Antibacterial activity of Cymbopogon species essential oils against Xanthomonas citri and their use in post-harvest treatment for citrus canker management.

Citrus canker is a disease caused by the gram-negative bacterium Xanthomonas citri subp. citri (X. citri), which affects all commercially important varieties of citrus and can lead to significant losses. Fruit sanitization with products such as chlorine-based ones can reduce the spread of the disease. While effective, their use raises concerns about safety of the workers. This work proposes essential oils (EOs) as viable alternatives for fruit sanitization. EOs from Cymbopogon species were evaluated as to their antibacterial activity, their effect on the bacterial membrane, and their ability to sanitize citrus fruit. The in vitro assays revealed that the EOs from C. schoenanthus and C. citratus had a lower bactericidal concentration at 312 mg L-1, followed by 625 mg L-1 for C. martini and C. winterianus. Microscopy assay revealed that the bacterial cell membranes were disrupted after 15 min of contact with all EOs tested. Regarding the sanitizing potential, the EOs with higher proportions of geraniol were more effective in sanitizing acid limes. Fruit treated with C. shoenanthus and C. martini showed a reduction of ∼68% in the recovery of viable bacterial cells. Therefore, these EOs can be used as viable natural alternatives in citrus fruit disinfection.

Antibacterial Activity of Isobavachalcone (IBC) Is Associated with Membrane Disruption.

Isobavachalcone (IBC) is a natural prenylated chalcone with a broad spectrum of pharmacological properties. In this work, we newly synthesized and investigated the antibacterial activity of IBC against Gram-positive, Gram-negative and mycobacterial species. IBC was active against Gram-positive bacteria, mainly against Methicillin-Susceptible Staphylococcus aureus (MSSA) and Methicillin-Resistant Staphylococcus aureus (MRSA), with minimum inhibitory concentration (MIC) values of 1.56 and 3.12 µg/mL, respectively. On the other hand, IBC was not able to act against Gram-negative species (MIC > 400 µg/mL). IBC displayed activity against mycobacterial species (MIC = 64 µg/mL), including Mycobacterium tuberculosis, Mycobacterium avium and Mycobacterium kansasii. IBC was able to inhibit more than 50% of MSSA and MRSA biofilm formation at 0.78 µg/mL. Its antibiofilm activity was similar to vancomycin, which was active at 0.74 µg/mL. In order to study the mechanism of the action by fluorescence microscopy, the propidium iodide (PI) and SYTO9 fluorophores indicated that IBC disrupted the membrane of Bacillus subtilis. Toxicity assays using human keratinocytes (HaCaT cell line) showed that IBC did not have the capacity to reduce the cell viability. These results suggested that IBC is a promising antibacterial agent with an elucidated mode of action and potential applications as an antibacterial drug and a medical device coating.