Biofilm-forming ability and resistance to industrial disinfectants of Staphylococcus aureus isolated from fishery products

Adhesion and biofilm-forming ability of twenty six S. aureus strains previously isolated from fishery products on stainless steel was assessed. All strains reached counts higher than 10⁴ CFU/cm² after 5 h at 25 °C. Most strains also showed a biofilm-forming ability higher than S. aureus ATCC 6538 – reference strain in bactericidal standard tests – by crystal violet staining. In addition, it seems that food-processing could have produced a selective pressure and strains with a high biofilm-forming ability were more likely found in highly handled and processed products.The efficacy of the industrial disinfectants benzalkonium chloride (BAC), sodium hypochlorite (NaClO) and peracetic acid (PAA) against biofilms and planktonic counterparts was also examined in terms of minimum biofilm eradication concentration (MBEC) and minimum bactericidal concentration (MBC), respectively. Biofilms showed an antimicrobial resistance higher than planktonic cells in all cases. However, no correlation was found between MBEC and MBC, likely due to differences in biofilm extracellular matrix (composition, content and architecture) between strains. BAC resistance increased as biofilms aged. Generally, biofilm formation seemed to attenuate the effect of low temperatures on BAC resistance. PAA was found to be most effective against both biofilms and planktonic cells, followed by NaClO and BAC. Resistance did not follow the same order for each biocide, which remarks the need of using a wide collection of strains in standard tests of bactericidal activity to ensure a proper application of disinfectants. Doses recommended by manufacturers for BAC, PAA and NaClO to disinfect food-contact surfaces were lower than doses for complete biofilm removal (i.e. MBEC) under some environmental conditions common in the food industry, which questions bactericidal standard tests and promotes the search for new strategies for biofilm removal.     

Effects of spray volume, type of surface tissue and inoculum level on the survival of Escherichia coli on beef sprayed with 5% lactic acid

Membrane, fat and cut muscle surfaces of beef were inoculated with Escherichia coli at numbers about 4, 1 or −1 log cfu/cm². The inoculated meat was sprayed with water or 5% lactic acid at volumes of 0.5, 0.1 or 0.02 ml/cm². Spraying with water reduced the numbers of E. coli on membrane surfaces by up to 1 log unit, but had little effect on the numbers of E. coli on fat or cut muscle surfaces. Spraying with 5% lactic acid reduced the highest numbers of E. coli on membrane surfaces by up to 4 log units; but those numbers on fat or cut muscle surfaces were reduced by ≤1.5 log unit, and the reductions declined with decreasing volumes of 5% lactic acid. With inocula of 1 log cfu/cm², spraying lactic acid in any volume reduced the numbers of E. coli on membrane or fat surfaces by about 1 log unit, and the numbers on cut muscle surfaces by between 0.8 and 0.2 log unit. E. coli were detected in enrichment cultures of samples from all surfaces inoculated with E. coli at −1 log cfu/cm² and sprayed with 5% lactic acid at 0.5 ml/cm². The findings indicate that spraying relatively heavily contaminated cuts or trimmings with 5% lactic acid at ≥0.1 ml/cm² can be expected to reduce numbers of E. coli and, presumably, associated pathogens by between 0.5 and 1 log unit. However, such a treatment is likely to be at best marginally effective for reduce the numbers of these organisms on lightly contaminated product.     

Removal of Listeria monocytogenes, Staphylococcus aureus and Escherichia coli O157:H7 biofilms on stainless steel using scallop shell powder

Biofilms on steel surfaces containing Listeria monocytogenes, Staphylococcus aureus and Escherichia coli O157:H7 continue to threaten dairy and meat processors. In this study, the ability of scallop shell powder (SSP) to remove biofilms formed by these three pathogens on stainless steel plates was examined. Whey powder solution (WPS) and bench wash water (BWW) provided by dairy and meat factories, respectively, were inoculated with L. monocytogenes, S. aureus or E. coli O157:H7 (9 log₁₀ CFU/ml). Stainless steel plates (10 cm²) were placed in the inoculated fluids and incubated at 20 °C at 48 h to form biofilms. After drying and washing in sterile water, the plates were treated with 0.0, 0.25, or 0.50% (w/v) SSP slurries for 1, 5, or 10 min and then quantitatively examined for the three pathogens. Both 0.25 and 0.50% SSP reduced L. monocytogenes on the plates by 4 log CFU/cm² with a 1 min exposure to 0.50% SSP decreasing S. aureus by 5 logs CFU/cm². After 1 min in 0.25 and 0.50% SSP, E. coli O157:H7 populations in WPS and BWW biofilms decreased 4 and 6 log CFU/cm² and 3 and 5 log CFU/cm², respectively. Increasing the concentration of SSP led to significantly increased efficacy against the tested pathogens (P < 0.05). In conclusion, this study showed that SSP slurries could significantly reduce the numbers of L. monocytogenes, S. aureus and E. coli O157:H7 in biofilms on stainless steel surfaces.     

Adherence of cold-adapted Escherichia coli O157:H7 to stainless steel and glass surfaces

The effects of previous cold-induced cell elongation on adherence of Escherichia coli O157:H7 to glass slides and stainless steel surfaces was evaluated at 4 °C for ≤48 h. Planktonic E. coli O157:H7 with and without cold adaptation were prepared at 15 and 37 °C, respectively, and planktonic E. coli O157:H7 containing elongated (>4 ≤ 10 μm) and filamentous (>10 μm) cells were prepared at 6 °C. Despite morphological differences in planktonic E. coli O157:H7 preparations, all three cell types attached to a greater extent to glass than to the stainless steel surfaces. E. coli O157:H7 cells adapted to growth at 15 °C attached better to both glass and stainless steel surfaces (3.2 and 2.6 log cfu/cm², respectively) than cells of the other treatments at ≥24 h. Cells adapted at 6 °C attached to glass slides and stainless steel coupons at levels of 3.0 and 1.8 log cfu/cm², respectively, while E. coli O157:H7 cells grown at 37 °C attached to these surfaces at levels of 2.0 and 1.7 log cfu/cm², respectively. No further attachment of cells from any of the treatments was noted between 24 and 48 h at 4 °C. These results suggest that E. coli O157:H7 cells adapted at 6 °C–15 °C have greater potential to attach to food contact surfaces than those grown at higher temperature. The enhanced biofilm-forming ability of 6 °C or 15 °C-adapted, elongated and filamentous E. coli O157:H7 cells did not appear to be related to the greater entanglement of longer cells within biofilm matrices.     

The incidence of significant foodborne pathogens in domestic refrigerators

The interior surfaces of household refrigerators are at risk of becoming contaminated with foodborne pathogens, increasing the risks of cross-contamination to other food items, including higher risk ready-to-eat foods. This study determined the incidence of a number of significant foodborne pathogens, and the general hygienic status (as estimated by total viable counts (TVCs), and total coliform counts (TCCs)) on the interior surfaces of domestic refrigerators (n = 342). Campylobacter spp., Salmonella spp. and Escherichia coli O157:H7 were not recovered from any refrigerators, but Staphylococcus aureus was recovered from 6.4%, Listeria monocytogenes and E. coli from 1.2% and Yersinia enterocolitica from 0.6% of examined refrigerators. As the recovered species can survive and grow under refrigeration or conditions of mild temperature abuse, such pathogens may transfer to (and develop to clinically significant numbers in) food in domestic fridges. Such risks are of particular concern in relation to “ready-to-eat” foods, which will not receive further bactericidal treatments (cooking) before consumption. The study estimated TVCs ranging from 2.91 log₁₀ cfu/cm² to 8.78 log₁₀ cfu/cm² and TTCs ranging from 0.045 log₁₀ cfu/cm² to 5.96 log₁₀ cfu/cm² indicating very poor standards of consumer refrigerator management and hygiene, and posing risks to consumer health. The study findings highlight the importance of adequate temperature control and thorough, regular cleaning of domestic refrigerators to ensure food safety, and of effective cooking as the last link in the domestic food service chain.     

Resistances to UV-C irradiation of Salmonella Typhimurium and Staphylococcus aureus in wet and dried suspensions on surface with egg residues

To clarify the effects of food sediments on ultraviolet-C (254 nm) sanitation in food-related environments, we examined the resistance of pathogenic bacteria (Salmonella Typhimurium and Staphylococcus aureus) cells, in wet and dried suspensions adhered with 1.5-15% w/v egg albumen, 1.5-15% yolk or 3.0-30% whole egg solutions, against UV-C irradiation. Bacterial suspensions (0.1 ml of 8 log CFU/ml) were put on 47 mmφ glass dishes and dried at room temperature (20-24 °C) for 180 min in a bio safety cabinet with ventilation. Viable S. Typhimurium and S. aureus cells in distilled water decreased during the drying period from 7.2 to 3.2 and from 8.0 to 6.5 log CFU/dish, respectively,. On the other hand, the bacteria cells were protected from drying by egg compounds, even by the lowest concentration. The UV-C treatment (0.16 mW/cm² for 10 min) showed a clear bactericidal effect in the absence of egg compounds. However, the bactericidal effect was inhibited by 15% yolk and 30% whole egg. Results in this study suggested that the small food sediment protect bacteria on the surfaces from dryness and UV-C irradiation and it might introduce cross contamination.     

Effect of neutral electrolyzed water on lux-tagged Listeria monocytogenes EGDe biofilms adhered to stainless steel and visualization with destructive and non-destructive microscopy techniques

The presence of Listeria monocytogenes in food processing environment is a risk of food contamination by persistent cells due to their ability to attach to stainless steel and other surfaces. We aimed to study biofilms formation of lux-tagged L. monocytogenes EGDe on stainless steel surfaces and their control using neutral electrolyzed water (NEW), where biofilms development was monitored using destructive and non-destructive microscopy techniques. The development of biofilms was monitored for 5 days on stainless steel chips. We used two sources of NEW, commercial (NEW-1) and from a prototype (NEW-2) for treatments of free and biofilm L. monocytogenes EGDe cells. Complete inhibition of L. monocytogenes EGDe free cells was observed after 1 min contact time for both NEW sources, but NEW-1 concentration used (9 mg/L total available chlorine, TAC) was 1.8 times higher. Cells within biofilms were more resistant to NEW compared to planktonic cells. Same concentration of both NEW sources (70 mg/L TAC) exhibited complete inhibition of biofilm cells after 3 min contact time. However, using a sub-lethal dose of 40 mg/L TAC, NEW-2 reduced about 2 log CFU/cm² biofilm cells while NEW-1 inhibited 0.3 log CFU/cm² only. Biofilms formation and antagonistic effect of NEW could be visualized by epifluorescence and scanning electron microscopy, revealing significant biofilms structure. The disinfectant effect of NEW may be attributed to the combined antimicrobial effect of available chlorine and high ORP exhibited by its oxidizing compounds. NEW does not promote metal equipment corrosion due to its neutral pH, and is also environmentally friendly.     

Attachment of foodborne pathogens to banana (Musa sp.) leaves

Fresh produce, and in particular minimally processed leafy green vegetables, have been recognized as a source of transmission for foodborne pathogens of animal origin. In South-East Asian and other cultures the leaves of the banana plant (Musa sp.) are widely used as food wrappings or as serving plates because of their waxy surfaces and represent a largely uninvestigated leafy green product. This study was undertaken to quantify the attachment of two strains of each of the three bacterial foodborne pathogens Staphylococcus aureus, Salmonella enterica and Escherichia coli to the top side and underside of banana leaf surfaces. All bacteria tested attached to banana leaves at levels in the range of 3.5–4.5 cfu/cm². Differences in attachment to leaves between strains were apparent. Most notably both Salmonella strains attached to the top side of leaf surfaces in significantly (p < 0.05) lower numbers than two (one E. coli and one S. aureus) of the other four strains. Furthermore, the two S. aureus strains attached to the undersides of leaves in significantly (p < 0.05) lower numbers than to the top side of leaves. Despite the waxy nature of the banana leaf surfaces the bacteria tested were capable of attaching to them in numbers equivalent to the attachment of bacterial pathogens to other leafy green produce.     

Growth inhibition of Listeria monocytogenes by bacteriocin-producing Staphylococcus equorum SE3 in cheese models

The anti-listerial activity of Staphylococcus equorum SE3 isolated from cheese brine was tested in two different model cheese systems to ascertain its potential for use as a protective culture for smear cheese ripening. Co-cultivation of Listeria monocytogenes L129 and the antilisterial S. equorum SE3 was performed on “milk agar” or “modified milk agar” (model cheese surface systems). S. equorum inoculated at concentrations of 10⁶ cfu/cm² completely inhibited growth of L. monocytogenes inoculated at 10–500 cfu/cm² on modified milk agar within 24 h of incubation, in the negative controls L129 grew to >10⁷ cfu/cm². At a higher inoculation level, growth inhibition was still more than 7 log units after 24 h. L. monocytogenes strains of different serotypes were also inhibited. Co-cultivation of S. equorum SE3 with other smear bacteria or yeasts, however, showed no growth inhibition of these important ripening microorganisms. The antilisterial effect was not diminished on the modified milk agar when co-cultivation was performed with the added smear cheese microbiota. However, on milk agar with no adjuncts (“green cheese model”), only a slight (<1 log unit) growth inhibition of L. monocytogenes was observed. Addition of peptides or amino acids to milk agar could restore growth inhibition of listeriae at different levels.     

Prediction of Escherichia coli O157:H7 adhesion and potential to form biofilm under experimental conditions

Escherichia coli O:157:H7 adhesion and potential to form biofilm on three different surfaces commonly used in the food industry was evaluated using probabilistic models; the surfaces tested were stainless steel 304 (SS304), poly(vinyl chloride) film covered with thick cloth (PVC1) and poly(vinyl chloride) film covered with thin cloth (PVC2). Using a Central Composite Rotational Design (CCRD), the effect of contact time (0 h, 7 h, 24 h, 41 h and 48 h) and temperature (12 °C, 17 °C, 28 °C, 39 °C and 44 °C) on the probability of achieving a particular adherent cell count (Log₁₀ CFU cm⁻²) was determined. By analyzing response surface plots and their corresponding contour plots and by determining quadratic equations for each surface, experimental values were shown to be significant in accordance with predicted values in all cases. The adjusted determination coefficient (R_adj²) was 90.5%, 97.2% and 98.9% for SS304, PVC1 and PVC2, respectively, and the level of significance was P ≤ 0.001. The bias factor (B_f) and accuracy factor (A_f) both approached 1.0 for the three surfaces evaluated. The model equations for predicting optimum response values were verified effectively by a validation data set for all surfaces evaluated. Therefore, an RSM provides a useful and accurate method for predicting E. coli O157:H7 adhesion and potential to form biofilm on SS304, PVC1 and PVC2 and could be considered to be a standard way to ensure food safety with respect to E. coli O157:H7 contamination through adhesion and biofilm formation.     

Synergistic effect of a combination of ultraviolet–C irradiation and sodium hypochlorite to reduce Listeria monocytogenes biofilms on stainless steel and eggshell surfaces

The present study demonstrates the synergistic effect of treatment with a combination of ultraviolet (UV)C (300, 600, 1200, and 1800 mWs/cm²) irradiation and sodium hypochlorite (NaOCl) (50, 100, 150, and 200 ppm) on Listeria monocytogenes ATCC 19113 biofilms formed on stainless steel and eggshell surfaces. The synergistic effect was not dependent on the dose of UV–C irradiation or the concentration of NaOCl. Synergistic reductions of biofilms after UV–C/NaOCl treatment on stainless steel and eggshells were 0.95–3.68 log CFU/cm² and –0.22 to 1.02 log CFU/cm², respectively. The largest synergistic reductions of biofilms were 3.68 log CFU/cm² using 1800 mWs/cm² UV–C and 200 ppm NaOCl on stainless steel, and 1.02 CFU/cm² using 600 mWs/cm² UV–C and 50 ppm NaOCl on eggshells. The Hunter colors of L*, a*, and b* were not significantly (P > 0.05) changed on eggshell surfaces treated with all single and combined treatments. The results in this study indicate that the combination treatment of 1800 mWs/cm² UV–C/200 ppm NaOCl could be feasible for use on stainless steel surfaces in industrial kitchens, facilities, and restaurants. In addition, the combination of 600 mWs/cm² UV–C/500 ppm NaOCl could possibly be used in egg production, processing, and distribution processes to enhance food safety without causing changes to the eggshell surface color.     

Rutin inhibits mono and multi-species biofilm formation by foodborne drug resistant Escherichia coli and Staphylococcus aureus

Considering the role of biofilm in food spoilage and the food industry, inhibition of biofilm formation by natural agents is expected to be safe and could also enhance the efficacy of other antimicrobial strategies for controlling microbial food spoilage. Plant flavonoids are known for their diverse biological activity including antimicrobial. Therefore, rutin was investigated for its biofilm inhibitory activity at sub-minimum inhibitory concentrations (sub-MICs) values against common foodborne pathogens (Escherichia coli and Staphylococcus aureus). Minimum inhibitory concentrations (MIC) ranged from 400 to 1600 μg/ml against the selected strains. Sub-MICs (1/16 × MIC to 1/2 × MIC) were used to assess the inhibition of biofilm formed by E. coli and S. aureus in microtitre plate assay. Mono strain biofilm formation by Escherichia coli and Staphylococcus aureus was greatly reduced by rutin at their respective 1/2 × MIC. For multi-species (E. coli: and S. aureus) biofilm formation, the reduction in biofilm production was concentration dependent. No significant bacteria mass reduction was recorded for any sub-MIC. SEM images of biofilm inhibition on steel chips confirmed the reduction in number of microcolonies. Exopolysaccharide production responsible for adherence and maturation of biofilms was also significantly (p ≤ 0.05) reduced at respective concentrations of rutin in tested strains. To the best of our knowledge, this is the first study describing the effect of flavonoid, rutin on multi-species biofilms consisting of S. aureus and E. coli. Findings of the study indicate a potential application of rutin in the prevention of biofilm on industrial equipment and food contact surfaces and prevent food contamination and spoilage.     

Adhesion of Staphylococcus aureus and Staphylococcus xylosus to materials commonly found in catering and domestic kitchens

The aim of this work was to investigate the adhesion of Staphylococcus aureus and Staphylococcus xylosus on marble, granite, polypropylene, stainless steel 304 and stainless steel 316. The results showed that S. aureus adhered to all substratums. The maximum was observed on marble (30 10⁶ CFU/cm²) and, on polypropylene (30,2 10⁶ CFU/cm²). The results showed also that S. xylosus revealed a high ability to adhere to all substratum. This strains adhere more on marble (32.8 10⁶ CFU/cm²) and granite (16,3 10⁶ CFU/cm²) than to others substratum. The highest extent of adhesion of S. aureus and S. xylosus occurred to marble, polypropylene and granite. A correlation between substratum physicochemical properties and bacterial adhesion was also examined. A good correlation was observed between S. xylosus adhesion and their acid-base character. The topography of substratum surface was investigated using AFM. A good correlation was obtained between roughness and bacterial adhesion.     

Use of epifluorescence microscopy to assess the effectiveness of phage P100 in controlling Listeria monocytogenes biofilms on stainless steel surfaces

One source of foodborne listeriosis is related to a virulent strain established in the food-processing environment. We used direct epifluorescence microscopy (DEM) to evaluate the effectiveness of phage P100 in controlling Listeria monocytogenes biofilms on stainless steel surfaces, in wet conditions at room temperature. Biofilms that were allowed to develop for 72 h were subsequently treated with different concentrations of phage (5, 6, 7, and 8 log PFU/ml). L. monocytogenes was monitored up to 48 h, using both DEM and cultivation method. When L. monocytogenes was monitored using a cultivation method, the first significant reduction was observed after phage treatment with 7 and 8 log PFU/ml at 8 h. Subsequently, these treatments achieved undetectable levels of pathogen at 48 h, with a mean reduction of 5.29 log CFU/cm². Conversely, when samples were evaluated using DEM in treatments with 6, 7, and 8 log PFU/ml, although disaggregation of biofilms could be observed after 8 h, viable cells were still present up to 48 h (maximal reduction 1.5 log units). The phage titer remained stable or increased up to 2.59 log units during the study period. In conclusion, phage P100 may provide an adjuvant measure to control L. monocytogenes biofilm on stainless steel surfaces. However, phage treatment must be used in combination with other hygienization measures to increase efficacy. In this study, DEM was a good tool to quickly and accurately assess the real effect of phage P100 on L. monocytogenes biofilms.     

Adherence kinetics, resistance to benzalkonium chloride and microscopic analysis of mixed biofilms formed by Listeria monocytogenes and Pseudomonas putida

Comparison between the resistance to BAC and the microscopic structure between mixed-species biofilms formed by different strains of Listeria monocytogenes and Pseudomonas putida CECT 845 under different scenarios and that obtained by the corresponding monospecies L. monocytogenes biofilm was carried out. The association of P. putida with L. monocytogenes quickens biofilm formation and increases significantly (p < 0.05) the BAC-resistance of the biofilm after 4 days of incubation at 25 °C respecting to that formed by monospecies biofilms. According with the adherence profiles of P. putida, two different patterns of association between both species (A and B) were identified, being type A pattern found in the mixed biofilms much more resistant to BAC. After 11 days of incubation, a destructuration of mixed biofilms occurred in all experimental assays, being in 2 out of 5 experimental cases (4032 and BAC-adapted 5873 on polypropylene) accompanied by a sharp decrease in the number of adhered cells. Microscopic analyses demonstrated that complex three-dimensional microscopic structure showed the highest resistance to BAC (4032-SS). Obtained results clearly highlight that to improve disinfection protocols for assuring food safety, it is necessary to mimick those bacterial association that occur in nature.     

Comparison of three sampling methods for the quantification of methicillin-resistant Staphylococcus aureus on the surface of pig carcases

The objective was to determine the appropriate sampling method to quantify methicillin-resistant Staphylococcus aureus (MRSA) on the surface of pig carcases. Carcases were contaminated using brain-heart infusion broth containing MRSA at 5 log and 7 log cfu/ml. Samples were collected using three methods: a wet/dry double swab (100 cm²⁾, an abrasive sponge (100 cm²) and a die cutter (2×5 cm²). Quantification of MRSA was done using decimal dilutions and cultivation in duplicate on chromogenic selective agar.The wet/dry double swab method was not sufficiently sensitive with 8 out of 10 samples being negative at low contamination levels. The abrasive sponge had reproducible results in all samples at both contamination levels. In low contamination areas 5 of the 10 die cutter samples were negative, while in the heavily contaminated areas, results of the die cutter and the abrasive sponge were comparable. It is concluded that with expected low contamination levels using the abrasive sponge is superior to the other two methods and should be used whenever contamination levels are not known.     

Influence of food soiling matrix on cleaning and disinfection efficiency on surface attached Listeria monocytogenes

Cleaning and disinfection are essential steps in preventing contamination of foods with pathogenic and spoilage bacteria. The efficacy of cleaning and disinfection products differ depending on target bacteria and type of soiling. We evaluated the effectiveness of cleaning and disinfecting products against Listeria monocytogenes attached on food soiled inert surfaces in a laboratory model. The number of bacteria on surfaces before and after treatment was quantified using indirect conductometric measurements. L. monocytogenes attached to stainless steel surfaces in fish broth systems reached approx. 10⁴ CFU/cm². However, all cleaning and disinfection products were equally effective in this system since all bacteria were removed or killed. When the steel disks were immersed in a fish or meat emulsions, a level of approx. 10⁵–10⁶ L. monocytogenes per cm² was reached after 2–3 days at 20 °C. In this case, 2–3 log were removed or killed by alkaline cleaning products (MC103 or FC140). Direct use of a peracetic acid based disinfectant (Oxivit Active Plus) killed all bacteria when attached in salmon emulsions, whereas only 1–2 log were removed or killed in the meat emulsion system. Thus, the efficiency of cleaning and disinfection products against L. monocytogenes is strongly influenced by the food matrix.     

Effectiveness of a phage cocktail as a biocontrol agent against L. monocytogenes biofilms

Listeria monocytogenes can persist and form biofilms in a food environment which are difficult to eradicate because biofilms are inherently resistant to a variety of antimicrobial treatments. Therefore, alternative approaches such as bacteriophages have been suggested as a promising biocontrol agent against biofilms. The aim of this study was to evaluate the efficacy of a cocktail bacteriophage product (ListShield™) against L. monocytogenes biofilms. These biofilms were established on lettuce, stainless steel, rubber, and a MBEC biofilm device and exposed to the ListShield™ phage preparation (1 × 10⁸ PFU/mL) for 2 h. ListShield™ had sufficient potency to significantly reduce the biofilm (P < 0.05) in all cases. Biofilm reduction achieved after ListShield™ treatment on the stainless steel coupon was 1.9–2.4 log CFU/cm² and on the rubber surface approximately 1.0 log CFU/cm². Phage application on lettuce inactivated biofilm bacteria up to 0.7 log CFU/cm². These results suggest that bacteriophage preparation ListShield™ is an effective tool for the inactivation of L. monocytogenes biofilms in the food industry.     

Lactobacillus sakei 1 and its bacteriocin influence adhesion of Listeria monocytogenes on stainless steel surface

Listeria monocytogenes is of particular concern for the food industry due to its psychrotolerant and ubiquitous nature. In this work, the ability of L. monocytogenes culturable cells to adhere to stainless steel coupons was studied in co-culture with the bacteriocin-producing food isolate Lactobacillus sakei 1 as well as in the presence of the cell-free neutralized supernatant of L. sakei 1 (CFSN-S1) containing sakacin 1. Results were compared with counts obtained using a non bacteriocin-producing strain (L. sakei ATCC 15521) and its bacteriocin free supernatant (CFSN-SA). Culturable adherent L. monocytogenes and lactobacilli cells were enumerated respectively on PALCAM and MRS agars at 3-h intervals for up to 12 h and after 24 and 48 h of incubation. Bacteriocin activity was evaluated by critical dilution method. After 6 h of incubation, the number of adhered L. monocytogenes cells in pure culture increased from 3.8 to 5.3 log CFU/cm² (48h). Co-culture with L. sakei 1 decreased the number of adhered L. monocytogenes cells (P < 0.001) during all sampling times with counts lower than 3.0 log CFU/cm². The CFNS-S1 also led to a significant and similar reduction in culturable adhered L. monocytogenes counts for up to 24 h of incubation, however after 48 h of incubation, re-growth of L. monocytogenes number of adhered cells was observed, likely due to lack of competition for nutrients. L. sakei ATCC 15521 or its supernatant (CFNS-SA) did not reduce the number of adhered L. monocytogenes cells on stainless steel surface and from 6 h of incubation, listerial counts were between 4.3 and 4.5 log CFU/cm². These results indicate that L. sakei 1 and its bacteriocin sakacin 1 may be useful to inhibit early stages of L. monocytogenes adherence to abiotic surface.     

Growth of Escherichia coli,Salmonella enterica and Listeria spp., and their inactivation using ultraviolet energy and electrolyzed water,on ‘Rocha’ fresh-cut pears

The present study aimed at evaluating the growth of Escherichia coli, Salmonella enterica, and Listeria spp. and studying the efficacy of Ultraviolet-C (UV-C) irradiation, acidic electrolyzed (AEW) and neutral electrolyzed (NEW) waters in the reduction of these bacteria on ‘Rocha’ pear. Fresh-cut pieces were inoculated and incubated at 4–20 °C for 8 days. Inoculated pears were treated with UV-C (2.5–10 kJ/m²), AEW, NEW and sodium hypochlorite (SH) and microbiological and quality parameters were evaluated. The three bacteria, inoculated at 6.1–6.2 log cfu/g, grew on the pear at high growth rates at 12 and 20 °C reaching populations of 8.1–8.6 log cfu/g, in 24 h. At 8 °C the microorganisms increased their populations by at least 1 log cfu/g in three days. At 4 °C adaptation phases of less than 24 h for Listeria spp. were measured before exponential growth occurred and the enterobacteria did not grow despite having survived for 8 days. AEW and NEW caused microbial reductions similar to SH, of approximately 1 log cfu/g, while the best UV-C dose (7.5 kJ/m²) of at least 2.4 log cfu/g. Fresh-cut pears were a good substrate for foodborne bacteria emphasizing the importance of preventing contaminations and cross contaminations. The UV-C was more effective than the chemical decontaminations, as it provided superior microbial reductions without greatly affecting the quality of pears.