Antimicrobial activity of essential oils against Vancomycin-Resistant enterococci (VRE) and Escherichia coli O157: H7 in feta soft cheese and minced beef meat

Eleven essential oils (EOs) were evaluated for their antibacterial properties, against Vancomycin-Resistant Enterococci (VRE) and E. coli O157:H7. EOs were introduced into Brain Heart Infusion agar (BHI) (15ml) at a concentration of 0.25 to 2 percent (vol/vol) to determine the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) for each pathogen evaluated. Results showed that the most active essential oils against bacteria tested were thyme oil, with MIC90 and MBC90 for the VRA strains of 0.25 percent and 0.5 percent, respectively. Eucalyptus, juniper and clove oils were the least potent agent, with MIC90 and MBC90 of 2 percent. Furthermore, the inhibitory effect of these EO were evaluated against VRE and E. coli O157:H7, experimentally inoculated (10³ cfu/g) in Feta soft cheese and minced beef meat, which was mixed with different concentrations (0.1 percent, 0.5 percent and 1 percent) of the EO and stored at 7 ºC for 14 days. Out of eucalyptus, juniper, mint, rosemary, sage, clove and thyme oils tested against target bacteria sage and thyme showed the best results. Clove and mint did not show any effect on VRE and E. coli O157:H7 in both kinds of studied foods. The addition of thyme oil at concentrations of 0.5 and 1 percent caused best significant reduction in the growth rate of VRE and E. coli O157:H7 in cheese and meat at 7 ºC. It is concluded that selected plant EOs can act as potent inhibitors of both microorganisms in a food product. The results revealed the potential of thyme oil as a natural preservative in feta soft cheese and minced beef meat against VRE and E. coli O157:H7 contamination.

Capacidad de formación de biopelículas de cepas de Listeria monocytogenes aisladas a partir de queso tierno de origen costarricense / Biofilm formation capacity of Listeria monocytogens strains isolated from soft cheese from Costa Rica

Listeria monocytogenes, además de ser un género capaz de producir una enfermedad infecciosa grave en el hombre, puede formar biopelículas en distintas superficies relacionadas con el ambiente de producción alimentario. Éstas constituyen un serio problema debido a que son una fuente importante y constante de contaminación para los alimentos y el ambiente de producción, además de que las bacterias presentes en ellas poseen una aumentada resistencia hacia agentes físicos y químicos de uso frecuente. En el presente trabajo se estudió la capacidad de formación de biopelícula de cepas de L. monocytogenes previamente aisladas a partir de queso tierno bajo diferentes condiciones de temperatura y cultivo. Se utilizó una técnica de microplaca con diferentes medios de cultivo (CICC, CTS 1:20 y suero de queso) a diferentes temperaturas de incubación (refrigeración, ambiente y 35ºC). La capacidad de formación de biopelícula fue clasificada según la densidad óptica obtenida a 620 nm. Ninguna de las cepas evaluadas fue clasificada como formadora fuerte de biopelicula bajo ninguna de las variables estudiadas, sí se detectaron formadoras débiles y moderadas. Los resultados obtenidos ponen de manifiesto la influencia del contenido de nutrientes en el medio de cultivo sobre la formación de biopelícula, no obstante, el CICC fue el único medio que permitió la expresión de formadores moderados. Por el contrario, el suero de queso resultó poco favorecedor. La formación de biopelícula es un proceso multifactorial, donde el nivel de adsorción depende de gran cantidad de variables y cuyo estudio debe fomentarse, de manera que se desarrollen metodologías que permitan su reducción o eliminación, de manera que las industrias alimentarias aseguren productos inocuos y de buena calidad microbiológica.

Listeria monocytogenes is a bacteria associated with the production of severe infectious disease in human being, but also with the formation of biofilms in different surfaces related to the food production environment. Biofilm represents a serious problem in food industry, since it is a constant and important contamination source and also, bacteria present in it have an increased resistance towards physical and chemical agents of common use. The capacity of biofilm formation of L. monocytogenes strains previously isolated from soft cheese samples from Costa Rica was studied under different temperature and culture conditions. The microplate technique was performed using different culture media (BHIB, TSB 1:20 and cheese serum) and at different incubation temperatures (refrigeration, environmental and 35ºC). Biofilm formation capacity was classified according to the optical density obtained at 620nm. None of the strains evaluated was classified as strong biofilm former under any of the variables studied, nevertheless, weak and moderate formers were detected. The results obtained show the influence of the nutrient content of the culture media used over biofilm formation; BHIB was the only culture media that allowed the expression of moderate biofilm forms, contrary to cheese serum that did not promote biofilm production. Biofilm formation is a multifactorial process, where adsorption level depends on several variables and its study must be promoted in order to develop methodologies that allow its reduction or elimination, so food industries may offer safe food products to consumers.