Panagiotis N. Skandamis

A study of the minimum inhibitory concentration and mode of action of oregano essential oil, thymol and carvacrol.

Aims: The minimum inhibitory concentration (MIC) of oregano essential oil (OEO) and two of its principle components, i.e. thymol and carvacrol, against Pseudomonas aeruginosa and Staphylococcus aureus was assessed by using an innovative technique. The mechanism of action of the above substances was also investigated.

Development and Evaluation of a Model Predicting the Survival of Escherichia coli O157:H7 NCTC 12900 in Homemade Eggplant Salad at Various Temperatures, pHs, and Oregano Essential Oil Concentrations

ABSTRACT Homemade eggplant salad, a traditional Greek appetizer, was inoculated with Escherichia coli O157:H7 NCTC 12900 supplemented with different concentrations of oregano essential oil (0.0, 0.7, 1.4, and 2.1% [vol/wt]) and stored at different temperatures (0, 5, 10, and 15°C). The products pH was adjusted to 4.0, 4.5, or 5.0 with lemon juice. For each combination of the environmental factors, the bacterial counts were modeled, using the Baranyi model, as a function of time to estimate the kinetic parameters of the pathogen. A reduction of more than 1 log unit in E. coli O157:H7 counts was observed in all cases, and the death rate depended on the pH, the storage temperature, and the essential oil concentration. Separate quadratic models were developed with natural logarithms of the shoulder period and death rate as estimated by the growth model, as a function of temperature, pH, and oregano essential oil concentrations. These were further used to predict the population of E. coli O157:H7 NCTC 12900 from other inoculated eggplant salads at random conditions of temperature, pH, and oregano oil concentration. The predicted values were compared with viable-count measurements for validation.

Disinfectant test against monoculture and mixed‐culture biofilms composed of technological, spoilage and pathogenic bacteria: bactericidal effect of essential oil and hydrosol of Satureja thymbra and comparison with standard acid–base sanitizers

Aims:  To assess the antimicrobial action of three natural‐derived products (essential oil, decoction and hydrosol of Satureja thymbra) against biofilms, composed of useful, spoilage and pathogenic bacteria (formed as monoculture or/and mixed‐culture), and to compare their efficiency with three standard acid and alkaline chemical disinfectants.

Survival of Escherichia coli O157:H7 during the fermentation of Spanish-style green table olives (conservolea variety) supplemented with different carbon sources.

Olives were supplemented with two different carbon sources (sucrose and glucose) at five different levels (0%, 0.1%, 0.3%, 0.5% and 1%) and the fermentation procedure was followed after inoculation with or without starter culture. Inhibition of Escherichia coli O157:H7 was evident in all fermentation procedures regardless of the treatment used. The pathogen numbers declined but did not die out during fermentation. It was evident that the rate of death was higher in samples supplemented with starter cultures compared to natural fermentation. The production of acids during fermentation seems to be the main factor that governs the behaviour of this pathogen under such stress conditions. The HPLC analysis revealed that lactate, formate and acetate were among the end-products during the fermentation of green olives.

Quorum Sensing in the Context of Food Microbiology

ABSTRACT Food spoilage may be defined as a process that renders a product undesirable or unacceptable for consumption and is the outcome of the biochemical activity of a microbial community that eventually dominates according to the prevailing ecological determinants. Although limited information are reported, this activity has been attributed to quorum sensing (QS). Consequently, the potential role of cell-to-cell communication in food spoilage and food safety should be more extensively elucidated. Such information would be helpful in designing approaches for manipulating these communication systems, thereby reducing or preventing, for instance, spoilage reactions or even controlling the expression of virulence factors. Due to the many reports in the literature on the fundamental features of QS, e.g., chemistry and definitions of QS compounds, in this minireview, we only allude to the types and chemistry of QS signaling molecules per se and to the (bioassay-based) methods of their detection and quantification, avoiding extensive documentation. Conversely, we attempt to provide insights into (i) the role of QS in food spoilage, (ii) the factors that may quench the activity of QS in foods and review the potential QS inhibitors that might “mislead” the bacterial coordination of spoilage activities and thus may be used as biopreservatives, and (iii) the future experimental approaches that need to be undertaken in order to explore the “gray” or “black” areas of QS, increase our understanding of how QS affects microbial behavior in foods, and assist in finding answers as to how we can exploit QS for the benefit of food preservation and food safety.

Modelling the combined effect of temperature, pH and aw on the growth rate of Monascus ruber, a heat-resistant fungus isolated from green table olives

Aims: Growth modes predicting the effect of pH (3·5–5·0), NaCl (2–10%), i.e. aw (0·937–0·970) and temperature (20–40°C) on the colony growth rate of Monascus ruber, a fungus isolated from thermally‐processed olives of the Conservolea variety, were developed on a solid culture medium.

Study of the effect of lethal and sublethal pH and aw stresses on the inactivation or growth of Listeria monocytogenes and Salmonella Typhimurium

During food processing, microorganisms are commonly exposed to multiple sublethal or lethal stresses (commonly a(w), pH) sequentially or simultaneously. The objectives of the present study were: (i) to comparatively evaluate the survival of Listeria monocytogenes and Salmonella Typhimurium in lethal acid (pH 4.0 and 4.5 with lactic acid) or osmotic conditions (15 and 20% NaCl), applied singly, sequentially (pH then NaCl or NaCl then pH), or simultaneously at 5 and 10 degrees C; and, (ii) to quantify the effect of osmotic shifts at pH 7.0, 6.0 or 5.0 (adjusted with lactic acid) on the lag phase and growth rate of L. monocytogenes at 10 degrees C. In sequential lethal stress applications, the second stress was applied 2 or 3 days after the first for Salmonella and L. monocytogenes, respectively. Acid tolerance of L. monocytogenes was higher than osmotic tolerance and the opposite was observed for Salmonella. Higher inactivation was observed after exposure to pH 4.0 compared to pH 4.5 as well as after exposure to 20% NaCl compared to 15% NaCl. Exposure to stresses sequentially resulted in faster (P<0.05) reductions than the exposure to single or double stresses applied simultaneously. The pH then NaCl sequence was more detrimental for pathogens than the reverse sequence. Incubation temperature (5 and 10 degrees C) did not show any profound (P<0.05) effect on microbial inactivation. When L. monocytogenes was incubated at a(w) 0.930 or 0.995 at 30 degrees C, then the lag phase increased both in subsequent osmotic downshift and upshift, respectively, at 10 degrees C. Shorter lag phase and higher ability to initiate growth at lower a(w) was observed after pre-adaptation at pH 6.0 or 5.0 compared to neutral pH. The results may contribute to the review of critical limits in low pH (with lactic acid) and water activity products, considering the risk of L. monocytogenes and Salmonella survival. In addition, the present indications may address the points in processing where stricter sanitation procedures should be applied in order to minimize the risk of survivors.

Effect of NaCl and KCl on fate and growth/no growth interfaces of Listeria monocytogenes Scott A at different pH and nisin concentrations

Aims:  The fate of Listeria monocytogenes Scott A, was studied in broth, at different aws (by adding NaCl or KCl from 0·0 to 1·4 mol l−1), pHs (from 4·0 to 7·3 by adding lactic acid), and nisin concentrations (from 0 to 100 IU ml−1).

Efficiency of different sanitation methods on Listeria monocytogenes biofilms formed under various environmental conditions

The resistance of Listeria monocytogenes biofilms formed under food processing conditions, against various sanitizing agents and disinfection procedures was evaluated in the present study. The first sanitation procedure included biofilm formation on stainless steel coupons (SS) placed in tryptic soy broth supplemented with 0.6% yeast extract (TSBYE) of various concentrations of NaCl (0.5, 7.5 and 9.5%) at different temperatures (5 and 20 °C). The biofilms formed were exposed to warm (60 °C) water for 20 min, or to peroxyacetic acid (2% PAA) for 1, 2, 3 and 6 min. Treatment with warm water caused no significant (P ≥ 0.05) reductions in the attached populations. Conversely, surviving bacteria on SS coupons decreased as the exposure time to 2% PAA increased and could not be detected by culture after 6 min of exposure. Biofilms formed at 20°C were more resistant to PAA than biofilms formed at 5 °C. Salt concentration in the growth medium had no marked impact on the resistance to PAA. The second sanitation procedure included biofilm formation of nonadapted (NA) and acid-adapted (AA) cells in TSBYE of pH 5.0 and 7.0 (i.e., NA-5.0, NA-7.0 and AA-5.0, AA-7.0) at 4 °C. Coupons bearing attached cells of L. monocytogenes were periodically exposed to chlorine (0.465% Cl(-)), quaternary ammonium compound (1% QAC) and 2% PAA. The resistance of attached cells to QAC, PAA and Cl(-) followed the order: AA-5.0>NA-7.0 ≥ AA-7.0>NA-5.0. The most effective sanitizer was QAC followed by PAA and Cl(-). The results can lead to the development of efficient sanitation strategies in order to eliminate L. monocytogenes from the processing environment. Furthermore, such results may explain the presence of L. monocytogenes after sanitation as a result of cell attachment history.

Listeria monocytogenes Attachment to and Detachment from Stainless Steel Surfaces in a Simulated Dairy Processing Environment

ABSTRACT The presence of pathogens in dairy products is often associated with contamination via bacteria attached to food-processing equipment, especially from areas where cleaning/sanitation is difficult. In this study, the attachment of Listeria monocytogenes on stainless steel (SS), followed by detachment and growth in foods, was evaluated under conditions simulating a dairy processing environment. Initially, SS coupons were immersed in milk, vanilla custard, and yogurt inoculated with the pathogen (107 CFU/ml or CFU/g) and incubated at two temperatures (5 and 20°C) for 7 days. By the end of incubation, cells were mechanically detached from coupons and used to inoculate freshly pasteurized milk which was subsequently stored at 5°C for 20 days. The suspended cells in all three products in which SS coupons were immersed were also used to inoculate freshly pasteurized milk (5°C for 20 days). When SS coupons were immersed in milk, shorter lag phases were obtained for detached than for planktonically grown cells, regardless of the preincubation temperature (5 or 20°C). The opposite was observed when custard incubated at 20°C was used to prepare the two types of inocula. However, in this case, a significant increase in growth rate was also evident when the inoculum was derived from detached cells. In another parallel study, while L. monocytogenes was not detectable on SS coupons after 7 days of incubation (at 5°C) in inoculated yogurt, marked detachment and growth were observed when these coupons were subsequently transferred and incubated at 5°C in fresh milk or/and custard. Overall, the results obtained extend our knowledge on the risk related to contamination of dairy products with detached L. monocytogenes cells.