Listeria monocytogenes Impact on Mature or Old Pseudomonas fluorescens Biofilms During Growth at 4 and 20°C

Influence of slope, material, and temperature on Listeria monocytogenes and Pseudomonas aeruginosa mono- and dual-species biofilms

Understanding factors influencing Listeria monocytogenes biofilms aid in developing more effective elimination/prevention strategies. This study examined the effect of temperature (4 °C, 21 °C, 30 °C), materials (stainless steel 316 L with 2B and 2 R finishes, glass, and polypropylene), and slope (0°/horizontal or 90°/vertical) on mono- and dual-species biofilms using two L. monocytogenes strains and one Pseudomonas aeruginosa strain. All biofilms were grown in 10% TSB for 24 h and analyzed using culture-based methods. Additionally, the architecture of monospecies biofilms was studied using fluorescence microscopy. Overall, P. aeruginosa showed higher biofilm formation potential (6.2 log CFU/cm2) than L. monocytogenes (4.0 log CFU/cm2). Temperature greatly influenced P. aeruginosa and varied for L. monocytogenes. The slope predominantly influenced L. monocytogenes monospecies biofilms, with cell counts increasing by up to 2 log CFU/cm2. Surface material had little impact on biofilm formation. The study highlights the varying effects of different parameters on multispecies biofilms and the importance of surface geometry.

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.

Transcriptome Reveals Regulation of Quorum Sensing of Hafnia alvei H4 on the Coculture System of Hafnia alvei H4 and Pseudomonas fluorescens ATCC13525

In the food industry, foodborne spoilage bacteria often live in mixed species and attach to each other, leading to changes in spoilage characteristics. Quorum sensing (QS) has been reported to be a regulating mechanism for food spoiling by certain kinds of bacteria. Here, the contents of biofilm, extracellular polysaccharides, and biogenic amines in the coculture system of Hafnia alvei H4 and Pseudomonas fluorescens ATCC13525 were significantly reduced when the QS element of H. alvei H4 was deleted, confirming that QS of H. alvei H4 is involved in the dual-species interactions. Then, transcriptomics was used to explore the regulatory mechanism at the mRNA molecular level. The deletion of the QS element decreased the transcript levels of genes related to chemotaxis, flagellar assembly, and the two-component system pathway of H. alvei H4 in the coculture system. Furthermore, a total of 732 DEGs of P. fluorescens ATCC13525 were regulated in the dual species, which were primarily concerned with biofilm formation, ATP-binding cassette transporters, and amino acid metabolism. Taken together, the absence of the QS element of H. alvei H4 weakened the mutual cooperation of the two bacteria in the coculture system, making it a good target for managing infection with H. alvei and P. fluorescens.

Listeria monocytogenes in food businesses: From persistence strategies to intervention/prevention strategies-A review

In 2023, Listeria monocytogenes persistence remains a problem in the food business. A profound understanding of how this pathogen persists may lead to better aimed intervention/prevention strategies. The lack of a uniform definition of persistence makes the comparison between studies complex. Harborage sites offer protection against adverse environmental conditions and form the ideal habitat for the formation of biofilms, one of the major persistence strategies. A retarded growth rate, disinfectant resistance/tolerance, desiccation resistance/tolerance, and protozoan protection complete the list of persistence strategies for Listeria monocytogenes and can occur on themselves or in combination with biofilms. Based on the discussed persistence strategies, intervention strategies are proposed. By enhancing the focus on four precaution principles (cleaning and disinfection, infrastructure/hygienic design, technical maintenance, and work methodology) as mentioned in Regulation (EC) No. 852/2004, the risk of persistence can be decreased. All of the intervention strategies result in obtaining and maintaining a good general hygiene status throughout the establishment at all levels ranging from separate equipment to the entire building.

Synergistic inhibition of Pseudomonas fluorescens growth and proteases activities via sodium chlorite-based oxyhalogen

Pseudomonas fluorescens is considered among the main spoilage microorganisms due to its ability to produce proteases. Food deterioration caused by spoilage microorganisms has a major impact on food quality and the environment. The inactivation of Pseudomonas fluorescens growth and protease production was intensively investigated with the use of Salmide®, A Sodium Chlorite-Based Oxy-halogen Disinfectant. A unique M9 media was also developed to assure sufficient protease productions with different mutants of Pseudomonas fluorescens as a microbioreactor. Mutations were induced by classical whole-cell mutagenesis using N-methyl-N'- nitro-N-nitrosoguanidine (NTG). A dramatic decrease occurred in protease activity when different Salmide concentrations (5, 10, and 15 ppm) were added to the growth culture followed by a complete inhibition concentration (20, 25, 50, and 100 ppm) of Salmide. However, no significant inhibition occurred once it is secreted out of cells. Some mutants were resistant and remains highly stable with high protease production under stressful conditions of Sodium Chlorite-Based Oxy-halogen. The production of the protease showed a linear correlation with the increase in incubation time using a continuous culture bioreactor system and recorded maximum protease activity after 40 h. Our findings would offer alternative antimicrobial procedures for food and industrial sectors.

Pseudomonas fluorescens group bacterial strains interact differently with pathogens during dual-species biofilm formation on stainless steel surfaces in milk

In order to develop strategies for preventing biofilm formation in the dairy industry, a deeper understanding of the interaction between different species during biofilm formation is necessary. Bacterial strains of the P. fluorescens group are known as the most important biofilm-formers on the surface of dairy processing equipment that may attract and/or shelter other spoilage or pathogenic bacteria. The present study used different strains of the P. fluorescens group as background microbiota of milk, and evaluated their interaction with Staphylococcus aureus, Bacillus cereus, Escherichia coli O157:H7, and Salmonella Typhimurium during dual-species biofilm formation on stainless steel surfaces. Two separate scenarios for dual-species biofilms were considered: concurrent inoculation of Pseudomonas and pathogen (CI), and delayed inoculation of pathogen to the pre-formed Pseudomonas biofilm (DI). The gram-positive pathogens used in this study did not form dual-species biofilms with P. fluorescens strains unless they were simultaneously inoculated with Pseudomonas strains. E. coli O157:H7 was able to form dual-species biofilms with all seven P. fluorescens group strains, both in concurrent (CI) and delayed (DI) inoculation. However, the percentage of contribution varied depending on the P. fluorescens strains and the inoculation scenario. S. Typhimurium contributed to biofilm formation with all seven P. fluorescens group strains under the CI scenario, with varying degrees of contribution. However, under the DI scenario, S. Typhimurium did not contribute to the biofilm formed by three of the seven P. fluorescens group strains. Overall, these are the first results to illustrate that the strains within the P. fluorescens group have significant differences in the formation of mono-or dual-species biofilms with pathogenic bacteria. Furthermore, the possibility of forming dual-species biofilms with pathogens depends on whether the pathogens form the biofilm simultaneously with the P. fluorescens group strains or whether these strains have already formed a biofilm.

Biofilm Formation of Listeria monocytogenes and Pseudomonas aeruginosa in a Simulated Chicken Processing Environment

This study aims to investigate the mono- and dual-species biofilm formation of Listeria monocytogenes and Pseudomonas aeruginosa incubated in different culture mediums, inoculum ratios, and incubation time. The planktonic cell population and motility were examined to understand the correlation with biofilm formation. The results showed that chicken juice significantly inhibited the biofilm formation of L. monocytogenes (p < 0.05). Pseudomonas aeruginosa was the dominant bacteria in the dual-species biofilm formation in the trypticase soy broth medium. The dynamic changes in biofilm formation were not consistent with the different culture conditions. The growth of planktonic L. monocytogenes and P. aeruginosa in the suspension was inconsistent with their growth in the biofilms. There was no significant correlation between motility and biofilm formation of L. monocytogenes and P. aeruginosa. Moreover, scanning electron microscopy (SEM) results revealed that the biofilm structure of L. monocytogenes was loose. At the same time, P. aeruginosa formed a relatively dense network in mono-species biofilms in an initial adhesion stage (24 h). SEM results also showed that P. aeruginosa was dominant in the dual-species biofilms. Overall, these results could provide a theoretical reference for preventing and controlling the biofilm formation of L. monocytogenes and P. aeruginosa in the food processing environment in the future.

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.

Control of bacterial biofilms in red meat - A systematic review

Biofilm formation is a serious threat in the meat industry, mainly since it aids food-borne pathogen survival. Biofilms are often difficult to eliminate, and it is essential to understand the best possible deployable measures to remove or inactivate biofilms. We systematically reviewed the published in vitro studies that investigated various methods for removing biofilms in red meat. Publicly available databases, including Google Scholar and PubMed, were queried for relevant studies. The search was restricted to articles published in the English language from 2010 to 2021. We mined a total of 394 studies, of which 12 articles were included in this review. In summary, the studies demonstrated the inhibitory effect of various methods, including the use of bacteriophages, dry heat, cold atmospheric pressure, ozone gas, oils, and acids, on red meat extract or red meat culture. This systematic review suggests that in addition to existing sanitation and antibiotic procedures, other methods, such as the use of phage cocktails and different oils as nanoparticles, yield positive outcomes and may be taken from the in vitro setting to industry with prior validation of the techniques.

In Vitro Preformed Biofilms of Bacillus safensis Inhibit the Adhesion and Subsequent Development of Listeria monocytogenes on Stainless-Steel Surfaces

Listeria monocytogenes continues to be one of the most important public health challenges for the meat sector. Many attempts have been made to establish the most efficient cleaning and disinfection protocols, but there is still the need for the sector to develop plans with different lines of action. In this regard, an interesting strategy could be based on the control of this type of foodborne pathogen through the resident microbiota naturally established on the surfaces. A potential inhibitor, Bacillus safensis, was found in a previous study that screened the interaction between the resident microbiota and L. monocytogenes in an Iberian pig processing plant. The aim of the present study was to evaluate the effect of preformed biofilms of Bacillus safensis on the adhesion and implantation of 22 strains of L. monocytogenes. Mature preformed B. safensis biofilms can inhibit adhesion and the biofilm formation of multiple L. monocytogenes strains, eliminating the pathogen by a currently unidentified mechanism. Due to the non-enterotoxigenic properties of B. safensis, its presence on certain meat industry surfaces should be favored and it could represent a new way to fight against the persistence of L. monocytogenes in accordance with other bacterial inhibitors and hygiene operations.

Metagenomic characterization of bacterial biofilm in four food processing plants in Colombia

Bacteria inside biofilms are more persistent and resistant to stress conditions found in the production environment of food processing plants, thus representing a constant risk for product safety and quality. Therefore, the aim of this study was to characterize, using 16S rRNA sequencing, the bacterial communities from biofilms found in four food processing plants (P1, P2, P3, and P4). In total, 50 samples from these four processing plants were taken after cleaning and disinfection processes. Four phyla: Proteobacteria, Firmicutes, Actinobacteria, and Bacteroides represented over 94% of the operational taxonomic units found across these four plants. A total of 102 families and 189 genera were identified. Two genera, Pseudomonas spp. and Acinetobacter spp., were the most frequently found (93.47%) across the four plants. In P1, Pseudomonas spp. and Lactobacillus spp. were the dominant genera, whereas Lactobacillus spp. and Streptococcus spp. were identified in P2. On the other hand, biofilms found in P3 and P4 mainly consisted of Pseudomonas spp. and Acinetobacter spp. Our results indicate that different bacterial genera of interest to the food industry due to their ability to form biofilm and affect food quality can coexist inside biofilms, and as such, persist in production environments, representing a constant risk for manufactured foods. In addition, the core microbiota identified across processing plants evaluated was probably influenced by type of food produced and cleaning and disinfection processes performed in each one of these.

Role of Lactobacillus biofilms in Listeria monocytogenes adhesion to glass surfaces

Listeria monocytogenes can form long-lasting biofilms on food-contact surfaces. Lactic acid bacteria (LAB) have shown promise in antagonizing this microorganism in liquid media. However, the ecological relationships differ when cells are forming biofilms. In this work, we propose the use of Lactobacillus biofilms as surface "conditioners" to modulate the adhesion of L. monocytogenes. For this, the biofilm formation ability of Lactobacillus fermentum MP26 and Lactobacillus salivarius MP14 (human milk origin), fluorescently labeled by transfer of the mCherry-encoding pRCR12 plasmid, was first evaluated. Then, mature biofilms of these strains transformed with pRCR12 for expressing the fluorescent protein mCherry were used as adhesion substrate for GFP-tagged L. monocytogenes Scott A. The resulting biofilms were studied in terms of cellular population and attached biomass (cells plus matrix). Species distribution inside the biofilm structure was revealed by confocal laser scanning microscopy (CLSM). Although none of the Lactobacillus spp. strains reduced the adhesion of L. monocytogenes Scott A, species interactions seem to interfere with the synthesis of extracellular polymeric substances and species distribution inside the biofilms. In dual-species biofilms, CLSM images revealed that Lactobacillus cells were trapping those of L. monocytogenes Scott A. When surfaces were conditioned with Lactobacillus biofilms, the spatial distribution of L. monocytogenes Scott A cells was species-specific, suggesting these interactions are governing the ultimate biofilm structure. The results here obtained open new possibilities for controlling L. monocytogenes dispersal using these Lactobacillus spp. biofilms as a "natural" immobilization way. Whether species interactions could modify the virulence of L. monocytogenes still remains unclear.

Genetic Subtyping, Biofilm-Forming Ability and Biocide Susceptibility of Listeria monocytogenes Strains Isolated from a Ready-to-Eat Food Industry

Listeria monocytogenes is a foodborne pathogen of special concern for ready-to-eat food producers. The control of its presence is a critical step in which food-grade sanitizers play an essential role. L. monocytogenes is believed to persist in food processing environments in biofilms, exhibiting less susceptibility to sanitizers than planktonic cells. This study aimed to test the susceptibility of L. monocytogenes in planktonic culture and biofilm to three commercial food-grade sanitizers and to benzalkonium chloride; together with the genetic subtyping of the isolates. L. monocytogenes isolates were collected from raw materials, final products and food-contact surfaces during a 6-year period from a ready-to-eat meat-producing food industry and genetically characterized. Serogrouping and pulsed-field gel electrophoresis (PFGE) revealed genetic variability and differentiated L. monocytogenes isolates in three clusters. The biofilm-forming ability assay revealed that the isolates were weak biofilm producers. L. monocytogenes strains were susceptible both in the planktonic and biofilm form to oxidizing and ethanol-based compounds and to benzalkonium chloride, but not to quaternary ammonium compound. A positive association of biofilm-forming ability and LD₉₀ values for quaternary ammonium compound and benzalkonium chloride was found. This study highlights the need for preventive measures improvement and for a conscious selection and use of sanitizers in food-related environments to control Listeria monocytogenes.

Identification of biofilm hotspots in a meat processing environment: Detection of spoilage bacteria in multi-species biofilms

Biofilms are comprised of microorganisms embedded in a self-produced matrix that normally adhere to a surface. In the food processing environment they are suggested to be a source of contamination leading to food spoilage or the transmission of food-borne pathogens. To date, research has mainly focused on the presence of (biofilm-forming) bacteria within food processing environments, without measuring the associated biofilm matrix components. Here, we assessed the presence of biofilms within a meat processing environment, processing pork, poultry and beef, by the detection of microorganisms and at least two biofilm matrix components. Sampling included 47 food contact surfaces and 61 non-food contact surfaces from eleven rooms within an Austrian meat processing plant, either during operation or after cleaning and disinfection. The 108 samples were analysed for the presence of microorganisms by cultivation and targeted quantitative real-time PCR based on 16S rRNA. Furthermore, the presence of the major matrix components carbohydrates, extracellular DNA and proteins was evaluated. Overall, we identified ten biofilm hotspots, among them seven of which were sampled during operation and three after cleaning and disinfection. Five biofilms were detected on food contact surfaces (cutters and associated equipment and a screw conveyor) and five on non-food contact surfaces (drains and water hoses) resulting in 9.3 % of the sites being classified as biofilm positive. From these biofilm positive samples, we cultivated bacteria of 29 different genera. The most prevalent bacteria belonged to the genera Brochothrix (present in 80 % of biofilms), Pseudomonas and Psychrobacter (isolated from 70 % biofilms). From each biofilm we isolated bacteria from four to twelve different genera, indicating the presence of multi-species biofilms. This work ultimately determined the presence of multi-species biofilms within the meat processing environment, thereby identifying various sources of potential contamination. Especially the identification of biofilms in water hoses and associated parts highlights the need of a frequent monitoring at these sites. The knowledge gained about the presence and composition of biofilms (i.e. chemical and microbiological) will help to prevent and reduce biofilm formation within food processing environments.

Communication is the key: biofilms, quorum sensing, formation and prevention

Antibiotic resistance is a relevant topic nowadays, representing one of the main causes of infection-related mortality and morbidity at a global level. This phenomenon is worrisome and represents an area of interest for both clinical practice and fundamental research. One important mechanism whereby bacteria acquire resistance to antibiotics and evade the immune system is by forming biofilms. It is estimated that ~80% of the bacteria producing chronic infections can form biofilms. During the process of biofilm formation microorganisms have the ability to communicate with each other through quorum sensing. Quorum sensing regulates the metabolic activity of planktonic cells, and it can induce microbial biofilm formation and increased virulence. In this review we describe the biofilm formation process, quorum sensing, quorum quenching, several key infectious bacteria producing biofilm, methods of prevention and their challenges and limitations. Although progress has been made in the prevention and treatment of biofilm-driven infections, new strategies are required and have to be further developed.

Characterization of biofilm production by Pseudomonas fluorescens isolated from refrigerated raw buffalo milk

Pseudomonas fluorescens can often be isolated from refrigerated raw milk. Two strains of P. fluorescens PL5.4 and PL7.1, isolated from raw buffalo milk, were evaluated for their proteolytic capacity, exopolysaccharide production and biofilm production. Proteolytic activity was observed in both strains. The P. fluorescens PL5.4 strain presented fluorescence in the presence of calcofluor, indicating exopolysaccharide production. Both strains were able to produce biofilm at 7 °C for 72 h. For the biofilm production test on stainless steel, adherent cell counts of up to 7.1, 7.3 and 8.8 log CFU/cm² at 7, 23 and 30 °C were obtained. Through scanning electron microscopy, it was possible to observe the biofilm produced by the P. fluorescens PL5.4 strain. Proper cleaning and disinfection practices in order are important to reduce bacterial contamination and extend the useful life of raw material and its derivatives.

Competitive interaction on dual-species biofilm formation by spoilage bacteria, Shewanella baltica and Pseudomonas fluorescens

AIMS

This study aims to characterize the biofilm produced by mono- and dual-species of Shewanella baltica and Pseudomonas fluorescens as fish spoilers at the different incubation temperature, and to elucidate the interactive behaviour of dual-species biofilm development.

METHODS AND RESULTS

The mono- and dual-species biofilm formation and adhesion characteristics of S. baltica and P. fluorescens were evaluated by using crystal violet staining, scanning electron microscopy and confocal laser scanning microscopy. Results showed that P. fluorescens had significantly higher biofilm biomass and polysaccharides production than S. baltica, and two isolates reached the maximum biofilm biomass during the early stationary phase. Lower biomass and polysaccharides in dual-species biofilms were observed compared to mono-species of P. fluorescens. Meanwhile, S. baltica and P. fluorescens formed fragile and viscous pellicles with different spatial architectures respectively. In dual-species pellicle few large microcolonies were dominated by P. fluorescens. Compared to mono-species of PF07, adherent cell population and biofilm thickness at the developing phase significantly decreased, and biofilm-forming cycle prolonged in the dual-species biofilms. Biofilm formation and adhesion of mono- and dual-species at 4 or 15°C were significantly higher than at 30°C during the same phase. The culture supernatant extracts of the two spoilage strains greatly inhibited biofilm development to each other.

CONCLUSIONS

Shewanella baltica and P. fluorescens had different biofilm and pellicle characteristics, and the inhibitory development on dual-species biofilm was associated with the competitive interaction by the two psychrotrophic spoilage bacteria.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work contributes to a better understanding of interactive behaviour of multispecies biofilm communities by psychrotrophic spoilage bacteria at low temperature, which could contribute to further control contamination of spoilage organism during the preservation and processing of aquatic products.

Characteristics of Biofilms Formed by Co-Culture of Listeria monocytogenes with Pseudomonas aeruginosa at Low Temperatures and Their Sensitivity to Antibacterial Substances

We assessed the properties of biofilms (BFs) formed by mono- and co-cultures of Listeria monocytogenes and Pseudomonas aeruginosa (L+P-BF) at low temperatures and examined their sensitivity to several antibacterial substances. L. monocytogenes viable counts comprised only 1-10% of total L+P-BF viable counts at 10℃ and 15℃, indicating the significant prevalence of P. aeruginosa in co-cultures. L+P-BF formed at 10℃ and 15℃ showed very high resistance to antibiotics and NaClO. Examination of the effects of nattokinase and nisin, natural food additives with antibacterial properties, showed that their application alone failed to inhibit L+P-BF development at 10℃ and 15℃. However, a combined treatment with nisin and ethylenediaminetetraacetic acid, a food additive that can be used as a permeabilizing agent, suppressed the formation of L+P-BF at 10℃ and 15℃. Microscopy observations of L+P-BF did not reveal pronounced morphological changes in bacterial cell morphology. We also noted that P. aeruginosa resistance to the action of nisin during BF formation was higher when it was maintained in co-culture with L. monocytogenes. The results of the present study are an important step toward developing a safe formulation of acceptable food additives that could be used for suppression of BFs formed by pathogenic bacteria during food storage.

Listeria monocytogenes Colonizes Pseudomonas fluorescens Biofilms and Induces Matrix Over-Production

In food facilities, biofilms or their debris might act as helpers for attracting free floating microorganisms. In this sense, Pseudomonas fluorescens, a dense biofilm producer frequently isolated from food contact surfaces, could be a good candidate for sheltering other microorganisms, such as Listeria monocytogenes. The main objective of this work was to evaluate the ability of L. monocytogenes to colonize pre-established Pseudomonas biofilms. For this, the movement throughout mature Pseudomonas biofilms of a green fluorescent protein (GFP) - tagged strain of L. monocytogenes was tracked for 24 h by confocal laser scanning microscopy (CLSM). Moreover, in order to check the effect of the incorporation of Listeria on the overall matrix production, attached populations of both microorganisms and total biomass (cells + matrix) of the resulting biofilms were measured over time. Planktonic cells of L. monocytogenes efficiently migrated to preformed P. fluorescens biofilms. Moreover, they moved preferentially toward the bottom layers of these structures, suggesting some kind of tropism. When preformed P. fluorescens biofilms were conditioning the surfaces, the L. monocytogenes attached population was on average, 1-2 Log higher than when this organism grew on bare coupons. Furthermore, the arrival of L. monocytogenes to the already established P. fluorescens biofilms led to a matrix over-production. Indeed, biomass values [optical density (OD595nm)] of the resulting biofilms were double those of the ordinary L. monocytogenes-P. fluorescens mixed biofilms (1.40 vs. 0.6). The fact that L. monocytogenes cells accumulate in the bottom layers of preformed biofilms provides this microorganism an extra protection toward physical-chemical damages. This might partly explain why this microorganism can persist in food industry environments.

Deficient Lipid A Remodeling by the arnB Gene Promotes Biofilm Formation in Antimicrobial Peptide Susceptible Pseudomonas aeruginosa

Multidrug resistant bacteria possess various mechanisms that can sense environmental stresses such as antibiotics and antimicrobial peptides and rapidly respond to defend themselves. Two known defense strategies are biofilm formation and lipopolysaccharide (LPS) modification. Though LPS modifications are observed in biofilm-embedded bacteria, their effect on biofilm formation is unknown. Using biochemical and biophysical methods coupled with confocal microscopy, atomic force microscopy, and transmission electron microscopy, we show that biofilm formation is promoted in a Pseudomonas aeruginosa PAO1 strain with a loss of function mutation in the arnB gene. This loss of function prevents the addition of the positively charged sugar 4-amino-4-deoxy-l-arabinose to lipid A of LPS under restrictive magnesium conditions. The data reveal that the arnB mutant, which is susceptible to antimicrobial peptides, forms a biofilm that is more robust than that of the wild type. This is in line with the observations that the arnB mutant exhibits outer surface properties such as hydrophobicity and net negative charge that promote the formation of biofilms. Moreover, when grown under Mg²⁺ limitation, both the wild type and the arnB mutant exhibited a reduction in the level of membrane-bound polysaccharides. The data suggest that the loss of polysaccharides exposes the membrane and alters its biophysical properties, which in turn leads to more biofilm formation. In summary, we show for the first time that blocking a specific lipid A modification promotes biofilm formation, suggesting a trade-off between LPS remodeling and resistance mechanisms of biofilm formation.

Dynamics of biofilm formation by Listeria monocytogenes on stainless steel under mono-species and mixed-culture simulated fish processing conditions and chemical disinfection challenges

The progressive ability of a six-strains L. monocytogenes cocktail to form biofilm on stainless steel (SS), under fish-processing simulated conditions, was investigated, together with the biocide tolerance of the developed sessile communities. To do this, the pathogenic bacteria were left to form biofilms on SS coupons incubated at 15°C, for up to 240h, in periodically renewable model fish juice substrate, prepared by aquatic extraction of sea bream flesh, under both mono-species and mixed-culture conditions. In the latter case, L. monocytogenes cells were left to produce biofilms together with either a five-strains cocktail of four Pseudomonas species (fragi, savastanoi, putida and fluorescens), or whole fish indigenous microflora. The biofilm populations of L. monocytogenes, Pseudomonas spp., Enterobacteriaceae, H₂S producing and aerobic plate count (APC) bacteria, both before and after disinfection, were enumerated by selective agar plating, following their removal from surfaces through bead vortexing. Scanning electron microscopy was also applied to monitor biofilm formation dynamics and anti-biofilm biocidal actions. Results revealed the clear dominance of Pseudomonas spp. bacteria in all the mixed-culture sessile communities throughout the whole incubation period, with the in parallel sole presence of L. monocytogenes cells to further increase (ca. 10-fold) their sessile growth. With respect to L. monocytogenes and under mono-species conditions, its maximum biofilm population (ca. 6logCFU/cm²) was reached at 192h of incubation, whereas when solely Pseudomonas spp. cells were also present, its biofilm formation was either slightly hindered or favored, depending on the incubation day. However, when all the fish indigenous microflora was present, biofilm formation by the pathogen was greatly hampered and never exceeded 3logCFU/cm², while under the same conditions, APC biofilm counts had already surpassed 7logCFU/cm² by the end of the first 96h of incubation. All here tested disinfection treatments, composed of two common food industry biocides gradually applied for 15 to 30min, were insufficient against L. monocytogenes mono-species biofilm communities, with the resistance of the latter to significantly increase from the 3rd to 7th day of incubation. However, all these treatments resulted in no detectable L. monocytogenes cells upon their application against the mixed-culture sessile communities also containing the fish indigenous microflora, something probably associated with the low attached population level of these pathogenic cells before disinfection (<10²CFU/cm²) under such mixed-culture conditions. Taken together, all these results expand our knowledge on both the population dynamics and resistance of L. monocytogenes biofilm cells under conditions resembling those encountered within the seafood industry and should be considered upon designing and applying effective anti-biofilm strategies.