Essential oil of Plectranthus amboinicus (Lour.) Spreng: Antimicrobial properties against Escherichia coli and Listeria monocytogenes in vitro and in meat matrix.

Name: Bruno Dutra da Silva
Type: MSc dissertation
Publication date: 24/05/2021

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Jackline Freitas Brilhante de São José External Examiner *

Summary: Essential oils have been investigated for their antimicrobial activity and their potential application as a natural ingredient in food processing. Research reports its application to ensure microbiological quality and safety in replacement of preservatives traditionally used in processed foods. The hamburger is usually sold raw, refrigerated, or frozen, which is the main method to ensure the conservation of this meat product. The addition of essential oils with proven antimicrobial activity represents an alternative to improve the conservation of the hamburger. Plant sources are scientifically investigated, with a wide variety of species whose antimicrobial potential and application in food are being or have not yet been studied. In this context, the antimicrobial activity of essential oil from Plectranthus amboinicus (Lour.) Spreng. (PAEO) was investigated in vitro, and beef hamburgers during refrigerated storage. The phytochemical composition of PAEO was characterized by gas chromatography. Minimum inhibitory (MIC) and bactericidal (MBC) concentrations of PAEO against Escherichia coli, Listeria monocytogenes, Pseudomonas aeruginosa, Salmonella Typhimurium, and Staphylococcus aureus were evaluated. Later, E. coli (Gram-negative) and L. monocytogenes (Gram-positive) were used to verify the antimicrobial activity in vitro, analyzing the parameters time versus death and the antiadhesion activity of PAEO. The beef hamburger was used to evaluate the sub-MIC and MIC concentrations of PAEO against E. coli and L. monocytogenes during storage at 7 °C for 15 days. The major constituents found in the PAEO were thymol (45.64%), p-Cymene (19.46%), &#946;-Myrcene (12.59%), and &#945;-Terpinolene (9.86%). The MIC for the bacteria evaluated was 0.25 mg.mL-1 (S. aureus), 1 mg.mL-1 (P. aeuroginosa), and 2 mg.mL-1 (E. coli, Salmonella Typhimurium and L. monocytogenes). Regarding MBC, the values obtained for E. coli and P. aeruginosa were equal to the MIC, which is indicative of only the bactericidal action of PAEO. For the other bacteria evaluated, the MBC was higher than the MIC, which highlights the distinction of the inhibitory and bactericidal effect at different concentrations. In the time versus death assay for E. coli, PAEO at a concentration of 2 mg.mL-1 showed a reduction in the microbial load with a count <10 CFU.mL-1 in 1 hour of incubation. For L. monocytogenes treated with 4 mg.mL-1, this reduction occurred after 4 hours. For the antiadhesion assay, at sub-inhibitory concentrations of 1 mg.mL-1, there was a reduction in the adhesion of 30.6% for E. coli and 13.7% for L. monocytogenes. At concentrations 8 times lower than MIC (0.25 mg . mL-1) significant effects were also observed. In the assay on the removal of adhered cells, the concentration of 2 mg.mL-1 of PAEO removed 48% of E. coli cells and 60.6% of L. monocytogenes within 24 hours of incubation. In bovine hamburgers, PAEO showed antimicrobial activity lower than that compared to the results of in vitro tests. However, at the concentration of 2 mg.g-1 there was a significant reduction in the count of E. coli and L. monocytogenes from the 6th day on compared to the control treatment, with a reduction in the count up to the 12th day of storage. After the 12th day of storage, an increase in the population of E. coli and L. monocytogenes was observed for all concentrations of PAEO tested. It is noteworthy that at the concentration of 2 mg.g-1 of PAEO this increase was significantly smaller when compared to concentrations of 1 mg. g-1 and E. coli and L. monocytogenes counts in the burger without the addition of PAEO. The results, therefore, confirm the potential application of PAEO as a natural preservative to ensure the microbiological safety of refrigerated hamburgers.

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