Variations in biofilms harbouring Listeria monocytogenes in dual and triplex cultures with Pseudomonas fluorescens and Lactobacillus plantarum produced under a model system of simulated meat processing conditions, and their resistance to benzalkonium chloride

Pseudomonadaceae increased the tolerance of Listeria monocytogenes to sanitizers in multi-species biofilms

The persistence of the foodborne pathogen Listeria monocytogenes in food processing facilities may be facilitated by the formation of multi-species biofilms by environmental microbiota. This study aimed to determine whether multi-species biofilm formation results in an increased tolerance of L. monocytogenes in biofilms to the sanitizers benzalkonium chloride (BAC) and peroxyacetic acid (PAA) at concentrations commonly used in food processing facilities. Biofilms composed of microbiota previously shown to co-occur with L. monocytogenes in tree fruit packing facilities (i.e., Pseudomonadaceae, Xanthomonadaceae, Flavobacteriaceae, and Microbacteriaceae) were formed with L. monocytogenes in single- and multi-family assemblages. Multi-family biofilms were exposed to 250 or 500 ppm of PAA, or 200 ppm of BAC to determine the die-off kinetics of L. monocytogenes. Furthermore, the ability of a commercial biofilm remover to disrupt biofilms and inhibit bacteria in the formed single- and multi-family assemblage biofilms was assessed. The die-off kinetics of total bacteria and L. monocytogenes in biofilm assemblages throughout the exposure to a sanitizer was determined using the aerobic plate count and the most probable number methods, respectively. Biofilm assemblages that included Pseudomonadaceae resulted in an increased tolerance of L. monocytogenes to BAC and PAA compared to biofilm assemblages without Pseudomonadaceae. Further, the use of the biofilm remover significantly disrupted biofilms and reduced the concentration of L. monocytogenes in single- and multi-family biofilms by 5 or more logarithmic units. These findings highlight the need to improve the control of biofilm-forming microbiota in food processing facilities to mitigate the persistence of L. monocytogenes.

Listeria monocytogenes and Listeriosis: The Global Enigma

Listeria monocytogenes is an intracellular, Gram-positive, non-spore-forming, non-encapsulated, facultative anaerobic, rod-shaped, and psychrotrophic food-borne pathogen that causes the infection, listeriosis, thus it attracts great attention following listeriosis outbreaks, which are often associated with high mortality rates. The prevalence of listeriosis is quite low globally; however, the most recent and deadliest outbreak occurred in South Africa, during which 216 persons lost their lives. L. monocytogenes is endowed with the potential to multiply through a wide range of harsh environmental conditions, forming biofilms on varying surfaces in the food industry, as well as having persistent and antibiotic-resistant cells, which pose a major threat and burden to the ready-to-eat food industry. A more frustrating characteristic of this bacterium is its strain divergence, alongside an increased level of antibiotic resistance registered among the strains of L. monocytogenes recovered from food, humans, and environmental sources, especially to those antibiotics involved in the treatment of human listeriosis. Antibiotic resistance exerted by and among pathogenic food-borne microbes is an ongoing public health menace that continues to be an issue. Against this background, a thorough search into different databases using various search engines was performed, which led to the gathering of salient information that was organised, chronologically, based on Listeria monocytogenes and listeriosis. Altogether, the findings elaborated in this study present up-to date knowledge on different aspects of this pathogen which will improve our understanding of the mystery associated with it and the ways to prevent and control its dissemination through ready-to-eat foods. In addition, constant monitoring of the antibiotic resistance profiles of strains of L. monocytogenes from varying sources detected changes, giving an update on the trend in antibiotic resistance. Overall, monitoring of bacterial contamination serves as the key aspect in the control of the food safety output in the food industry.

The effect of commercial sanitizers on Listeria monocytogenes (planktonic and biofilm forms) experimentally inoculated materials commonly used during tree-fruit harvesting

This study compares the efficacy of commercially available sanitizers in reducing Listeria monocytogenes planktonic and biofilm form on surfaces commonly used during tree fruit harvesting. Planktonic L. monocytogenes cells were grown to create a bacterial lawn and inoculated on high-density polyethylene plastic, wood, or nylon. Biofilms were grown for 96 h in a Centers for Disease Control reactor on the same surfaces. Bacteria were exposed to free chlorine, peroxyacetic acid, silver-dihydrogen citrate, steam, and chlorine dioxide. In planktonic forms, peroxyacetic acid and silver-dihydrogen citrate, applied for 2 min resulted in ≥3-log reduction regardless surface type (P < 0.05). Steam and chlorine dioxide efficacy differed significantly by surface type. When applied to Listeria biofilms, steam, chlorine, peroxyacetic acid, and chlorine dioxide resulted in an overall significant difference compared to untreated coupons for all surfaces (P < 0.05). Silver-dihydrogen citrate significantly affected population on plastic (P < 0.05), but not on wood or nylon. For porous surfaces few treatments, such as peroxyacetic acid (for nylon) and chlorine dioxide (for wood), resulted in a ≥3 log reduction of both forms. Interactions between sanitizer, surfaces, and exposure time should be considered by tree fruit harvesting facilities when determining sanitation strategies to be implemented.

Recent advances in examining the factors influencing the efficacy of biocides against Listeria monocytogenes biofilms in the food industry: A systematic review

Controlling Listeria monocytogenes and its associated biofilms in the food industry requires various disinfection techniques, including physical, chemical, and biological treatments. Biocides, owing to their ease of use, cost-effectiveness, dissolvability in water, and efficacy against a wide range of microorganisms, are frequently selected options. Nonetheless, concerns have been raised about their efficacy in controlling L. monocytogenes biofilm, as laboratory-based and commercial studies have reported the persistence of this bacterium after cleaning and disinfection. This review systematically examined scientific studies, sourced from the Web of Science, Scopus, and PubMed databases between January 2010 and May 2024, that investigated the effectiveness of the most commonly used biocides in the food industry against L. monocytogenes biofilms. A total of 92 articles which met the screening criteria, were included, with studies utilizing biocides containing sodium hypochlorite, quaternary ammonium compounds, and peroxyacetic acid being predominant. Studies indicated that several key factors may potentially influence biocides' efficacy against L. monocytogenes biofilms. These factors included strain type (persistent, sporadic), serotype, strain origin (clinical, environmental, or food), surface type (biotic or abiotic), surface material (stainless steel, polystyrene, etc.), incubation time (biofilm age) and temperature, presence of organic matter, biocide's active agent, and the co-culture of L. monocytogenes with other bacteria. The induction of the viable but nonculturable (VBNC) state following disinfection is also a critical concern. This review aims to provide a global understanding of how L. monocytogenes biofilms respond to biocides under different treatment conditions, facilitating the development of effective cleaning and disinfection strategies in the food industry.

Environmental Monitoring of Nebraska Ready-to-eat Meat Processing Establishments Resulted in the Isolation of Listeria Alongside Pseudomonas Highly Resistant to Quaternary Ammonia Sanitizer

Robust environmental monitoring for Listeria monocytogenes often may not be feasible for small and very small meat processors in the United States due to the limitations in finances, staffing, or expertise. Three small/very small processors in Nebraska were sampled using sponge applicators in nonfood contact surface areas to determine if biofilm and sanitizer resistance behaviors of Pseudomonas could relate to the prevalence of L. monocytogenes and Listeria spp. in ready-to-eat meat processing environments. Samples were 3.3% (3/90) positive for L. monocytogenes, and 12.2% (11/90) of samples were positive for Listeria spp. Pseudomonas spp. were also isolated. When Listeria spp. and Pseudomonas spp. were assayed for biofilm production and resistance to a quaternary ammonia sanitizer, multiple isolates belonging to both genera capable of forming biofilms were identified. Four Pseudomonas spp. isolates resisted the 200 ppm manufacturer-recommended sanitizer concentration for food contact surface sanitation, and one Pseudomonas spp. isolated from a drain sample that was also positive for L. monocytogenes demonstrated a sanitizer minimum bactericidal concentration of 1000 ppm. These findings further support the need for monitoring of small and very small meat processors for L. monocytogenes as well as highlight the need to identify other bacteria in these processing environments, like Pseudomonas, that are resistant to environmental stressors.

Legionella colonization and 3D spatial location within a Pseudomonas biofilm

Biofilms are known to be critical for Legionella settlement in engineered water systems and are often associated with Legionnaire's Disease events. One of the key features of biofilms is their heterogeneous three-dimensional structure which supports the establishment of microbial interactions and confers protection to microorganisms. This work addresses the impact of Legionella pneumophila colonization of a Pseudomonas fluorescens biofilm, as information about the interactions between Legionella and biofilm structures is scarce. It combines a set of meso- and microscale biofilm analyses (Optical Coherence Tomography, Episcopic Differential Interference Contrast coupled with Epifluorescence Microscopy and Confocal Laser Scanning Microscopy) with PNA-FISH labelled L. pneumophila to tackle the following questions: (a) does the biofilm structure change upon L. pneumophila biofilm colonization?; (b) what happens to L. pneumophila within the biofilm over time and (c) where is L. pneumophila preferentially located within the biofilm? Results showed that P. fluorescens structure did not significantly change upon L. pneumophila colonization, indicating the competitive advantage of the first colonizer. Imaging of PNA-labelled L. pneumophila showed that compared to standard culture recovery it colonized to a greater extent the 3-day-old P. fluorescens biofilms, presumably entering in VBNC state by the end of the experiment. L. pneumophila was mostly located in the bottom regions of the biofilm, which is consistent with the physiological requirements of both bacteria and confers enhanced Legionella protection against external aggressions. The present study provides an expedited methodological approach to address specific systematic laboratory studies concerning the interactions between L. pneumophila and biofilm structure that can provide, in the future, insights for public health Legionella management of water systems.

Airborne signals of Pseudomonas fluorescens modulate swimming motility and biofilm formation of Listeria monocytogenes in a contactless coculture system

Bacterial volatile compounds (BVCs) facilitate interspecies communication in socio-microbiology across physical barriers, thereby influencing interactions between diverse species. The impact of BVCs emitted from Pseudomonas on the biofilm formation characteristics of Listeria monocytogenes within the same ecological niche has been scarcely investigated under practical conditions of food processing. The objective of this study was to explore the motility and biofilm formation characteristics of L. monocytogenes under the impact of Pseudomonas BVCs. It was revealed that BVCs of P. fluorescens, P. lundensis, and P. fragi significantly promoted swimming motility of L. monocytogenes (P < 0.05). As evidenced by crystal violet staining, the L. monocytogenes biofilms reached a maximum OD570 value of approximately 3.78 at 4 d, which was 0.65 units markedly higher than that of the control group (P < 0.05). Despite a decrease in adherent cells of L. monocytogenes biofilms among the BVCs groups, there was a remarkable increase in the abundance of extracellular polysaccharides and proteins with 3.58 and 4.90 μg/cm2, respectively (P < 0.05), contributing to more compact matrix architectures, which suggested that the BVCs of P. fluorescens enhanced L. monocytogenes biofilm formation through promoting the secretion of extracellular polymers. Moreover, the prominent up-regulated expression of virulence genes further revealed the positive regulation of L. monocytogenes under the influence of BVCs. Additionally, the presence of BVCs significantly elevated the pH and TVB-N levels in both the swimming medium and biofilm broth, thereby exhibiting a strong positive correlation with increased motility and biofilm formation of L. monocytogenes. It highlighted the crucial signaling regulatory role of BVCs in bacterial interactions, while also emphasizing the potential food safety risk associated with the hitchhiking behavior of L. monocytogenes, thereby shedding light on advancements in control strategies for food processing.

Multi-species biofilms of environmental microbiota isolated from fruit packing facilities promoted tolerance of Listeria monocytogenes to benzalkonium chloride

Listeria monocytogenes may survive and persist in food processing environments due to formation of complex multi-species biofilms of environmental microbiota that co-exists in these environments. This study aimed to determine the effect of selected environmental microbiota on biofilm formation and tolerance of L. monocytogenes to benzalkonium chloride in formed biofilms. The studied microbiota included bacterial families previously shown to co-occur with L. monocytogenes in tree fruit packing facilities, including Pseudomonadaceae, Xanthomonadaceae, Microbacteriaceae, and Flavobacteriaceae. Biofilm formation ability and the effect of formed biofilms on the tolerance of L. monocytogenes to benzalkonium chloride was measured in single- and multi-family assemblages. Biofilms were grown statically on polystyrene pegs submerged in a R2A broth. Biofilm formation was quantified using a crystal violet assay, spread-plating, confocal laser scanning microscopy, and its composition was assessed using amplicon sequencing. The concentration of L. monocytogenes in biofilms was determined using the most probable number method. Biofilms were exposed to the sanitizer benzalkonium chloride, and the death kinetics of L. monocytogenes were quantified using a most probable number method. A total of 8, 8, 6, and 3 strains of Pseudomonadaceae, Xanthomonadaceae, Microbacteriaceae, and Flavobacteriaceae, respectively, were isolated from the environmental microbiota of tree fruit packing facilities and were used in this study. Biofilms formed by Pseudomonadaceae, Xanthomonadaceae, and all multi-family assemblages had significantly higher concentration of bacteria, as well as L. monocytogenes, compared to biofilms formed by L. monocytogenes alone. Furthermore, multi-family assemblage biofilms increased the tolerance of L. monocytogenes to benzalkonium chloride compared to L. monocytogenes mono-species biofilms and planktonic multi-family assemblages. These findings suggest that L. monocytogenes control strategies should focus not only on assessing the efficacy of sanitizers against L. monocytogenes, but also against biofilm-forming microorganisms that reside in the food processing built environment, such as Pseudomonadaceae or Xanthomonadaceae.

Degradation of Listeria monocytogenes biofilm by phages belonging to the genus Pecentumvirus

Listeria monocytogenes is a pathogenic foodborne bacterium that is a significant cause of mortality associated with foodborne illness and causes many food recalls attributed to a bacteriological cause. Their ability to form biofilms contributes to the persistence of Listeria spp. in food processing environments. When growing as biofilms, L. monocytogenes are more resistant to sanitizers used in the food industry, such as benzalkonium chloride (BAC), as well as to physical stresses like desiccation and starvation. Lytic phages of Listeria are antagonistic to a broad range of Listeria spp. and may, therefore, have utility in reducing the occurrence of Listeria-associated food recalls by preventing food contamination. We screened nine closely related Listeria phages, including the commercially available Listex P100, for host range and ability to degrade microtiter plate biofilms of L. monocytogenes ATCC 19111 (serovar 1/2a). One phage, CKA15, was selected and shown to rapidly adsorb to its host under conditions relevant to applying the phage in dairy processing environments. Under simulated dairy processing conditions (SDPC), CKA15 caused a 2-log reduction in Lm19111 biofilm bacteria. This work supports the biosanitation potential of phage CKA15 and provides a basis for further investigation of phage-bacteria interactions in biofilms grown under SDPC.

IMPORTANCE

Listeria monocytogenes is a pathogenic bacterium that is especially dangerous for children, the elderly, pregnant women, and immune-compromised people. Because of this, the food industry takes its presence in their plants seriously. Food recalls due to L. monocytogenes are common with a high associated economic cost. In food-processing plants, Listeria spp. typically reside in biofilms, which are structures produced by bacteria that shield them from environmental stressors and are often attached to surfaces. The significance of our work is that we show a bacteriophage-a virus-infecting bacteria-can reduce Listeria counts by two orders of magnitude when the bacterial biofilms were grown under simulated dairy processing conditions. This work provides insights into how phages may be tested and used to develop biosanitizers that are effective but are not harmful to the environment or human health.

Biofilm formation and desiccation survival of Listeria monocytogenes with microbiota on mushroom processing surfaces and the effect of cleaning and disinfection

Microbial multispecies communities consisting of background microbiota and Listeria monocytogenes could be established on materials used in food processing environments. The presence, abundance and diversity of the strains within these microbial multispecies communities may be affected by mutual interactions and differences in resistance towards regular cleaning and disinfection (C&D) procedures. Therefore, this study aimed to characterize the growth and diversity of a L. monocytogenes strain cocktail (n = 6) during biofilm formation on polyvinyl chloride (PVC) and stainless steel (SS) without and with the presence of a diverse set of background microbiota (n = 18). L. monocytogenes and background microbiota strains were isolated from mushroom processing environments and experiments were conducted in simulated mushroom processing environmental conditions using mushroom extract as growth medium and ambient temperature (20 °C) as culturing temperature. The L. monocytogenes strains applied during monospecies biofilm incubation formed biofilms on both PVC and SS coupons, and four cycles of C&D treatment were applied with a chlorinated alkaline cleaning agent and a disinfection agent based on peracetic acid and hydrogen peroxide. After each C&D treatment, the coupons were re-incubated for two days during an incubation period for 8 days in total, and C&D resulted in effective removal of biofilms from SS (reduction of 4.5 log CFU/cm2 or less, resulting in counts below detection limit of 1.5 log CFU/cm2 after every C&D treatment), while C&D treatments on biofilms formed on PVC resulted in limited reductions (reductions between 1.2 and 2.4 log CFU/cm2, which equals a reduction of 93.7 % and 99.6 %, respectively). Incubation of the L. monocytogenes strains with the microbiota during multispecies biofilm incubation led to the establishment of L. monocytogenes in the biofilm after 48 h incubation with corresponding high L. monocytogenes strain diversity in the multispecies biofilm on SS and PVC. C&D treatments removed L. monocytogenes from multispecies biofilm communities on SS (reduction of 3.5 log CFU/cm2 or less, resulting in counts below detection limit of 1.5 log CFU/cm2 after every C&D treatment), with varying dominance of microbiota species during different C&D cycles. However, C&D treatments of multispecies biofilm on PVC resulted in lower reductions of L. monocytogenes (between 0.2 and 2.4 log CFU/cm2) compared to single species biofilm, and subsequent regrowth of L. monocytogenes and stable dominance of Enterobacteriaceae and Pseudomonas. In addition, planktonic cultures of L. monocytogenes were deposited and desiccated on dry surfaces without and with the presence of planktonic background microbiota cultures. The observed decline of desiccated cell counts over time was faster on SS compared to PVC. However, the application of C&D resulted in counts below the detection limit of 1.7 log CFU/coupon on both surfaces (reduction of 5.9 log CFU/coupon or less). This study shows that L. monocytogenes is able to form single and multispecies biofilms on PVC with high strain diversity following C&D treatments. This highlights the need to apply more stringent C&D regime treatments for especially PVC and similar surfaces to efficiently remove biofilm cells from food processing surfaces.

Sources and contamination routes of seafood with human pathogenic Vibrio spp.: A Farm-to-Fork approach

Vibrio spp., known human foodborne pathogens, thrive in freshwater, estuaries, and marine settings, causing vibriosis upon ingestion. The rising global vibriosis cases due to climate change necessitate a deeper understanding of Vibrio epidemiology and human transmission. This review delves into Vibrio contamination in seafood, scrutinizing its sources and pathways. We comprehensively assess the contamination of human-pathogenic Vibrio in the seafood chain, covering raw materials to processed products. A "Farm-to-Fork" approach, aligned with the One Health concept, is essential for grasping the complex nature of Vibrio contamination. Vibrio's widespread presence in natural and farmed aquatic environments establishes them as potential entry points into the seafood chain. Environmental factors, including climate, human activities, and wildlife, influence contamination sources and routes, underscoring the need to understand the origin and transmission of pathogens in raw seafood. Once within the seafood chain, the formation of protective biofilms on various surfaces in production and processing poses significant food safety risks, necessitating proper cleaning and disinfection to prevent microbial residue. In addition, inadequate seafood handling, from inappropriate processing procedures to cross-contamination via pests or seafood handlers, significantly contributes to Vibrio food contamination, thus warranting attention to reduce risks. Information presented here support the imperative for proactive measures, robust research, and interdisciplinary collaboration in order to effectively mitigate the risks posed by human pathogenic Vibrio contamination, safeguarding public health and global food security. This review serves as a crucial resource for researchers, industrials, and policymakers, equipping them with the knowledge to develop biosecurity measures associated with Vibrio-contaminated seafood.

Growth performance of Listeria monocytogenes and background microbiota from mushroom processing environments

Interaction between Listeria monocytogenes and resident background microbiota may occur in food processing environments and may influence the survival of this pathogen in a factory environment. Therefore the aim of this study was to characterize the growth performance of microbiota isolated from the processing environments of frozen sliced mushrooms, and to investigate the competitive performance of L. monocytogenes when co-cultured with accompanying environmental microbiota. Acinetobacter, Enterobacteriaceae, Lactococcus and Pseudomonas were the most prominent background microbiota isolated from the processing environment of frozen sliced mushrooms. All individual microbiota strains were able to grow and form biofilm in filter-sterilized mushroom medium, with the mannitol-consumers Raoultella and Ewingella as top performers, reaching up to 9.6 and 9.8 log CFU/mL after 48 h incubation at room temperature. When L. monocytogenes mushroom isolates were co-cultured with the microbiota strains, L. monocytogenes counts ranged from 7.6 to 8.9 log CFU/mL after 24 h of incubation, while counts of the microbiota strains ranged from 5.5 to 9.0 log CFU/mL. Prolonged incubation up to 48 h resulted in further increase of L. monocytogenes counts when co-cultured with non-acidifying species Pseudomonas and Acinetobacter reaching 9.1 to 9.2 log CFU/mL, while a decrease of L. monocytogenes counts reaching 5.8 to 7.7 log CFU/mL was observed in co-culture with Enterobacteriaceae and acidifying Lactococcus representatives. In addition, L. monocytogenes grew also in spent mushroom media of the microbiota strains, except in acidified spent media of Lactococcus strains. These results highlight the competitive ability of L. monocytogenes during co-incubation with microbiota in fresh and in spent mushroom medium, indicative of its invasion and persistence capacity in food processing factory environments.

The mixed biofilm formed by Listeria monocytogenes and other bacteria: Formation, interaction and control strategies

Listeria monocytogenes is an important foodborne pathogen. It can adhere to food or food contact surface for a long time and form biofilm, which will lead to equipment damage, food deterioration, and even human diseases. As the main form of bacteria to survive, the mixed biofilms often exhibit higher resistance to disinfectants and antibiotics, including the mixed biofilms formed by L. monocytogenes and other bacteria. However, the structure and interspecific interaction of the mixed biofilms are very complex. It remains to be explored what role the mixed biofilm could play in the food industry. In this review, we summarized the formation and influence factors of the mixed biofilm developed by L. monocytogenes and other bacteria, as well as the interspecific interactions and the novel control measures in recent years. Moreover, the future control strategies are prospected, in order to provide theoretical basis and reference for the research of the mixed biofilms and the targeted control measures.

The background microbiota and sanitization agent drive the fate of Listeria monocytogenes in multispecies biofilms formed on a plasma-polymerized coating applied on stainless steel

The present study evaluates the anti-biofilm activity of a coating applied with an atmospheric-pressure plasma jet system on AISI 316 stainless steel (SS) against multispecies biofilms containing Listeria monocytogenes (using background microbiota from three different meat industries) using culture-dependent and culture-independent approaches. Also, the disinfection effectiveness and biofilm evolution after sanitization with two food industry biocides were assessed. The anti-biofilm activity of the coating against L. monocytogenes, observed on mono-species biofilms (p < 0.05), was lost on the multispecies biofilms developed for 7 days at 12 °C (p > 0.05), with L. monocytogenes counts ranging from 5.5 ± 0.7 to 6.1 ± 0.5 CFU/cm² on the uncoated SS and from 4.4 ± 0.2 to 6.4 ± 0.5 CFU/cm² on the coated SS. The taxonomic composition of the formed biofilms was highly dependent on the industry but not affected by the artificial inoculation with L. monocytogenes and the nature of the surface (coated vs uncoated SS). When L. monocytogenes was artificially inoculated, its growth was partially controlled in the biofilms developed, with the magnitude of this effect being lower (p < 0.05 on coated SS) for the industry with the lowest taxonomy richness and diversity (3.8 ± 0.2 CFU/cm²), as compared the other two sampled industries (2.4 ± 0.4 and 1.6 ± 0.2 CFU/cm²). The 15-min disinfection treatments with either sodium hypochlorite or peracetic acid at 0.5 % resulted in total viable and L. monocytogenes counts below the limit of detection in most cases, immediately after treatment. The subsequent incubation of the sanitized plates for another 7 days at 12 °C in fresh BHI media led to the development of biofilms with lower bacterial richness and alpha diversity, and higher beta diversity. Even though sodium hypochlorite was in general slightly less effective than peracetic acid immediately after application, it caused a stronger growth control (p < 0.05) of the naturally present L. monocytogenes on the multispecies biofilms developed. This finding highlights the importance of understanding the interspecific competitive relationships between the members of the background microbiota and L. monocytogenes for the long-term control of this pathogen in food processing facilities.

Impact of Growth Conditions on Pseudomonas fluorescens Morphology Characterized by Atomic Force Microscopy

This work is dedicated to the characterization by Atomic Force Microscopy (AFM) of Pseudomonas fluorescens, bacteria having high potential in biotechnology. They were first studied first in optimal conditions in terms of culture medium and temperature. AFM revealed a more-or-less elongated morphology with typical dimensions in the micrometer range, and an organization of the outer membrane characterized by the presence of long and randomly distributed ripples, which are likely related to the organization of lipopolysaccharides (LPS). The outer membrane also presents invaginations, some of them showing a reorganization of ripples, which could be the first sign of a bacterial stress response. In a second step, bacteria grown under unfavorable conditions were characterized. The choice of the medium appeared to be more critical in the case of the second generation of cells, the less adapted medium inducing not only changes in the membrane organization but also larger damages in bacteria. An increased growth temperature affected both the usual “swollen” morphology and the organization of the outer membrane. Here also, LPS likely contribute to membrane remodelling, which makes them potential markers to track cell state changes.

N-chlorosuccinimide enhancing the antimicrobial effect of benzalkonium chloride on biofilm Pseudomonas aeruginosa and its interaction mechanisms

This study investigated the influence of N-chlorosuccinimide (NCS) pretreatment on the antimicrobial effect of benzalkonium chloride (BZC, representative of QACs) against biofilm bacteria and its mechanisms. Results show that 0.04 - 0.07 mmol/L NCS pretreatment significantly increased the antimicrobial efficacy of 0.03 mmol/L BZC on biofilm cells by 30% - 70%. The main mechanisms involved membrane permeability, oxidative damage, and metabolic disorder. More precisely, NCS pretreatment increased the permeability of bacteria and reduced the activity of the electron transport system (ETS) and dehydrogenase (DHA). At the same time, the oxidative damage of both endogenous and exogenous ROS and the disorder of the antioxidant enzymes (superoxide dismutase and catalase) further improved their combined antibacterial ability. Moreover, NCS pretreatment greatly reduced the resistance of biofilm Pseudomonas aeruginosa to BZC. The findings of the study provide a new method to effectively enhance the antimicrobial efficiency of quaternary ammonium cationic surfactants (e.g., BZC) and reduce bacterial resistance, as well as a scientific guidance for the development of new antimicrobial products.

Architectural Features and Resistance to Food-Grade Disinfectants in Listeria monocytogenes-Pseudomonas spp. Dual-Species Biofilms

Listeria monocytogenes is considered a foodborne pathogen of serious concern capable of forming multispecies biofilms with other bacterial species, such as Pseudomonas spp., adhered onto stainless steel (SS) surfaces. In an attempt to link the biofilms' morphology and resistance to biocides, dual-species biofilms of L. monocytogenes, in co-culture with either Pseudomonas aeruginosa, Pseudomonas fluorescens, or Pseudomonas putida, were assayed to ascertain their morphological characteristics and resistance toward benzalkonium chloride (BAC) and neutral electrolyzed water (NEW). Epifluorescence microscopy analysis revealed that each dual-species biofilm was distributed differently over the SS surface and that these differences were attributable to the presence of Pseudomonas spp. Confocal laser scanning microscopy (CLSM) assays demonstrated that despite these differences in distribution, all biofilms had similar maximum thicknesses. Along with this, colocalization analyses showed a strong trend of L. monocytogenes to share location within the biofilm with all Pseudomonas assayed whilst the latter distributed throughout the surface independently of the presence of L. monocytogenes, a fact that was especially evident in those biofilms in which cell clusters were present. Finally, a modified Gompertz equation was used to fit biofilms' BAC and NEW dose-response data. Outcomes demonstrated that L. monocytogenes was less susceptible to BAC when co-cultured with P. aeruginosa or P. fluorescens, whereas susceptibility to NEW was reduced in all three dual-species biofilms, which can be attributable to both the mechanism of action of the biocide and the architectural features of each biofilm. Therefore, the results herein provided can be used to optimize already existing and develop novel target-specific sanitation treatments based on the mechanism of action of the biocide and the biofilms' species composition and structure.

Photocatalytic inactivation of dual- and mono-species biofilms by immobilized TiO2

Biofilms formed by different bacterial species are likely to play key roles in photocatalytic resistance. This study aims to evaluate the efficacy of a photocatalytic immobilized nanotube system (TiO₂-NT) (IS) and suspended nanoparticles (TiO₂-NP) (SS) against mono- and dual-species biofilms developed by Gram-negative and Gram-positive strains. Two main factors were corroborated to significantly affect the biofilm resistance during photocatalytic inactivation, i.e., the biofilm-growth conditions and biofilm-forming surfaces. Gram-positive bacteria showed great photosensitivity when forming dual-species biofilms in comparison with the Gram-positive bacteria in single communities. When grown onto TiO₂-NT (IS) surfaces for immobilized photocatalytic systems, mono- and dual-species biofilms did not exhibit differences in photocatalytic inactivation according to kinetic constant values (p > 0.05) but led to a reduction of ca. 3-4 log₁₀. However, TiO₂-NT (IS) surfaces did affect biofilm colonization as the growth of mono-species biofilms of Gram-negative and Gram-positive bacteria is significantly (p ≤ 0.05) favored compared to co-culturing; although, the photocatalytic inactivation rate did not show initial bacterial concentration dependence. The biofilm growth surface (which depends on the photocatalytic configuration) also favored resistance of mono-species biofilms of Gram-positive bacteria compared to that of Gram-negative in immobilized photocatalytic systems, but opposite behavior was confirmed with suspended TiO₂ (p ≤ 0.05). Successful efficacy of immobilized TiO₂ for inactivation of mono- and dual-species biofilms was accomplished, making it feasible to transfer this technology into real scenarios in water treatment and food processing.

Interactions between L. monocytogenes and P. fluorescens in Dual-Species Biofilms under Simulated Dairy Processing Conditions

In dairy processing environments, many bacterial species adhere and form biofilms on surfaces and equipment, leading to foodborne illness and food spoilage. Among them, Listeria monocytogenes and Pseudomonas spp. could be present in mixed-species biofilms. This study aimed to evaluate the interactions between L. monocytogenes and P. fluorescens in biofilms simulating dairy processing conditions, as well as the capability of P. fluorescens in co-culture to produce the blue pigment in a Ricotta-based model system. The biofilm-forming capability of single- and mixed-cultures was evaluated on polystyrene (PS) and stainless steel (SS) surfaces at 12 °C for 168 h. The biofilm biomass was measured, the planktonic and sessile cells and the carbohydrates in biofilms were quantified. The biofilms were also observed through Confocal Laser Scanning Microscopy analysis. Results showed that only P. fluorescens was able to form biofilms on PS. Moreover, in dual-species biofilms at the end of the incubation time (168 h at 12 °C), a lower biomass compared to P. fluorescens mono-species was observed on PS. On SS, the biofilm cell population of L. monocytogenes was higher in the dual-species than in mono-species, particularly after 48 h. Carbohydrates quantity in the dual-species system was higher than in mono-species and was revealed also at 168 h. The production of blue pigment by P. fluorescens was revealed both in single- and co-culture after 72 h of incubation (12 °C). This work highlights the interactions between the two species, under the experimental conditions studied in the present research, which can influence biofilm formation (biomass and sessile cells) but not the capability of P. fluorescens to produce blue pigment.