly homologous to PctA, a chemoreceptor from Pseudomonas aeruginosa that detects several amino acids. Therefore, LBDs of the same family can bind diverse compounds, confirming that experimental approaches are required to define accurate LBD‐binding molecules or signals.

Essential oils are known to exhibit diverse antimicrobial properties, showing their value as a natural resource. Our work aimed to investigate the primary mode of action of Cuban Lippia graveolens (Kunth) essential oil (EO) against Salmonella enterica subsp. enterica serovar Typhimurium (S. enterica ser. Typhimurium). We assessed cell integrity through various assays, including time-kill bacteriolysis, loss of cell material with absorption at 260 and 280 nm, total protein leakage, and transmission electron microscopy (TEM). The impact of L. graveolens EO on membrane depolarization was monitored and levels of intracellular and extracellular ATP were measured by fluorescence intensity. The minimum inhibitory and bactericidal concentrations (MIC and MBC) of L. graveolens EO were 0.4 and 0.8 mg/mL, respectively. This EO exhibited notable bactericidal effects on treated cells within 15 min without lysis or leakage of cellular material. TEM showed distinct alterations in cellular ultrastructure, including membrane shrinkage and cytoplasmic content redistribution. We also observed disruption of the membrane potential along with reduced intracellular and extracellular ATP concentrations. These findings show that L. graveolens EO induces the death of S. enterica ser. Typhimurium, important information that can be used to combat this foodborne disease-causing agent.

Honey is known for its medicinal uses, with compounds such as hydrogen peroxide or phenolic compounds. For the first time, the antibacterial activity of protected designation of origin (PDO) “Miel de Corse – Mele di Corsica” commercial Corsican honeys was investigated. According to the PDO recommendation, melissopalynological analysis were used to classify 11 Corsican honeys. They have been tested for antimicrobial action on seven foodborne and nosocomial bacterial strains by Minimum Inhibitory Concentration (MIC₉₀) then statistical analyses helped to correlate their action on the different strains. The varietal range of honey samples were obtained by melissopalynological analysis leading to 2: “Spring”, 3 “Chestnut grove”, 1 “Summer maquis”, 2 “Honeydew maquis” and 1 “Autumn maquis” and 2 samples classified as “generic” Corsican honeys. 29 MIC values determined were under 10%. Only Enterococcus faecalis didn’t seem sensitive to the action of the honey. However, “Honeydew maquis”, “generic” Corsican honeys and 2 “Chestnut grove” honeys appeared as the most effective (MIC values ranging from under 5 to 9% on Pseudomonas aeruginosa and Staphylococcus aureus). The antimicrobial activity of PDO “Miel de Corse – Mele di Corsica” honeys demonstrate bacterial growth inhibition on 6 of the 7 bacterial strains. Now, an in-depth study must be performed to identify the mode of action of the active honeys.

Indoor environments may impact human health significantly since microbial diversity made of various bacteria, fungi, and other microorganisms constitutes the habitat’s microbiome. These microorganisms originate from diverse sources, such as the environment, humans, and pets. Understanding such microbial diversity is crucial for assessing related health impacts. The aim of this study was to investigate bacterial and fungal microorganisms from different surfaces in a set of houses on Corsica Island (southeastern France). We conducted sampling and high-throughput sequencing of PCR-amplified microbial DNA from 40 habitats (eight different surfaces tested for each) located on diverse regions of the island. Following nucleic acid recovery, we performed PCR targeting V3–V4 regions of 16S rRNA for bacteria and ITS1 region for fungi. Bioinformatics processing allowed the identification of distinct microbial species and the analysis of their distribution. Our findings highlighted that the most represented bacterial genera in the Corsican homes tested were Ralstonia (16.9%), Staphylococcus (4.8%), Corynebacterium (2.7%), Enhydrobacter (6.8%), and Methylorubrum (6.0%). Regarding fungal diversity, the most common genera identified were Dipodascaceae (21.6%), Rhodotorula (10.9%), Aspergillus (8.9%), Clavispora (7.3%), and Candida (6.8%). Our results also showed differences in microbial composition among the habitats studied, while a higher bacterial and fungal diversity was observed in rural habitats and in those with pets. IMPORTANCE: This study provides valuable insights into the microbial diversity present in indoor environments of Corsican homes, specifically highlighting bacterial and fungal communities on various household surfaces. By identifying the predominant microbial genera and revealing differences linked to habitat characteristics, such as rural settings and pet ownership, this research enhances our understanding of how indoor microbial communities vary.