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Irradiation with deep-ultraviolet light-emitting diodes (DUV LEDs) is emerging as a low energy, chemical-free approach to mitigate microbial contamination, but the effect of surface conditions on treatment effectiveness is not well understood. Here, inactivation of L. innocua and E. coli ATCC25922, as examples of Gram-positive and Gram-negative bacteria, respectively, by DUV LED of 280 nm wavelength was studied. Surface scenarios commonly encountered in environmental, clinical or food processing environments were used: nutrient rich surfaces, thin liquid films (TLF), and stainless steel surfaces (SS). DUV LED exposure achieved 5-log reduction for both strains within 10 min in most scenarios, except for TLF thicker than 0.6 mm. Inactivation kinetics in TLF and on dry SS followed the Weibull model (0.96 ≤ R2 ≤ 0.99), but the model overestimated inactivation by small-dose DUV on wet SS. Confocal microscopy revealed in situ that bacteria formed a dense outer layer at the liquid-air interface of the liquid droplet, protecting the cells inside the droplet from the bactericidal DUV. This resulted in lower than anticipated inactivation on wet SS at small DUV doses, and deviation from the Weibull model. These findings can be used to design effective DUV LED disinfection strategies for various surface conditions and applications.

Staphylococcus epidermidis and Staphylococcus aureus, two bacterial strains commonly associated with biofilm-related medical infections and food poisoning, can rapidly colonize biotic and abiotic surfaces. The present study investigates the ability of anodic alumina surfaces with nanoporous surface topography to minimize the attachment and biofilm formation mediated by these pathogenic bacterial strains. Early attachment and subsequent biofilm development were retarded on surfaces with nanopores of 15-25 nm in diameter compared to surfaces with 50-100 nm pore diameter and nanosmooth surfaces. After 30 min of incubation in nutritive media, the biomass accumulation per unit surface area was 2·93 ± 1·72 µm3 µm-2 for the 15 nm, 3·49 ± 1·97 µm3 µm-2 for the 25 nm, as compared to 14·04 ± 6·39 µm3 µm-2 for the nanosmooth, 11·88 ± 9·72 µm3 µm-2 for the 50 nm and 12·09 ± 11·84 µm3 µm-2 for the 100 nm surfaces respectively. These findings suggest that anodic alumina with small size nanoscale pores could reduce the incidence of staphylococcal biofilms and infections, and shows promise as a material for a variety of medical applications and food contact surfaces. SIGNIFICANCE AND IMPACT OF THE STUDY: This paper reports on a simple, robust and scientifically sound method to reduce attachment and biofilm formation by Staphylococcus aureus and Staphylococcus epidermidis to abiotic surfaces using a carefully designed nanoscale topography. This approach can help to reduce the incidence of staphylococcal biofilms and infections without imposing selective stresses on bacteria, thus preventing the creation of resistant strains.

Bacterial spores present in milk can cause quality and shelf-life issues for dairy products. The objectives of this study were to evaluate the effectiveness of microfiltration (MF) in removing Bacillus licheniformis and Geobacillus sp. spores from skim milk using membranes with pore sizes of 1.4 and 1.2 µm, and to investigate the role of spore surface properties in MF removal. Cell sizes were determined by scanning electron microscopy, surface charge by zeta potential analysis, and surface hydrophobicity by contact angle measurements. Commercially pasteurized skim milk was inoculated with a spore suspension at about 106 cfu/mL, and then processed by MF using ceramic membranes at 6°C, a cross-flow velocity of 4.1 m/s, and transmembrane pressure of 69 to 74 kPa. Total aerobic plate and spore counts in the milk were determined before and after MF. All processing runs and surface and product analyses were conducted in triplicate, and data were analyzed statistically. For the same strain, spores were shorter and wider than vegetative cells, averaging 1.37 to 1.59 µm in length and 0.64 to 0.81 µm in width. Reduction of B. licheniformis spores significantly increased with a reduction in MF pore size, from 2.17 log for 1.4-µm pore size, to 4.57 log for 1.2-µm pore size. Both pore sizes resulted in almost complete removal of Geobacillus sp. spores. All spores and the ceramic membrane had a negative surface charge at milk pH, indicating an electrostatic repulsion between them. Bacillus licheniformis spores were hydrophilic, whereas Geobacillus sp. spores were hydrophobic. Consequently, Geobacillus sp. spores had a tendency to cluster in skim milk, preventing their passage even through the 1.4-µm MF membrane, whereas some B. licheniformis spores could still pass through a 1.2-µm membrane. This study demonstrates that efficient removal of spores from skim milk by cold MF may require a smaller membrane pore size than required for removal of vegetative cells of the same species, and that cell surface properties may interfere with the outcome of filtration as would be anticipated based on size alone.

Hydrophilic surfaces of both abiotic and biological origin have been shown to bear particle-exclusion zones as large as hundreds of micrometers at liquid-solid interfaces. Here we present the first systematic investigation and evidence for bacteria-free exclusion zones for several bacterial strains, including pathogens associated with hospital infections and/or foodborne outbreaks: Staphylococcus aureus, Escherichia coli O157:H7, and Listeria monocytogenes. Tests were carried out both in a phosphate buffer, as well as triptic soy broth (TSB) of high ionic strength. Bacterial cell density distribution at the Nafion-liquid interface was visualized using confocal laser scanning microscopy. A robust image analysis method was developed to generate a profile of cell concentration near the interface and quantify EZ size. Results revealed an exclusion zone (EZ) of 40-60μm and a transition zone (TZ) of 40-80μm for bacterial cells suspended in tryptic soy broth. There were no statistical differences in the size of EZ and TZ for the bacterial strains tested with the same substrate, but differences existed for different substrates tested, implying a physicochemical underpinning for EZ. In a test conducted with E. coli, cells progressively penetrated EZ over 2days. Furthermore, EZ-bearing Nafion had 80% less biomass accumulation of E. coli over 2days compared to an EZ-less, hydrophilic, smooth aluminum oxide surface. This suggests that EZ may represent the first line of defense, spatially and temporally, against bacteria approaching certain hydrophilic surfaces. These findings could have important implications in developing biofouling-resistant material surfaces for applications sensitive to bacterial attachment and biofilm formation.

Postprocessing cross-contamination of cheese can lead to both food safety issues and significant losses due to spoilage. Pulsed light (PL) treatment, consisting of short, high-energy, broad-spectrum light pulses, has been proven effective in reducing the microbial load on cheese surface. As PL treatment effectiveness is limited by light-cheese interactions, the possibility to improve its effectiveness by combining it with the antimicrobial nisin was explored. The effect of natamycin, which is added to cheeses as an antifungal agent, on PL effectiveness was also investigated. Pseudomonas fluorescens, Escherichia coli ATCC 25922, and Listeria innocua were used as challenge microorganisms. Bacterial cultures in stationary growth phase were diluted to initial inoculum levels of 5 or 7 log cfu per cheese slice. Slices of sharp white Cheddar cheese and white American singles were cut in rectangles of 2.5 × 5 cm. For cheese slices receiving antimicrobial treatment before PL, slices were dipped in natamycin or nisin, spot inoculated with 100 μL of bacterial suspension, and then treated with PL. Cheese slices receiving PL treatment before antimicrobials were spot inoculated, treated with PL, and then treated with antimicrobials. The PL fluence levels from 1.02 to 12.29 J/cm2 were used. Survivors were enumerated by standard plate counting or the most probable number technique, as appropriate. All treatments were performed in triplicate, and the data were analyzed using a general linear model. Treatment with nisin or natamycin before PL decreased the effectiveness of PL for all bacteria tested. For instance, PL reduced P. fluorescens on Cheddar cheese by 2.19 ± 0.27 log after 6.14 J/cm2, whereas combination treatments at the same PL fluence yielded barely 1 log reduction. Inactivation of L. innocua on Cheddar was only 0.78 ± 0.01 log when using PL after nisin, compared with a 1.30 ± 0.76 log reduction by nisin alone. This was attributed to the absorption of UV light by the 2 antimicrobials, which diminished the UV fluence received by the bacteria. Increased inactivation was obtained when antimicrobials were applied after PL. On process cheese, a maximum reduction of 3.73 ± 0.96 log of L. innocua was obtained at 9.22 J/cm2 for PL followed by nisin, compared with 3.01 ± 0.48 by PL alone. This study demonstrates that antimicrobials may increase the antimicrobial effectiveness of PL on cheese surface, but the order of treatments is critical.

This study investigated the effect of sublethal temperatures on the efficacy of pulsed light (PL) treatment for the inactivation of Listeria innocua, Escherichia coli ATCC 25922, and Pseudomonas fluorescens. A thin layer of clear, liquid phosphate buffer inoculated with one of the challenge organisms, at a concentration of about 108 CFU/mL, was equilibrated to a temperature ranging from 5 °C to 50 °C and then treated with PL, at doses between 1.02 and 12.29 J/cm2. All treatments were performed in triplicate. In the temperature range of 5 °C to 40 °C, the average maximum reductions for L. innocua, E. coli, P. fluorescens were 6.27 ± 0.23 log CFU, 6.66 ± 0.36 log CFU, and 6.15 ± 0.19 log CFU, respectively. Temperature did not affect PL inactivation of E. coli or P. fluorescens, but a modest synergistic effect between PL and temperature was observed for L. innocua treated above 40 °C. Industrial relevance This study suggests that PL inactivation of bacteria is independent of temperature in the sublethal range 5 °C to 40 °C. The practical benefit of this finding is that PL treatments can be conducted in a wide range of environmental temperatures, without any change in the outcome of the treatment.

Pulsed Light (PL) is effective for inactivation of bacteria in food and non-food systems, yet the mechanisms of inactivation are not fully understood. In this work, the response of Listeria monocytogenes to PL and continuous ultraviolet light (UV) was investigated using whole genome DNA microarray analysis. Suspensions of L. monocytogenes were treated with 3.20 J/cm 2 of PL and 33 mJ/cm 2 of UV, respectively, which yielded comparable levels of inactivation. In a separate experiment, cells were exposed to 3.20 J/cm 2 of full spectrum PL and UV-blocked PL. Whole genome microarray analysis of L. monocytogenes was performed to identify differential gene expression after the treatments, using a 1.5 fold change cutoff and adjusted P

The main goal of this study was to evaluate the effectiveness of a combination of antimicrobial films and Pulsed Light (PL) on inactivation of Listeria innocua on Cheddar cheese surface, and maintaining of product quality during refrigerated storage. The properties of antimicrobial starch films (ASF) containing sodium benzoate (ASF-SB), citric acid (ASF-CA), and both (ASF-CASB), prepared by casting and irradiated with PL, were evaluated. These films were used as packaging for Cheddar cheese slices that were surface inoculated with L. innocua at an initial inoculum level of 7 log CFU per cheese slice. PL had no effects on the structure of ASF. PL applied in combination with ASF-CA achieved an average of 4.5 log CFU reduction of L. innocua after 3 days of storage at 4 °C. However, significant changes in physicochemical properties of cheese, including pH, moisture level and mechanical properties, were observed after 7 days of refrigerated storage. These results suggest that, while PL-ASF can be effective in reducing L. innocua on Cheddar cheese surface, this combination treatment may induce some quality changes in the product.

The objective of this investigation was to develop the mathematical models that included the effects of time, temperature, pH and substrate concentration on the non-enzymatic browning reaction. Glycine-glucose solutions were used as food models. Experimental data of 420-nm browning absorbance, which were converted into s-curved Xt values, were used for the modeling and validation processes. The impacts of multiple factors on browning were well expressed by a combined use of modified Gompertz, Arrhenius and interaction polynomial models and a data conversion technique (R2 ≥ 0.9441). The browning rates varied greatly (λ = 7.46–888 min and umax = 0.06 to 26.90 min−1), depending on temperature, pH and concentration. The activation energy (Ea) values of λ (62–120 kJ/mol) and umax (87–129 kJ/mol) varied, depending more on concentration than pH. Although non-enzymatic browning reactions are complex with a variety of pathways and reactive products in response to multiple factors, they are likely modellable so that the benefits (food quality based on color and aroma) and harmful properties (e.g., carcinogens) of browning can be better managed.

Cheese products are susceptible to postprocessing cross-contamination by bacterial surface contamination during slicing, handling, or packaging, which can lead to food safety issues and significant losses due to spoilage. This study examined the effectiveness of pulsed-light (PL) treatment on the inactivation of the spoilage microorganism Pseudomonas fluorescens, the nonenterohemorrhagic Escherichia coli ATCC 25922 (nonpathogenic surrogate of Escherichia coli O157:H7), and Listeria innocua (nonpathogenic surrogate of Listeria monocytogenes) on cheese surface. The effects of inoculum level and cheese surface topography and the presence of clear polyethylene packaging were evaluated in a full factorial experimental design. The challenge microorganisms were grown to early stationary phase and subsequently diluted to reach initial inoculum levels of either 5 or 7 log cfu/slice. White Cheddar and process cheeses were cut into 2.5×5 cm slices, which were spot-inoculated with 100 µL of bacterial suspension. Inoculated cheese samples were exposed to PL doses of 1.02 to 12.29 J/cm(2). Recovered survivors were enumerated by standard plate counting or the most probable number technique, as appropriate. The PL treatments were performed in triplicate and data were analyzed using a general linear model. Listeria innocua was the least sensitive to PL treatment, with a maximum inactivation level of 3.37±0.2 log, followed by P. fluorescens, with a maximum inactivation of 3.74±0.8 log. Escherichia coli was the most sensitive to PL, with a maximum reduction of 5.41±0.1 log. All PL inactivation curves were nonlinear, and inactivation reached a plateau after 3 pulses (3.07 J/cm(2)). The PL treatments through UV-transparent packaging and without packaging consistently resulted in similar inactivation levels. This study demonstrates that PL has strong potential for decontamination of the cheese surface.

This work reports on a simple, robust and scientifically sound method to develop surfaces able to reduce microbial attachment and biofilm development, with possible applications in medicine, dentistry, food processing, or water treatment. Anodic surfaces with cylindrical nanopores 15 to 100 nm in diameter were manufactured and incubated with Escherichia coli ATCC 25922 and Listeria innocua. Surfaces with 15 and 25 nm pore diameters significantly repressed attachment and biofilm formation. Surface-bacteria interaction forces calculated using the extended Derjaguin Landau Verwey-Overbeek (XDLVO) theory indicate that reduction in attachment and biofilm formation is due to a synergy between electrostatic repulsion and surface effective free energy. An attachment study using E. coli K12 strains unable to express appendages also suggests that the small-pore surfaces may inhibit flagella-dependent attachment. These results can have immediate, far-reaching implications and commercial applications, with substantial benefits for human health and life.

Pulsed light (PL) treatment has been viewed as an alternative to thermal treatments for the inactivation of pathogenic and spoilage microorganisms in recent years. The objectives of this study were to quantify the effectiveness of PL on inactivating Escherichia coli in cow milk and to evaluate the effect of total solids and fat content on inactivation. Samples of reconstituted milk with variable total solids levels (9.8, 25, and 45%) and commercial cow milk with different fat contents (skim milk, 2% fat, and whole milk) were inoculated with nonpathogenic E. coli ATCC 25922 at a concentration of 10(7)cfu/mL. One milliliter of the inoculated sample was placed in a thin layer in a glass chamber and exposed to PL doses of up to 14.9 J/cm(2), both in static mode and turbulent mode. Survivors were quantified using standard plate counting. All experiments were performed in triplicate. Pulsed light treatment of the concentrated milks of 25 and 45% solids content resulted in reductions of less than 1 log, even in turbulent mode, whereas for the milk with 9.8% solids content, reduction levels of 2.5 log cfu were obtained after treatment with 8.4 J/cm(2) in turbulent mode. In the skim milk, a 3.4 log cfu reduction at 14.9 J/cm(2) was obtained and a plateau of the inactivation curve typical of PL treatment was not achieved. Under the same conditions, both 2% and whole milk attained inactivation levels greater than 2.5 log cfu. These data indicate that PL is effective for the inactivation of E. coli in milk, but has limited effectiveness for microbial inactivation in concentrated milk, due to the absorption of light by the milk solids and shielding of the bacteria in the concentrated substrates. Milk fat also diminishes the effectiveness of PL to some extent, due to light-scattering effects.

The main objective of this work was to evaluate the effectiveness of pulsed light (PL) treatment for the inactivation of Escherichia coli in liquids with different levels of clarity. Nonpathogenic E. coli ATCC 25922 and pathogenic E. coli O157: H7 were used as challenge organisms. Butterfield's phosphate buffer (BPB), tryptic soy broth (TSB), apple juice, and apple cider were used as substrates. The inoculated liquids were placed in a thin layer (1.3 mm) into glass chambers (23 by 53 by 11 mm) and exposed to PL doses of up to 13.1 J/cm2. PL treatments were performed in a Xenon RS-3000C PL unit, both in static mode and under turbulence. Survivors were determined by standard plate counting or the most-probable-number technique. For static treatments, reduction levels exceeding 8.5 log were obtained in BPB for all strains and reduction levels of about 3.5 log were obtained in TSB. For apple juice, inactivation levels of 2.66 +/- 0.10 log were obtained for E. coli ATCC 25922 and 2.52 +/- 0.19 log for E. coli O157:H7. In cider, inactivation levels of 2.32 +/- 0.16 log and 3.22 +/- 0.29 log were obtained for the nonpathogenic and pathogenic strains, respectively. Inactivation kinetics was characterized using the Weibull model. Turbulent treatments resulted in 5.76 +/- 0.06 log reduction in cider and 7.15 +/- 0.22 log reduction in juice, which satisfies the U.S. Food and Drug Administration requirement of 5-log reduction of E. coli. These results show promise for the use of PL for the effective reduction of E. coli in apple juice and cider.

The risk of listeriosis associated with ready-to-eat foods is a major concern in the United States. Pulsed light (PL) treatment has been effective for killing Listeria. The possibility of enhancing the antilisterial capability of PL treatment by combining PL with an additional hurdle, the natural antimicrobial nisin, was explored in this study. First, the ability of Listeria innocua to mimic the response of Listeria monocytogenes to PL treatment was demonstrated. Subsequently, a series of inoculation studies was performed in which canned sausages were surface inoculated with L. innocua as a surrogate for L. monocytogenes and then treated with a commercial preparation of nisin (Nisaplin), PL, or a combination of the two treatments. The application of a Nisaplin dip alone resulted in an immediate reduction of L. innocua by 2.35 +/- 0.09 log CFU. PL reduced L. innocua by 1.37 +/- 0.30 log CFU after exposure to 9.4 J/cm2. A total reduction of 4.03 +/- 0.15 log CFU was recorded after the combined treatment of Nisaplin and PL for 48 h at 4 degrees C. The long-term survival of L. innocua was evaluated on sausages stored at 4 degrees C. Treatment with Nisaplin and PL resulted in a 4- to 5-log reduction for two replicate studies. The combination treatment resulted in no significant microbial growth during 28 and 48 days of refrigerated storage in the first and second replicates, respectively. These results suggest that this combination treatment can be used as an effective antilisterial step in the production of ready-to-eat foods.

Pulsed light (PL) treatment can effectively reduce microbial populations in clear substrates and on surfaces, but its effectiveness varies as a function of substrate or treatment-related factors. For PL to be successfully adopted by the food industry, all factors of influence, as well as the inactivation kinetics for the microorganisms of concern, must be elucidated. In this study, the inactivation kinetics of Listeria innocua and the effect of inoculum size on PL inactivation were investigated. Stainless steel coupons (50.8 by 101.6 mm) of defined surface properties and transparent glass chamber slides (25.4 by 50.8 by 10 mm) were each inoculated with 1 ml of aqueous suspensions of L. innocua containing inoculum populations of up to 10(9) CFU. The thickness of the liquid layer in the glass slides was 1.16 mm. The inoculated substrates were exposed to PL treatment of up to 17 J/cm2 in a static PL chamber equipped with a pulsed Xenon lamp. Survivors were recovered and enumerated by both standard plate counting and most-probable-number procedures. The data indicated that in clear liquids, PL resulted in more than a 6-log reduction of L. innocua after a 12-J/cm2 treatment, regardless of the initial inoculum size. For the stainless steel surfaces, less than a 4-log reduction after a 12-J/cm2 treatment and a noticeable effect of substrate characteristics and inoculum size on inactivation were observed. The survivor curves showed pronounced tailing for all substrates used in the study. The Weibull model accurately predicted the survivor ratios for the PL treatment of L. innocua in clear liquids, with a shape and scale parameter of 0.33 and 3.01, respectively. The Weibull model resulted in significant overestimation of PL effectiveness for the stainless steel substrates, where the influence of various substrate properties and inoculum level on inactivation was significant.

Thermal pasteurization can achieve the U. S. Food and Drug Administration-required 5-log reduction of pathogenic Escherichia coli O157:H7 and Cryptosporidium parvum in apple juice and cider, but it can also negatively affect the nutritional and organoleptic properties of the treated products. In addition, thermal pasteurization is only marginally effective against the acidophilic, thermophilic, and spore-forming bacteria Alicyclobacillus spp., which is known to cause off-flavors in juice products. In this study, the efficiency of a combined microfiltration (MF) and UV process as a nonthermal treatment for the reduction of pathogenic and nonpathogenic E. coli, C. parvum, and Alicyclobacillus acidoterrestris from apple cider was investigated. MF was used to physically remove suspended solids and microorganisms from apple cider, thus enhancing the effectiveness of UV and allowing a lower UV dose to be used. MF, with ceramic membranes (pore sizes, 0.8 and 1.4 μm), was performed at a temperature of 10 °C and a transmembrane pressure of 155 kPa. The subsequent UV treatment was conducted using at a low UV dose of 1.75 mJ/cm(2). The combined MF and UV achieved more than a 5-log reduction of E. coli, C. parvum, and A. acidoterrestris. MF with the 0.8-μm pore size performed better than the 1.4-μm pore size on removal of E. coli and A. acidoterrestris. The developed nonthermal hurdle treatment has the potential to significantly reduce pathogens, as well as spores, yeasts, molds, and protozoa in apple cider, and thus help juice processors improve the safety and quality of their products.

Pulsed light (PL) treatment has been proven effective for killing a wide variety of microorganisms on foods and food contact materials. However, there is concern regarding how shading may impact the effectiveness of PL when applied to imperfect surfaces. The main objective of this work was to examine how surface properties, particularly topography, influence the microbicidal effect of PL. Four types of stainless-steel surfaces were inoculated with Listeria innocua and treated with up to 12 pulses of light. The highest level of inactivation achieved was about a 4-log reduction. Initially, an increase in inactivation with increasing treatment intensity was observed, but the inactivation curves tailed off above 3 light pulses. The differences in inactivation levels among the 4 finishes at specific treatment levels were rather insignificant, but some interesting trends were observed. At low treatment levels, inactivation on the smoothest finish was slightly lower than for the other surfaces, due to clustering of the cells on the highly hydrophobic smooth surface and to its reflective nature. For the roughest surface, scanning electron microscopy (SEM) imaging confirmed the preferential location of the cells inside surface features, which also promoted a relatively uniform distribution of the cells across the surface. This counter balanced to some extent of the shading effects, and as a result inactivation on the roughest surface was comparable to inactivation on the smoother surfaces. These results demonstrate that PL can be effective on both smooth and rough surfaces, but also indicate a complex effect of various surface properties on inactivation.

Pulsed light (PL) treatment can effectively inactivate a large proportion of contaminating bacteria on surfaces and in clear solutions. An important issue that needs to be investigated is whether repeated exposure to PL treatment causes any changes to the growth and resistance behavior of the bacteria surviving the treatment. To test this, three challenge microorganisms were used: Listeria monocytogenes, Listeria innocua, and Escherichia coli. Cells of the challenge bacteria were treated with either low or high PL doses. Survivors of the PL treatment were enumerated, isolated, regrown, and exposed again to PL treatment. PL inactivation curves were generated for the survivors of each exposure cycle (as well as controls) to examine possible differences induced by repeated treatments. Growth curves of L. monocytogenes, L. innocua, and E. coli isolates recovered from exposure to either 1.1 or 10.1 J/cm(2) were not significantly different from the growth curves of untreated cells. Reduction levels of up to 4 and up to 6 log CFU were obtained after exposure to 1.1 and 10.1 J/cm(2), respectively, both for the controls and the repeatedly treated and recovered isolates. These results show that PL did not significantly change the growth kinetics or resistance to PL of the target microorganisms after up to 10 exposures. These findings have significance for the practical application of PL treatment, as they indicate that this technology does not select for microorganisms with increased resistance.

Attachment and biofilm formation by bacterial pathogens on surfaces in natural, industrial, and hospital settings lead to infections and illnesses and even death. Minimizing bacterial attachment to surfaces using controlled topography could reduce the spreading of pathogens and, thus, the incidence of illnesses and subsequent human and financial losses. In this context, the attachment of key microorganisms, including Escherichia coli , Listeria innocua , and Pseudomonas fluorescens , to silica and alumina surfaces with micron and nanoscale topography was investigated. The results suggest that orientation of the attached cells occurs preferentially such as to maximize their contact area with the surface. Moreover, the bacterial cells exhibited different morphologies, including different number and size of cellular appendages, depending on the topographical details of the surface to which they attached. This suggests that bacteria may utilize different mechanisms of attachment in response to surface topography. These results are important for the design of novel microbe-repellant materials.

Pulsed light (PL) treatment is an alternative to traditional thermal treatment that has the potential to achieve several log-cycle reductions in the concentration of microorganisms. One issue that is still debated is related to what specifically causes cell death after PL treatments. The main objective of this work was to elucidate which portions of the PL range are responsible for bacterial inactivation. Stainless steel coupons with controlled surface properties were inoculated with a known concentration of Listeria innocua in the stationary growth phase and treated with 1 to 12 pulses of light at a pulse rate of 3 pulses per s and a pulse width of 360 micros. The effects of the full spectrum (lambda = 180 to 1,100 nm) were compared with the effects obtained when only certain regions of UV, visible, and near-infrared light were used. The effectiveness of the treatments was determined in parallel by the standard plate count and most-probable-number techniques. At a fluence of about 6 J/cm(2), the full-spectrum PL treatment resulted in a 4.08-log reduction of L. innocua on a Mill finish surface, the removal of lambda200 nm diminished the reduction to only 1.64 log, and total elimination of UV light resulted in no lethal effects on L. innocua. Overwhelmingly, the portions of the PL spectrum responsible for bacterial death are the UV-B and UV-C spectral ranges (X300 nm), with some death taking place during exposure to UV-A radiation (300lambda400 nm) and no observable death upon exposure to visible and near-infrared light (lambda400 nm). This work provides additional supporting evidence that cell death in PL treatment is due to exposure to UV light. Additionally, it was shown that even a minor modification of the light path or the UV light spectrum in PL treatments can have a significant negative impact on the treatment intensity and effectiveness.