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This study focus on the main factors that affect the antimicrobial capacity of hydroxytyrosol, including the concentration (200, 400, and 1000 μg/mL), target strains, and the culture media (nutrient-rich and less-rich culture media). The potential HT degradation was also evaluated by HPLC-PAD. Kinetic parameters from growth curves showed that HT concentrations produced a doses-dependent shift when compared to the untreated control. In most of the cases, the highest tested dose (1000 μg/mL) was needed to inhibit growth of the selected strains. However, all the strains were able to grow even at the highest HT dose when cultivated in nutrient-rich culture media. It was observed that HT concentrations were reduced by about 15%, except for Escherichia coli 533 and 679 in Muller Hinton broth, where HT was reduced up to 35%. The results showed a limited antimicrobial activity, contrary to information previously published in some research papers.

Degradation of the quorum-sensing signal molecule N-acyl-L-homoserine lactone (AHL) in cocultures was verified with Bacillus cereus and Yersinia enterocolitica in culture medium and in pork extract. Results showed evidence of microbial interaction when the AHL-degrading bacterium and AHL-producing bacterium were cocultured in a food-simulating condition.The term "quorum sensing" has been proposed to describe the ability of bacteria to monitor their own population density and modulate gene expression accordingly (17). This communication system uses chemical signal molecules called autoinducers, which are produced and released by the bacterial cell. Intracellular response occurs when the concentration of autoinducers rises above the minimum concentration threshold. Some gram-negative food-associated bacteria can produce Nacyl-L-homoserine lactones (AHLs) as signal molecules. Examples include Aeromonas hydrophila, Pseudomonas aeruginosa, and Yersinia enterocolitica (11,12,20,24,26). In these bacteria, a relationship between AHL production and expression of some mechanisms such as biofilm formation, motility, and exoprotease production has been described previously (2,15,17,25).The signal molecule AHL is the key factor in this bacterial communication process. Degradation of the molecule prevents its sufficient accumulation in the immediate vicinity of the bacterial cell and will consequently lead to a disruption of the communication system. In several fields, blocking of quorum sensing by degradation of AHL molecules has been proposed as a promising alternative to diminish bacterial virulence (4,5,6,9,18). In food systems quorum sensing may have a role in food spoilage, the growth and/or toxin production of pathogens present in food, biofilm formation, bacteriocin production, virulence responses, etc. A possible role of some bacteria in biological degradation of AHLs has been suggested (5,6,7,10,13,14,19,22). Most of the reports on quorum-sensing degradation have been focused on plant pathogens (5, 6, 18). Although it is well known that the lactonase gene is widely spread among certain strains of food-borne Bacillus spp., no actual data are available on the interaction of mixed populations of AHL-producing and AHL-degrading bacteria in food-simulating conditions.The aim of the present study was to screen for the AHLdegrading capacity of Bacillus cereus and some other current and former Bacillus spp. and to establish whether degradation capacity is noted in the coculture environment. The AHLdegrading capacity of Bacillus sp. strains was evaluated with both synthetic AHL molecules and AHL produced by Y. enterocolitica. The AHL degradation ability was further assessed in one B. cereus strain inoculated in coculture with a Y. enterocolitica strain in culture medium and pork extract. Screening of synthetic-AHL degradation at 30°C. Twenty B. cereus and 29 other current and former Bacillus isolates were screened for their AHL degradation capacities. N-Hexanoyl-L-homoserine lactone (C 6 -HSL) (Biochemika Sigma-Aldrich) and ...

Cudemos E., Izquier A., Medina-Martínez M.S., Gómez-López V.M. (2013): Effects of shading and growth phase on the microbial inactivation by pulsed light. Czech J. Food Sci., 31: 189-193. Pulsed light is an emerging technology that kills microorganisms using pulses of an intense broad-spectrum light. This work aimed to determine the effect of population density and microbial growth phase on its microbicidal efficacy. To this, pseudomonas fluorescens cultures were grown, diluted to different population densities, flashed, plated, incubated, and enumerated, Also, cultures of p. fluorescens, Bacillus cereus, and saccharomyces cerevisiae were taken at different growth phases, diluted to the same population density, flashed, plated, incubated, and enumerated. Microbial inactivation was lower at high densities, probably as the consequence of the shading effect, and higher at the exponential phase. This study sets the background information useful for scientists and industrial implementation. The population density and growth phase must be taken into account in the planning experiments and comparing the literature. On the industrial scale, heavily contaminated solids are not suitable for pulsed light (PL) treatment; while liquids should receive several PL flashes under the flow conditions that assure that all microorganisms receive a PL treatment, that should be also designed in function of the growth phase of the microbial contaminant.Keywords: UV-C light; non-thermal methods; shading effect; decontamination; inactivation Pulsed light (PL) is a non-thermal emerging technology to decontaminate surfaces and transparent liquids by killing microorganisms using pulses of an intense broad-spectrum light where its UV-C part is the most lethal (Gómez-López et al. 2007).The shading effect is usually cited as a limiting factor of PL efficacy. It occurs when microorganisms are present one upon another, which causes that those at the top can be killed by PL but protect in turn the underlying microorganisms (Gómez-López et al. 2007). Its existence is claimed from the beginning of the use of PL as a germicidal method, however, just for specific microorganisms that readily absorb UV rays such as Aspergillus niger and not as a general limiting factor (Hiramoto 1984). Furthermore, the patent of Hiramoto (1984) also claimed that this phenomenon occurs with the conventional UV lamps and is overcome by the photothermal effect of PL. On the other side, Farrell et al. (2010) reported a population density effect in the inactivation curves of staphylococcus aureus and pseudomonas aeruginosa, where tailing was only observed at very high initial sizes.The susceptibility of microorganisms to inactivation by physical methods depends on several factors, which depend in turn on the specific method. The effect of the growth phase of the microor- 190Vol. 31, 2013, No. 2: 189-193 Czech J. Food Sci. PL equipment. PL processing was performed using a XeMaticA-1XL system (SteriBeam GmbH, Kehl, Germany), which had been described previously (I...