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Growing consumer demand for high-quality products coupled with minimally processed products and a minor use of synthetic food additives have increased the need to search for new sources of natural antimicrobials to ensure product safety. This study aimed to evaluate the antimicrobial activity of extracts from the brown algae Ericaria selaginoides against Bacillus cereus in typical Catalan fresh cheese (“mató”) by means of challenge testing. Three concentrations of a crude extract and its corresponding two subfractions (non-polar and mid-polar) obtained after purification showed an antimicrobial dose-dependent effect on B. cereus, from inhibition to inactivation. The best results were obtained with higher concentrations of the non-polar subfraction that caused a total inactivation of the inoculated pathogen after 2 or 4 days, followed by the mid-polar that inactivated B. cereus after 2 or 6 days. The results showed an improvement in the antimicrobial effect after purification compared with the effect observed when the crude extract was tested. Moreover, compounds of different chemical natures may be involved in this antimicrobial activity since it remained in both subfractions after purification. The results obtained in this work show the great potential of macroalgae extracts as natural food preservatives against B. cereus in fresh cheese.

[EN] The search for new sources of antimicrobial compounds has become an urgent need, due to the threat that the spread of bacterial resistance represents for global health and food safety. Brown macroalgae have been proposed as a great reservoir in the search for novel antimicrobial compounds. In this study, mid-polarity extracts were performed with a selection of 20 brown macroalgae species from northern Spain. The total polyphenol, carbohydrate and protein contents were quantified by spectrophotometry. The volatile organic compounds (VOCs) of whole macroalgae were also studied as a biomarker of their metabolic state in the representative species of the tested families by gas chromatography-mass spectrometry (GC-MS). The antimicrobial potential of the extracts was assessed by a disk diffusion assay against 20 target bacteria and further determinations of the minimum inhibitory (MIC) and minimum bactericidal concentrations (MBC) were performed by a microdilution assay for the active extracts. Ericaria selaginoides, Bifurcaria bifurcata and Dictyota dichotoma showed an antimicrobial effect against six Gram-positive strains: Bacillus cereus, Bacillus subtilis, Geobacillus stearothermophilus, Listeria monocytogenes, Staphylococcus aureus and Staphylococcus haemolyticus. The phenolic content was generally higher in the extracts that showed antimicrobial activity, followed by carbohydrates and low contents of proteins. The results obtained in this study reveal the potential of brown macroalgae as a promising alternative source of antimicrobial compounds as functional ingredients for the application in industrial fields.

High hydrostatic pressure (HP) processing is used in the food industry to enhance the safety and extend the shelf-life of food. Although a drastic decrease in microbial viability is achieved immediately after the application of HP treatments, under favorable conditions the injured bacteria can recover. The present study evaluated the inactivation and recovery of five strains of Listeria monocytogenes, Salmonella enterica and Staphylococcus aureus subjected to pressures of 400, 600, and 900 MPa under stressing and non-stressing conditions in a complex medium. Treatments at 400 and 600 MPa were found to greatly affect the viability of L. monocytogenes and S. enterica, but only a treatment of 5 min at 900 MPa decreased the levels of the three pathogens to below the detection limit (8–9 log units reduction). After HP treatment, not only the baroresistant S. aureus but also several replicates of L. monocytogenes and S. enterica strains recovered during subsequent incubation under favorable conditions. However, when HP was combined with low pH and nitrite but not with NaCl or lactate, the viability of pressurized S. aureus cells progressively decreased. As pathogenic bacteria can recover even after the application of very high pressure levels, the combination of HP with other hurdles for microbial growth, either intrinsically present in the food product or extrinsically applied, may be needed to guarantee the efficacy of technologies aimed at pathogen reduction and shelf-life extension. [Int Microbiol 2010; 13(3):105-112]

Postprint (published version)

The effects of enterocins A and B (produced by Enterococcus faecium CTC492), lactate, and ethylenediaminetetraacetic acid (EDTA) on the growth of Salmonella were modeled together with temperature using the response surface methodology. Six serovars of Salmonella enterica were inoculated (ca. 103 cells/ml) in brain-heart infusion broth with different levels of the studied factors and then incubated at different temperatures. The results showed that while Salmonella growth was affected by all the factors, temperature was the most important factor influencing the time to detection of the pathogen. All factors, including temperature, showed significant two-way interactions. The presence of enterocins A and B, lactate, and EDTA had an inhibitory effect that was enhanced at suboptimal temperatures for growth, thus delaying the time to detection. Moderate-low concentrations of lactate and EDTA increased the inhibitory effect of enterocins A and B. The effectiveness of these bacteriocins was not further enhanced by high concentrations of lactate (>3.6%) or EDTA(>200 mg/l). The mathematical model obtained from these analyses provides a useful tool to assess the effects of natural antimicrobials and their interactions with other growth-related factors on the growth response of Salmonella. The results can be applied to determine the most effective combination of hurdles to be used in the preservation of food products. [Int Microbiol 2008; 11(1):11-16]