Effect of the lactoperoxidase system on Listeria monocytogenes behavior in raw milk at refrigeration temperatures

Influence of treatment and refrigeration time on antimicrobial activity of goat and sheep colostrum

The aim of the studies presented in this research communication was to compare species of origin (goat and sheep) and the effect of treatment (pasteurization at 56, 63 and 72 °C, skimming and curding) and refrigeration time on colostrum antimicrobial activity (AnAc). Two experiments were performed. In experiment 1, twenty-four first milking colostrum samples were obtained (12 goats, 12 sheep) and an aliquot of each sample was subjected to 6 different treatments, control (untreated), pasteurization at 56, 63 and 72 °C, skimming and curding. Colostrum AnAc was tested directly against E. coli using disks in a Petri dish and Enrofloxacin (antibiotic) and saline serum as positive and negative control, respectively. Species had no effect (P > 0.05) on colostrum AnAc, and neither did pasteurization at different temperatures or skimming. However, curding showed the lowest colostrum AnAc (P < 0.05) in both species. In the second experiment, four treatments were assayed, control, pasteurization at 56 and 63 °C and skimming. An aliquot of twelve goat colostrum samples were refrigerated after treatments for 10 d at 4 °C. Colostrum AnAc was measured at 0, 2, 4, 6, 8, and 10 d. A reduction in colostrum AnAc was observed due to refrigeration time. The results suggest that if farmers use frozen colostrum for neonates, the process of curding colostrum or refrigeration at 4 °C longer than 4 d is not recommended.

Control of Listeria monocytogenes in whole milk using antimicrobials applied individually and in combination

Dairy product recalls and dairy-related illnesses are often the result of contamination with Listeria monocytogenes, which can occur throughout the dairy production and supply chains. The use of antimicrobial compounds is one practical approach for controlling pathogen survival and growth in foods. The goal of this study was to use fluid milk as a model system to identify listeristatic or listericidal treatments that show promise for application in fluid milk and for further evaluation in other dairy products (e.g., cheese). Caprylic acid (CA), ε-polylysine (EPL), hydrogen peroxide, lauric arginate (LAE), and sodium caprylate (SC) were added individually or in combination to whole milk inoculated with L. monocytogenes at ˜4 log₁₀ cfu/mL. Samples were stored at 7°C for 21 d, and L. monocytogenes counts were determined weekly. Inhibitory concentrations of LAE (800 mg/L) and EPL (100-400 mg/L), as well as SC and CA (3,200 mg/L each), were identified. The addition of EPL at 800 mg/L reduced L. monocytogenes counts by >3 log₁₀ cfu/mL from initial inoculation levels after 21 d. Addition of hydrogen peroxide to milk reduced counts by >3 log₁₀ cfu/mL from initial inoculation within 24 h (400 and 800 mg/L) or by d 7 (200 mg/L). Although the combinatory treatments of EPL + CA, EPL + LAE, and LAE + SC were characterized as indifferent, EPL + SC worked synergistically to reduce L. monocytogenes populations in milk over 21 d. Overall, these data identify potential antimicrobial treatments to control L. monocytogenes in milk and serve as a foundation for the continued development of antimicrobial controls for L. monocytogenes in dairy products.

Occurrence and growth of Listeria monocytogenes in packaged raw milk

The increased availability of packaged raw drinking milk necessitates the investigation of the occurrence and growth of Listeria monocytogenes in raw milk during distribution and storage. The occurrence of L. monocytogenes in 105 retailed raw milk bottles, 115 bulk tank milk samples, 23 in-line milk filter socks and in 50 environmental samples collected from an on-farm dairy establishment were investigated. Growth of inoculated low-level L. monocytogenes contamination was also investigated in two types of raw milk packaging, namely in 1-litre plastic bottles and 3-litre bag-in-boxes, both stored at three different storage temperatures of 6, 8 and 10°C. The occurrence of L. monocytogenes was higher (4.8%) in bottled raw milk stored until the use-by-date of the package compared to fresh bulk tank milk (1.7%). L. monocytogenes counts were ≤13CFU/ml in bottled raw milk and ≤1CFU/ml in bulk tank milk. L. monocytogenes was not detected in the packaging facility, but occurred very frequently (39%) in the milk filter socks. Subtyping of L. monocytogenes isolates using pulsed-field gel-electrophoresis revealed seven pulsotypes, of which two occurred in multiple samples. Targeted inoculum levels of 1-2CFU/ml yielded L. monocytogenes counts≥100CFU/ml within seven days of storage in 22% of the raw milk packages stored at 6°C, and in all of the raw milk packages stored at 8°C. The frequent occurrence of L. monocytogenes in raw milk and the ability of a low-level L. monocytogenes contamination to grow at refrigeration temperatures highlight the importance of consumer education regarding the appropriate raw milk storage and handling.

N2 Gas Flushing Limits the Rise of Antibiotic-Resistant Bacteria in Bovine Raw Milk during Cold Storage

Antibiotic resistance has been noted to be a major and increasing human health issue. Cold storage of raw milk promotes the thriving of psychrotrophic/psychrotolerant bacteria, which are well known for their ability to produce enzymes that are frequently heat stable. However, these bacteria also carry antibiotic resistance (AR) features. In places, where no cold chain facilities are available and despite existing recommendations numerous adulterants, including antibiotics, are added to raw milk. Previously, N2 gas flushing showed real potential for hindering bacterial growth in raw milk at a storage temperature ranging from 6 to 25°C. Here, the ability of N2 gas (N) to tackle antibiotic- resistant bacteria was tested and compared to that of the activated lactoperoxidase system (HT) for three raw milk samples that were stored at 6°C for 7 days. To that end, the mesophiles and psychrotrophs that were resistant to gentamycin (G), ceftazidime (Ce), levofloxacin (L), and trimethoprim-sulfamethoxazole (TS) were enumerated. For the log10 ratio (which is defined as the bacterial counts from a certain condition divided by the counts on the corresponding control), classical Analyses of Variance (ANOVA) was performed, followed by a mean comparison with the Ryan-Einot-Gabriel-Welsch multiple range test (REGWQ). If the storage "time" factor was the major determinant of the recorded effects, cold storage alone or in combination with HT or with N promoted a sample-dependent response in consideration of the AR levels. The efficiency of N in limiting the increase in AR was highest for fresh raw milk and was judged to be equivalent to that of HT for one sample and superior to that of HT for the two other samples; moreover, compared to HT, N seemed to favor a more diverse community at 6°C that was less heavily loaded with antibiotic multi-resistance features. Our results imply that N2 gas flushing could strengthen cold storage of raw milk by tackling the bacterial spoilage potential while simultaneously hindering the increase of bacteria carrying antibiotic resistance/multi-resistance features.

Responding to bioterror concerns by increasing milk pasteurization temperature would increase estimated annual deaths from listeriosis

In a 2005 analysis of a potential bioterror attack on the food supply involving a botulinum toxin release into the milk supply, the authors recommended adopting a toxin inactivation step during milk processing. In response, some dairy processors increased the times and temperatures of pasteurization well above the legal minimum for high temperature, short time pasteurization (72 °C for 15 s), with unknown implications for public health. The present study was conducted to determine whether an increase in high temperature, short time pasteurization temperature would affect the growth of Listeria monocytogenes, a potentially lethal foodborne pathogen normally eliminated with proper pasteurization but of concern when milk is contaminated postpasteurization. L. monocytogenes growth during refrigerated storage was higher in milk pasteurized at 82 °C than in milk pasteurized at 72 °C. Specifically, the time lag before exponential growth was decreased and the maximum population density was increased. The public health impact of this change in pasteurization was evaluated using a quantitative microbial risk assessment of deaths from listeriosis attributable to consumption of pasteurized fluid milk that was contaminated postprocessing. Conservative estimates of the effect of pasteurizing all fluid milk at 82 °C rather than 72 °C are that annual listeriosis deaths from consumption of this milk would increase from 18 to 670, a 38-fold increase (8.7- to 96-fold increase, 5th and 95th percentiles). These results exemplify a situation in which response to a rare bioterror threat may have the unintended consequence of putting the public at increased risk of a known, yet severe harm and illustrate the need for a paradigm shift toward multioutcome risk benefit analyses when proposing changes to established food safety practices.

Lactoperoxidase: structural insights into the function,ligand binding and inhibition

Lactoperoxidase (LPO) is a member of a large group of mammalian heme peroxidases that include myeloperoxidase (MPO), eosinophil peroxidase (EPO) and thyroid peroxidase (TPO). The LPO is found in exocrine secretions including milk. It is responsible for the inactivation of a wide range of micro-organisms and hence, is an important component of defense mechanism in the body. With the help of hydrogen peroxide, it catalyzes the oxidation of halides, pseudohalides and organic aromatic molecules. Historically, LPO was isolated in 1943, nearly seventy years ago but its three-dimensional crystal structure has been elucidated only recently. This review provides various details of this protein from its discovery to understanding its structure, function and applications. In order to highlight species dependent variations in the structure and function of LPO, a detailed comparison of sequence, structure and function of LPO from various species have been made. The structural basis of ligand binding and distinctions in the modes of binding of substrates and inhibitors have been analyzed extensively.

Thiocyanate transport in resting and IL-4-stimulated human bronchial epithelial cells: role of pendrin and anion channels

SCN(-) (thiocyanate) is an important physiological anion involved in innate defense of mucosal surfaces. SCN(-) is oxidized by H(2)O(2), a reaction catalyzed by lactoperoxidase, to produce OSCN(-) (hypothiocyanite), a molecule with antimicrobial activity. Given the importance of the availability of SCN(-) in the airway surface fluid, we studied transepithelial SCN(-) transport in the human bronchial epithelium. We found evidence for at least three mechanisms for basolateral to apical SCN(-) flux. cAMP and Ca(2+) regulatory pathways controlled SCN(-) transport through cystic fibrosis transmembrane conductance regulator and Ca(2+)-activated Cl(-) channels, respectively, the latter mechanism being significantly increased by treatment with IL-4. Stimulation with IL-4 also induced the strong up-regulation of an electroneutral SCN(-)/Cl(-) exchange. Global gene expression analysis with microarrays and functional studies indicated pendrin (SLC26A4) as the protein responsible for this SCN(-) transport. Measurements of H(2)O(2) production at the apical surface of bronchial cells indicated that the extent of SCN(-) transport is important to modulate the conversion of this oxidant molecule by the lactoperoxidase system. Our studies indicate that the human bronchial epithelium expresses various SCN(-) transport mechanisms under resting and stimulated conditions. Defects in SCN(-) transport in the airways may be responsible for susceptibility to infections and/or decreased ability to scavenge oxidants.

Evaluation of lactoperoxidase system treatment to reduce anthracnose, stem-end rot, and bacterial black spot development during storage of mangoes

The lactoperoxidase system (LPS) was evaluated for the prevention of postharvest diseases caused by Xanthomonas campestris, Botryodiplodia theobromae, and Colletotrichum gloeosporioides in 'Keitt' and 'Kent' mangoes. The LPS treatment significantly reduced the disease development on both cultivars after storage at 12 degrees C for 2 weeks, which was followed by a ripening at 25 degrees C. The LPS treatment did not alter the sensory quality of mango fruits (color, firmness, titrable acidity, and total soluble solids) when compared to untreated fruits. The LPS thus presents good potential alternative to the chemical fungicides traditionally used to improve the shelf life of mangoes.

Effect of the lactoperoxidase system against three major causal agents of disease in mangoes

The antibacterial activity of the lactoperoxidase system (LPS) on the growth of Xanthomonas campestris, the causal agent of bacterial black spot in mangoes, Botryodiplodia theobromae, the causal agent of stem-end rot disease in mangoes, and Colletotrichum gloeosporioides, the causal agent of anthracnose disease in mangoes, was determined during culture at 30 degrees C and at several pH values (4.5, 5.5, and 6.5). When the results of using the LPS were compared with those from control cultures without the LPS reagents, the growth of the three microorganisms was totally inhibited in all of the conditions tested. Viability tests enumerating cultivable cells of X. campestris showed that the LPS had a bactericidal effect, whatever the pH value. This effect is faster at pH 5.5, corroborating the results reported in the literature (optimal pH for the LPS efficiency). Further, we proved that hydrogen peroxide alone had little inhibition effect on the growth of the microorganisms studied. This compound is essentially used to convert thiocyanate into hypothiocyanate during the lactoperoxidase reaction. The potential of the LPS for the postharvest treatment of the fruits for controlling microbial diseases was thus demonstrated. Nevertheless, further studies are needed on fresh fruits before envisaging any application.

Analysis time and lactation stage influence on lactoperoxidase system components in dairy ewe milk

To study the effect of time elapsed from the moment of taking samples on lactoperoxidase system components, we analyzed the activity of the lactoperoxidase enzyme and the concentrations of thiocyanate and hydrogen peroxide in 46 individual samples of Manchega ewe milk. Samples were maintained at a temperature of 4 degrees C until analysis, which took place at 6, 12, 24, and 48 h after extraction. Decreases were observed in lactoperoxidase activity when the analyses were performed at 48 h and in the thiocyanate and hydrogen peroxide concentrations at 12 h compared with those carried out earlier. Consequently, when the components of the lactoperoxidase system or its antibacterial activity are studied, the time elapsed since the sampling commenced must be taken into account. Similarly, the time elapsed is important when carrying out bacterial counts or residue screening by microbiological methods, during which the lactoperoxidase system may interfere. To study the component changes in the lactoperoxidase system during lactation, samples obtained 15, 30, 45, 60, 75, 90, 105, 120, and 135 d postpartum from 48 Manchega ewes were used. Average lactoperoxidase activity, thiocyanate, and hydrogen peroxide concentrations were 3.46 U/ml, 6.89 mg/L, and 0.39 mg/L, respectively, with significant variations throughout lactation. The thiocyanate and hydrogen peroxide levels at different lactation stages seemed to be insufficient to activate the lactoperoxidase system. Nevertheless, this could be achieved by adding 5 mg/L of thiocyanate and 8 mg/L of hydrogen peroxide at any time during lactation.

Effects of combinations of lactoperoxidase system and nisin on the behaviour of Listeria monocytogenes ATCC 15313 in skim milk

Individual or combined effects of nisin (100 or 200 IU/ml) and the lactoperoxidase system (LPS) were analysed against 1 x 10(4) cfu/ml Listeria monocytogenes ATCC 15313 cells in skim milk, at 25 degrees C for 15 days. Nisin induced an immediate bactericidal effect and LPS a 48 h bacteriostatic phase which in both cases was followed by re-growth of L. monocytogenes. LPS and nisin added together at t0 showed a synergistic and lasting bactericidal effect which after 8 days and until 15 days resulted in no detectable cells in 1 ml of milk. When LPS was added to cells already in contact with 100 or 200 IU/ml nisin for a period of 4 h, the inhibitory activity was enhanced with no L. monocytogenes detectable after 72 or 48 h, respectively, and until 15 days. When LPS was added after 12 h, the nisin bactericidal phase was followed by re-growth. When nisin, 100 or 200 UI/ml, was added to cells already in contact with LPS over 24 h, L. monocytogenes was not detectable after 196 and 244 h, respectively, without any re-growth. For nisin addition after 72 h, cell counts were 8 log10 cycles lower than in the control milk after 196 h, but population levels were similar to the control within 15 days. The best combination to inhibit L. monocytogenes ATCC 15313 was nisin present at t0 followed by the LPS addition 4 h later, when the maximum inhibitory effect of nisin was reached.

Inactivation of Escherichia coli and Listeria innocua in milk by combined treatment with high hydrostatic pressure and the lactoperoxidase system

We have studied inactivation of four strains each of Escherichia coli and Listeria innocua in milk by the combined use of high hydrostatic pressure and the lactoperoxidase-thiocyanate-hydrogen peroxide system as a potential mild food preservation method. The lactoperoxidase system alone exerted a bacteriostatic effect on both species for at least 24 h at room temperature, but none of the strains was inactivated. Upon high-pressure treatment in the presence of the lactoperoxidase system, different results were obtained for E. coli and L. innocua. For none of the E. coli strains did the lactoperoxidase system increase the inactivation compared to a treatment with high pressure alone. However, a strong synergistic interaction of both treatments was observed for L. innocua. Inactivation exceeding 7 decades was achieved for all strains with a mild treatment (400 MPa, 15 min, 20 degrees C), which in the absence of the lactoperoxidase system caused only 2 to 5 decades of inactivation depending on the strain. Milk as a substrate was found to have a considerable effect protecting E. coli and L. innocua against pressure inactivation and reducing the effectiveness of the lactoperoxidase system under pressure on L. innocua. Time course experiments showed that L. innocua counts continued to decrease in the first hours after pressure treatment in the presence of the lactoperoxidase system. E. coli counts remained constant for at least 24 h, except after treatment at the highest pressure level (600 MPa, 15 min, 20 degrees C), in which case, in the presence of the lactoperoxidase system, a transient decrease was observed, indicating sublethal injury rather than true inactivation.

Use of a ground beef model to assess the effect of the lactoperoxidase system on the growth of Escherichia coli O157:H7, Listeria monocytogenes and Staphylococcus aureus in red meat

The ability to preserve food in a state that is both appetising and nutritious is a basic requirement for health. Food poisoning represents a major source of illness and loss of productivity in many developed countries. Of particular concern in recent years are outbreaks of food poisoning associated with Escherichia coli O157:H7 or Listeria monocytogenes, many of which have been associated with the consumption of ground meat. Many of the chemicals presently licensed for use as food preservatives are increasingly being questioned with regard to their effects on humans, creating pressure on food suppliers to consider the use of 'natural' alternatives to these chemical agents. The potential use of one such agent, the lactoperoxidase system (LPS), for use in ground meat preparations is examined in this study. The degree of inhibition of growth of E. coli O157:H7, L. monocytogenes L45 and S. aureus R37 by LPS was examined in a broth system at 37 degrees C and in a ground beef system at 0, 6 and 12 degrees C. The degree of inhibition by LPS of natural populations of microorganisms present in ground beef obtained from eight retail outlets and incubated at room temperature was also examined. For each of the strains examined, sensitivity from most to least sensitive followed the order L. monocytogenes L45, S. aureus R37 and E. coli O157:H7. In each case the ability of LPS to inhibit growth was highly temperature dependent and maximal at a temperature permissive but not optimal for growth of the test strain. The numbers of bacteria detected in ground beef obtained from retail outlets varied considerably between the eight samples. In all cases, numbers of bacteria increased markedly in the uninhibited control over the 4 h incubation time and, with the exception of one faecal coliform count, growth of the microbial populations was strongly inhibited by the presence of LPS. It was concluded that LPS could potentially be applied to a considerably wider range of food products than those to which it is presently restricted.

Response surface methodology, an approach to predict the effects of a lactoperoxidase system, Nisin, alone or in combination, on Listeria monocytogenes in skim milk

Experimental designs using Response Surface Methodology (RSM) were used to determine effects and interactions of Nisin (0-200 i.u. ml-1), pH values (5.4-6.6), incubation time (0-36 h or 0-144 h) and the lactoperoxidase-thiocyanate-hydrogen peroxide system (LPS) on Listeria monocytogenes CIP 82110 in skim milk, at 25 degrees C. The LPS varied from level 0-2; LPS at level 1 consisted of lactoperoxidase (35 mg l-1), thiocyanate (25 mg l-1) and H2O2, which was supplied exogenously by glucose-oxidase (1 mg l-1) and glucose (0.2 g l-1); LPS activity was dependent on LPS level and incubation time. In the presence of LPS at level 1, a bacteriostatic phase was followed by growth, whereas at a higher level, a bactericidic phase was observed. Nisin response was time- and pH-dependent. Nisin was bactericidic at acidic pH values and for a short incubation time (12 h) only; then, a re-growth phase was observed. Nisin and LPS in combination gave an original response which lacked the transitory bactericidal effect of Nisin and had a continuously bactericidal affect, leading to 10 cfu ml-1 of L. monocytogenes at 144 h; the response was greatly affected by incubation time. Predicted values were in good agreement with experimental values. Response Surface Methodology is a useful experimental approach for rapid testing of the effects of inhibitors.

Synergistic effect of nisin and the lactoperoxidase system on Listeria monocytogenes in skim milk

Nisin added at 10 or 100 IU/ml to ultra-high temperature processed (UHT) skim milk had no effect on counts of Listeria monocytogenes after 24 h at 30 degrees C, whereas addition of the lactoperoxidase system (LPS) resulted in counts of viable cells three log units lower than those of control milk after 24 h at 30 degrees C. Addition of nisin and LPS showed a synergistic effect and resulted in counts up to 5.6 log units lower than the control milk. When the two preservatives were added to actively growing cells of L. monocytogenes in two steps with a 2 h interval, their synergistic effect was enhanced. Counts of L. monocytogenes Ohio after 24 h at 30 degrees C in milk with nisin and LPS added together after 3 h of growth were 5.7 log units lower than the control milk. The difference in counts increased to 7.4 log units if LPS was added after 3 h and nisin after 5 h of growth. Similar but less pronounced effects were observed for the more resistant strain L. monocytogenes Scott A.

Psychrotrophs in dairy products: their effects and their control

Health concerns and technological effects of psychrotrophic bacteria in dairy products are reviewed, as well as methods to control their presence and development. The various Gram-negative and Gram-positive psychrotrophic species are listed and, with respect to pathogenic psychrotrophs, emphasis is given on Listeria monocytogenes, Yersinia enterocolitica, and Bacillus cereus. The influence of psychrotrophic bacteria on the quality of raw milk, pasteurized and UHT milks, butter, ice cream, cheese, and powders is examined. Public health considerations of Listeria monocytogenes, Yersinia enterocolitica, and Bacillus cereus of these various dairy products are also presented. Methods that can be used to eliminate or control the development of psychrotropic bacteria include low or high temperatures, chemicals, gases, the lactoperoxidase system, lactic acid bacteria, microfiltration, bactofugation, lactoferrin-related proteins, sanitation, flavors, and naturally occurring spore germinants.