Biofilm formation of Listeria monocytogenes and its resistance to quaternary ammonium compounds in a simulated salmon processing environment

Disinfectant efficacy of glabridin against dried and biofilm cells of Listeria monocytogenes and the impact of residual organic matter

Glabridin is an antimicrobial compound which can be extracted from plants, such as liquorice (Glycyrrhiza glabra) roots. Although its activity against foodborne pathogens and spoilage microorganisms has already been reported, the investigation of potential applications as a surface disinfectant is still largely unexplored. Hence, this study evaluated the disinfectant efficacy of glabridin against Listeria monocytogenes. The activity of glabridin was first tested in vitro in a nutrient-rich medium against eight strains of L. monocytogenes, including food isolates and the model strain EGDe. The tested strains showed similar susceptibility with minimal inhibitory and bactericidal concentrations of 12.5 µg/mL and 25 µg/mL, respectively. Subsequently, L. monocytogenes L6, FBR17 and EGDe were selected to assess the efficacy of glabridin against dried cells (according to the European standard EN 13697:2015 + A1:2019) and biofilm cells on stainless steel surfaces. Moreover, the impact of food residual organic matter was investigated using skim milk, cantaloupe and smoked salmon solution as soiling components. Our results showed that applying 200 µg/mL of glabridin resulted in a substantial reduction (>3 log10) of dried and biofilm cells of L. monocytogenes in standard conditions (i.e. low level of residual organic matter). Cantaloupe soiling components slightly reduced the activity of glabridin, while the efficacy of glabridin when tested with salmon and skim milk residuals was substantially affected. Comparative analysis using standardized protein contents provided evidence that the type of food matrices and type of proteins may impact the activity of glabridin as a disinfectant. Overall, this study showed low strain variability for the activity of glabridin against L. monocytogenes and shed light on the possible application of this natural antimicrobial compound as a surface disinfectant.

Inhibitory effect and mechanism of oregano essential oil on Listeria monocytogenes cells, toxins and biofilms

Listeria monocytogenes (L. monocytogenes) is a prevalent foodborne pathogen with a remarkable capacity to form biofilms on utensil surfaces. The Listeriolysin O (LLO) exhibits hemolytic activity, which is responsible for causing human infections. In this study, we investigated the inhibitory effect and mechanism of oregano essential oil (OEO) on L. monocytogenes, evaluated the effects on its biofilm removal and hemolytic activity. The minimum inhibitory concentration (MIC) of OEO against L. monocytogenes was 0.03% (v/v). L. monocytogenes was treated with OEO at 3/2 MIC for 30 min the bacteria was decreased below the detection limit (10 CFU/mL) in PBS and TSB (the initial bacterial load was about 6.5 log CFU/mL). The level of L. monocytogenes in minced pork co-cultured with OEO (15 MIC) about 2.5 log CFU/g lower than that in the untreated group. The inhibitory mechanisms of OEO against planktonic L. monocytogenes encompassed perturbation of cellular morphology, elevation in reactive oxygen species levels, augmentation of lipid oxidation extent, hyperpolarization of membrane potential, and reduction in intracellular ATP concentration. In addition, OEO reduced biofilm coverage on the surface of glass slides by 62.03% compared with the untreated group. Meanwhile, OEO (1/8 MIC) treatment reduced the hemolytic activity of L. monocytogenes to 24.6% compared with the positive control. Molecular docking suggested carvacrol and thymol might reduce the hemolytic activity of L. monocytogenes. The results of this study demonstrate that OEO exhibits inhibitory effects against L. monocytogenes, biofilms and LLO, which had potential as natural antimicrobial for the inhibition of L. monocytogenes.

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.

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.

A review of Spaulding's classification system for effective cleaning, disinfection and sterilization of reusable medical devices: Viewed through a modern-day lens that will inform and enable future sustainability

Despite advances in medicine and innovations in many underpinning fields including disease prevention and control, the Spaulding classification system, originally proposed in 1957, remains widely used for defining the disinfection and sterilization of contaminated re-usable medical devices and surgical instruments. Screening PubMed and Scopus databases using a PRISMA guiding framework generated 272 relevant publications that were used in this review. Findings revealed that there is a need to evolve how medical devices are designed, and processed by cleaning, disinfection (and/or sterilization) to mitigate patient risks, including acquiring an infection. This Spaulding Classification remains in use as it is logical, easily applied and understood by users (microbiologists, epidemiologists, manufacturers, industry) and by regulators. However, substantial changes have occurred over the past 65 years that challenge interpretation and application of this system that includes inter alia emergence of new pathogens (viruses, mycobacteria, protozoa, fungi), a greater understanding of innate and adaptive microbial tolerance to disinfection, toxicity risks, increased number of vulnerable patients and associated patient procedures, and greater complexity in design and use of medical devices. Common cited examples include endoscopes that enable non- or minimal invasive procedures but are highly sophisticated with various types of materials (polymers, electronic components etc), long narrow channels, right angle and heat-sensitive components and various accessories (e.g., values) that can be contaminated with high levels of microbial bioburden and patient tissues after use. Contaminated flexible duodenoscopes have been a source of several significant infection outbreaks, where at least 9 reported cases were caused by multidrug resistant organisms [MDROs] with no obvious breach in processing detected. Despite this, there is evidence of the lack of attention to cleaning and maintenance of these devices and associated equipment. Over the last few decades there is increasing genomic evidence of innate and adaptive resistance to chemical disinfectant methods along with adaptive tolerance to environmental stresses. To reduce these risks, it has been proposed to elevate classification of higher-risk flexible endoscopes (such as duodenoscopes) from semi-critical [contact with mucous membrane and intact skin] to critical use [contact with sterile tissue and blood] that entails a transition to using low-temperature sterilization modalities instead of routinely using high-level disinfection; thus, increasing the margin of safety for endoscope processing. This timely review addresses important issues surrounding use of the Spaulding classification system to meet modern-day needs. It specifically addresses the need for automated, robust cleaning and drying methods combined with using real-time monitoring of device processing. There is a need to understand entire end-to-end processing of devices instead of adopting silo approaches that in the future will be informed by artificial intelligence and deep-learning/machine learning. For example, combinational solutions that address the formation of complex biofilms that harbour pathogenic and opportunistic microorganisms on the surfaces of processed devices. Emerging trends are addressed including future sustainability for the medical devices sector that can be enabled via a new Quintuple Helix Hub approach that combines academia, industry, healthcare, regulators, and society to unlock real world solutions.

Evaluation of Listeria monocytogenes biofilms attachment and formation on different surfaces using a CDC biofilm reactor

Listeria monocytogenes can adapt, persist, and form biofilms on food premises surfaces, representing a challenge for food safety, since they led to disease transmission, food contamination and spoilage during production. Physical interventions (scrubbing and wiping) can help controlling formation, nevertheless when biofilms are formed, they are usually very resistant to current control strategies used in the food industry. Biofilm attachment and formation is influenced by environment characteristics, substrate properties and microbial motility. The purpose of this study was to evaluate the ability of L. monocytogenes to attach and form biofilms on different surfaces (wood, nylon, and polycarbonate) representative of the materials used during produce harvesting and storage. Multi-strain L. monocytogenes biofilms were grown in a CDC Biofilm reactor at 20 ± 2 °C up to 96-h and characterized for: a) attachment strength by enumerating cells after rinsing; b) hydrophobicity and interfacial tension by contact angle measurements; c) biofilm architecture by Laser Scanning Confocal Microscopy. All experiments were done in triplicate. Material, incubation, and solvent significantly affected the hydrophobicity and wetting properties of L. monocytogenes biofilms (P < 0.05). The type of material and incubation time significantly influenced hydrophobicity and wetting properties of L. monocytogenes biofilms (P < 0.05). Highest contact angle and lowest interfacial tension were observed on polycarbonate coupons. The data presented contributes to understanding Listeria biofilms grow on different surfaces commonly used in produce harvesting and storage. The data obtained in this study can be used when evaluating intervention strategies to control this pathogen in food premises.

Biofilm Formation From Listeria monocytogenes Isolated From Pangasius Fish-processing Plants

Biofilm formation of Listeria monocytogenes in food processing environments cause potential source of cross-contamination to foodstuffs; hence, the control of biofilm is currently addressed to find effective solutions for preventing biofilm formation or eliminating the established one. Forty-five strains of Listeria monocytogenes isolated from Pangasius fish-processing plants were studied for their capability to form a biofilm on 96-well microtiter plate by using the conventional crystal violet staining. Additionally, the inhibitory effect of biofilm formation by food additives including monascus pigment and ε-polylysine was examined. The average OD value showing biofilm mass of all 45 strains L. monocytogenes increased with an increasing temperature and time (p < 0.05). Monascus pigment and ε-polylysine significantly decreased biofilm formation by 80 ± 5.5% and 20 ± 5.9%, respectively, at the tested concentration (p < 0.05) Further, the effects of lysozyme (0.1 mg/mL) alone or in combination with slightly acidic hypochlorous water (SAHW) with 40 mg/L available chlorine or sodium hypochlorite (NaOCl) with 100 mg/L available chlorine against 7-d established biofilm of L. monocytogenes were investigated. The results indicated that slightly acidic hypochlorous water alone exhibited significant antibacterial activity (p < 0.05), decreasing the viable count by 5.2 ± 0.5 log CFU/mL. It seems that sequential treatment of lysozyme and SAHW showed an additional efficacy against biofilm of L. monocytogenes on polystyrene plate surface, reducing 70% of biomass of biofilm and 7.6 ± 0.3 log of biofilm viable cells (p < 0.05). Additionally, SAHW exhibited greater bactericidal activity against viable biofilm cells than NaOCl did. This result reveals that SAHW is a promising disinfectant agent against L. monocytogenes and the potential alternative to NaOCl in practice.

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.

Hurdle Technology Approach to Control Listeria monocytogenes Using Rhamnolipid Biosurfactant

This study evaluates the combination of mild heat with a natural surfactant for the inactivation of L. monocytogenes Scott A in low-water-activity (a_w) model systems. Glycerol or NaCl was used to reduce the a_w to 0.92, and different concentrations of rhamnolipid (RL) biosurfactant were added before heat treatment (60 °C, 5 min). Using glycerol, RL treatment (50-250 µg/mL) reduced bacterial population by less than 0.2 log and heat treatment up to 1.5 log, while the combination of both hurdles reached around 5.0 log reduction. In the NaCl medium, RL treatment displayed higher inactivation than in the glycerol medium at the same a_w level and a larger synergistic lethal effect when combined with heat, achieving ≥ 6.0 log reduction at 10-250 µg/mL RL concentrations. The growth inhibition activity of RL was enhanced by the presence of the monovalent salts NaCl and KCl, reducing MIC values from >2500 µg/mL (without salt) to 39 µg/mL (with 7.5% salt). The enhanced antimicrobial activity of RL promoted by the presence of salts was shown to be pH-dependent and more effective under neutral conditions. Overall, results demonstrate that RL can be exploited to design novel strategies based on hurdle approaches aiming to control L. monocytogenes.

The effect of disinfectants and antiseptics on co- and cross-selection of resistance to antibiotics in aquatic environments and wastewater treatment plants

The outbreak of the SARS-CoV-2 pandemic led to increased use of disinfectants and antiseptics (DAs), resulting in higher concentrations of these compounds in wastewaters, wastewater treatment plant (WWTP) effluents and receiving water bodies. Their constant presence in water bodies may lead to development and acquisition of resistance against the DAs. In addition, they may also promote antibiotic resistance (AR) due to cross- and co-selection of AR among bacteria that are exposed to the DAs, which is a highly important issue with regards to human and environmental health. This review addresses this issue and provides an overview of DAs structure together with their modes of action against microorganisms. Relevant examples of the most effective treatment techniques to increase the DAs removal efficiency from wastewater are discussed. Moreover, insight on the resistance mechanisms to DAs and the mechanism of DAs enhancement of cross- and co-selection of ARs are presented. Furthermore, this review discusses the impact of DAs on resistance against antibiotics, the occurrence of DAs in aquatic systems, and DA removal mechanisms in WWTPs, which in principle serve as the final barrier before releasing these compounds into the receiving environment. By recognition of important research gaps, research needs to determine the impact of the majority of DAs in WWTPs and the consequences of their presence and spread of antibiotic resistance were identified.

Microbial resistance to sanitizers in the food industry: review

Hygiene programs which comprise the cleaning and sanitization steps are part of the Good Hygiene Practices (GHP) and are considered essential to ensure food safety and quality. Inadequate hygiene practices may contribute to the occurrence of foodborne diseases, development of microbial resistance to sanitizers, and economic losses. In general, the sanitizer resistance is classified as intrinsic or acquired. The former is an inherent characteristic, naturally present in some microorganisms, whereas the latter is linked to genetic modifications that can occur at random or after continuous exposure to a nonnormal condition. The resistance mechanisms can involve changes in membrane permeability or in the efflux pump, and enzymatic activity. The efflux pump mechanism is the most elucidated in relation to the resistance caused by the use of different types of sanitizers. In addition, microbial resistance to sanitizers can also be favored in the presence of biofilms due to the protection given by the glycocalyx matrix and genetic changes. Therefore, this review aimed to show the main microbial resistance mechanisms to sanitizers, including genetic modifications, biofilm formation, and permeability barrier.

Microbial Biofilms at Meat-Processing Plant as Possible Places of Bacteria Survival

Biofilm contamination in food production threatens food quality and safety, and causes bacterial infections. Study of food biofilms (BF) is of great importance. The taxonomic composition and structural organization of five foods BF taken in different workshops of a meat-processing plant (Moscow, RF) were studied. Samples were taken from the surface of technological equipment and premises. Metagenomic analysis showed both similarities in the presented microorganisms dominating in different samples, and unique families prevailing on certain objects were noted. The bacteria found belonged to 11 phyla (no archaea). The dominant ones were Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria. The greatest diversity was in BFs taken from the cutting table of raw material. Biofilms’ bacteria may be the cause of meat, fish and dairy products spoilage possible representatives include Pseudomonas, Flavobacterium, Arcobacter, Vagococcus, Chryseobacterium, Carnobacterium, etc.). Opportunistic human and animal pathogens (possible representatives include Arcobacter, Corynebacterium, Kocuria, etc.) were also found. Electron-microscopic studies of BF thin sections revealed the following: (1) the diversity of cell morphotypes specific to multispecies BFs; (2) morphological similarity of cells in BFs from different samples, micro-colonial growth; (3) age heterogeneity of cells within the same microcolony (vegetative and autolyzed cells, resting forms); (4) heterogeneity of the polymer matrix chemical nature according to ruthenium red staining.

Antibiofilm and Antiquorum Sensing Potential of Lactiplantibacillus plantarum Z057 against Vibrio parahaemolyticus

Vibrio parahaemolyticus is a widespread foodborne pathogen that causes serious seafood-borne gastrointestinal infections. Biofilm and quorum sensing (QS) are critical in regulating these infections. In this study, first, the ability of Lactiplantibacillus plantarum Z057 to compete, exclude, and displace V. parahaemolyticus biofilm was evaluated. Then, the inhibitory effects of L. plantarum Z057 extract (Z057-E) on V. parahaemolyticus biofilm and QS were explored from the aspects of biofilm biomass, metabolic activity, physicochemical properties, extracellular polymer matrix content, QS signal AI-2 activity, biofilm microstructure, and the expression levels of biofilm and QS-related genes. Results showed that L. plantarum Z057 effectively inhibited biofilm formation of V. parahaemolyticus and interfered with the adhesion of V. parahaemolyticus on the carrier surface. In addition, the Z057-E could significantly reduce the biofilm biomass, metabolic activity, hydrophobicity, auto-aggregation ability, swimming and swarming migration diameter, AI-2 activity, extracellular polysaccharide (EPS), and extracellular protein content of V. parahaemolyticus. Fluorescence microscope and scanning electron microscope (SEM) images demonstrated that the Z057-E could efficiently inactivate the living cells, destroy the dense and complete biofilm architectures, and reduce the essential component of the extracellular polymer matrix. Real-time fluorescence quantitative PCR revealed that the Z057-E treatment down-regulated the expression of flagellum synthesis-related genes (flaA, flgM), EPS, and extracellular protein synthesis-related genes (cpsA, cpsQ, cpsR, ompW), QS-related genes (luxS, aphA, opaR), and hemolysin secretion-related genes (toxS, toxR) of V. parahaemolyticus. Thus, our results suggested that L. plantarum Z057 could represent an alternative biocontrol strategy against foodborne pathogens with anti-adhesive, antibiofilm, and antiquorum sensing activities.

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.

Synergism With ε-Polylysine Hydrochloride and Cinnamon Essential Oil Against Dual-Species Biofilms of Listeria monocytogenes and Pseudomonas lundensis

Various pathogenic and spoilage bacteria frequently coexist in meat processing environments and can form multispecies biofilms, causing significant health and economic issues. Despite the prevalence and coexistence, only less is known about possible interactions between Listeria monocytogenes (LM) and spoilers like Pseudomonas species, and their community-wide resistance against natural preservatives. This study evaluates the interactions between mono- or dual-species biofilms formed by LM and Pseudomonas lundensis (PL), as well as the sensitivity of these bacteria in dual-species biofilms to ε-polylysine hydrochloride (ε-PLH) alone or combined with cinnamon essential oil (CEO). The results showed that the biofilm cell density of P. lundensis in dual species was higher (p < 0.05) than LM, constituting about 85% of the total population. More biofilms and exopolysaccharide both in mono- or dual species of the two psychrotrophic strains were greatly produced at 15°C than at 30°C. The biomass, biovolume, and thickness of dual-species biofilms were significantly lower than single PL biofilm when tested using crystal violet staining, confocal laser scanning microscopy, and scanning electron microscopy, indicating the competitive interactions between them prevail. Additionally, ε-PLH significantly reduced the biofilm development as mono- and dual species in a concentration-dependent manner, especially single LM biofilm, which was consistent with the decrease in autoinducer-2 (AI-2) activity. LM as dual-species biofilms exhibited lower sensitivity to ε-PLH than its mono-biofilm probably due to protective effect conferred by PL. ε-PLH in combination with CEO, at the maximum sublethal concentrations (MSCs), showed enhanced inhibitory activity against dual-species biofilm formation, as evidenced by thin spare spatial structures and reduced AI-2 activity. In addition, the preformed dual biofilms were dramatically eradicated following treatment with ε-PLH combined with CEO at higher than minimum inhibitory concentration in comparison with either of the compounds used alone, indicating the synergistic antibiofilm of the two preservatives. This study reveals the competitive interactions between the two strains in dual-species biofilms, in which the dominant PL significantly contributed toward the tolerance of LM to ε-PLH, and the use of combined preservatives shows it is an effective strategy to control the multispecies biofilms in meat processing.

Antimicrobial Activity of Slow Pyrolysis Distillates from Pine Wood Biomass against Three Pathogens

The aim of this study was to evaluate the antimicrobial activity of wood distillates obtained from Scots pine (Pinus sylvestris) sawdust in order to explore new alternatives for the utilization of wood industry by-products. The distillates were produced by slow pyrolysis thermal conversion in three process phases with increasing temperatures, namely drying, torrefaction and pyrolysis, and three cooling units with different temperatures to condensate the distillates. This yielded nine different liquid fractions. The food-related pathogens, Salmonella, Listeria monocytogenes and Candida albicans, were evaluated for their susceptibility to the distillate fractions using an agar diffusion test. The antimicrobial activity was estimated by measuring the formed inhibition zones after the incubation period. In addition, the minimum inhibitory concentration (MIC) and microbicidic concentration were assayed for a selected fraction (T2) from the torrefaction phase with Bio-screen C. The results indicated that the distillates from the torrefaction and pyrolysis phases had antimicrobial activity against the tested microbes. The MIC value of the T2 fraction for all tested microbes was 0.83% (v/v). Furthermore, the T2 fraction was microbicidic for Salmonella and Listeria strains in 0.83% (v/v) solution and Candida strain in 1.67% (v/v) solution. In conclusion, Scots pine wood distillates obtained from slow pyrolysis have the potential to be developed as antimicrobial agents against pathogenic microbes. Next, research is needed to investigate the chemical composition of the distillates and to assess their safe use.

The elimination effects of lavender essential oil on Listeria monocytogenes biofilms developed at different temperatures and the induction of VBNC state

Listeria monocytogenes is a typical foodborne pathogen that causes hard-to-treat bacterial infections, mainly due to its ability to form biofilm and enter into a viable but non-culturable state (VBNC). In this study, we investigated the removal effects of four antimicrobial agents on L. monocytogenes biofilms formed at 32 ℃ and 10 ℃, analyzed the resistances of the mature biofilms to antimicrobial agents, and explored the VBNC state of cells in mature biofilms induced by lavender essential oil (LEO). The results showed that the growth of L. monocytogenes was completely inhibited when 1.6% (v/v) of the LEO was added. Meanwhile, the results of the crystal violet staining and XTT reduction method indicated that different concentrations of LEO significantly reduced L. monocytogenes biofilms biomass and metabolic activities, followed by sodium hypochlorite, lactic acid, and hydrogen peroxide. Moreover, the confocal laser scanning microscopy (CLSM) images confirmed that the treated biofilms became thinner, the structure was sparse, and the appearance was blurry. More interestingly, L. monocytogenes biofilms developed at 10 ℃ were less susceptible to the sanitizers than that formed at 32 ℃. In addition, LEO presented a more significant dispersing effect on the biofilm cells, and 1/2 MIC to 4 MIC of LEO could induce fewer VBNC state cells in biofilm and plankton compared with sodium hypochlorite. This study indicated that the LEO could be considered as an ideal antibiofilm agent for controlling L. monocytogenes. But we should pay attention to the resistance of the biofilms developed at low temperatures.

Probing antimicrobial resistance and sanitizer tolerance themes and their implications for the food industry through the Listeria monocytogenes lens

The development of antibiotic resistance is a serious public health crisis, reducing our ability to effectively combat infectious bacterial diseases. The parallel study of reduced susceptibility to sanitizers is growing, particularly for environmental foodborne pathogens, such as Listeria monocytogenes. As regulations demand a seek-and-destroy approach for L. monocytogenes, understanding sanitizer efficacy and its uses are critical for the food industry. Studies have reported the ability of L. monocytogenes to survive in sanitizer concentrations 10-1000 times lower than the manufacturer-recommended concentration (MRC). Notably, data show that at MRC and when applied according to the label instructions, sanitizers remain largely effective. Studies also report that variables such as the presence of organic material, application time/temperature, and bacterial attachment to surfaces can impact sanitizer effectiveness. Due to the lack of standardization in the methodology and definitions of sanitizer resistance, tolerance, and susceptibility, different messages are conveyed in different studies. In this review, we examine the diversity of definitions, terminology, and methodologies used in studies examining L. monocytogenes resistance and susceptibility to antimicrobials. Research available to date fails to demonstrate "resistance" of L. monocytogenes to recommended sanitizer treatments as prescribed by the label. As such, sanitizer tolerance would be a more accurate description of L. monocytogenes response to low sanitizer concentrations (i.e., sub-MRC). Conservative use of word "resistance" will reduce confusion and allow for concise messaging as sanitizer research findings are communicated to industry and regulators.

Assessment of the Microbiological Quality of Ready-to-Eat Salads-Are There Any Reasons for Concern about Public Health?

Ready-to-eat food products can be readily consumed without further preparation and are convenient for busy on-the-go consumers. The objective of the study was to assess the microbiological quality of ready-to-eat salads. Thirty RTE salads were tested for the presence of bacteria, yeasts, and molds using the TEMPO and agar plate method. The study demonstrated that most of the tested products were characterized by varying microbiological quality. The total number of mesophilic microbiotas was about 6 log CFU g^-1. The high number of microorganisms was due to yeast and molds or Enterobacteriaceae. Half of the salads were contaminated with E. coli and three salads were contaminated with S. aureus. LAB were also found, which can be explained mainly by a dairy ingredient. In some salads, Salmonella spp. and L. monocytogenes were detected (26.7% and 33.3% of the samples, respectively). Based on the conducted tests, it was found that the microbiological quality was not satisfactory. The results presented in this study indicate that there is a significant problem of the presence of pathogens. Manufacturers should strive to reduce the possibility of microbial contamination through the use of widely understood hygiene of the production process, using hurdle technology, including the modified atmosphere and refrigerated storage.

Bacteriophages: A weapon against mixed-species biofilms in the food processing environment

Mixed-species biofilms represent the most frequent actual lifestyles of microorganisms in food processing environments, and they are usually more resistant to control methods than single-species biofilms. The persistence of biofilms formed by foodborne pathogens is believed to cause serious human diseases. These challenges have encouraged researchers to search for novel, natural methods that are more effective towards mixed-species biofilms. Recently, the use of bacteriophages to control mixed-species biofilms have grown significantly in the food industry as an alternative to conventional methods. This review highlights a comprehensive introduction of mixed-species biofilms formed by foodborne pathogens and their enhanced resistance to anti-biofilm removal strategies. Additionally, several methods for controlling mixed-species biofilms briefly focused on applying bacteriophages in the food industry have also been discussed. This article concludes by suggesting that using bacteriophage, combined with other 'green' methods, could effectively control mixed-species biofilms in the food industry.

Bioactivity of Essential Oils for Mitigation of Listeria monocytogenes Isolated from Fresh Retail Chicken Meat

Listeria monocytogenes is one of the most severe foodborne pathogens found in several habitats. Therefore, this study aims to investigate the antilisterial activity of different essential oils (EOs) against multidrug-resistant (MDR) L. monocytogenes strains isolated from fresh chicken meat. Our results showed that the prevalence of L. monocytogenes in the examined samples was 48%. Seventy-eight isolates were identified as L. monocytogenes. Out of these, 64.1% were categorized as MDR and were categorized in 18 patterns with 50 MDR isolates. One isolate was selected randomly from each pattern to investigate their biofilm-forming ability, resistance, and virulence genes incidence. Out of 18 MDR isolates, 88.9% showed biofilm-forming ability. Moreover, the most prevalent resistance genes were ermB (72%), aadA (67%), penA (61%), and floR genes (61%). However, the most prevalent virulence genes were inlA (94.4%), prfA (88.9%), plcB (83.3%), and actaA (83.3%). The antilisterial activity of EOs showed that cinnamon bark oil (CBO) was the most effective antilisterial agent. CBO activity could be attributed to the bioactivity of cinnamaldehyde which effects cell viability by increasing the bacterial cell electrical conductivity, ion leakage, and salt tolerance capacity loss. Therefore, CBO could be an effective alternative natural agent for food safety applications.

Application of Whole Genome Sequencing to Understand Diversity and Presence of Genes Associated with Sanitizer Tolerance in Listeria monocytogenes from Produce Handling Sources

Recent listeriosis outbreaks linked to fresh produce suggest the need to better understand and mitigate L. monocytogenes contamination in packing and processing environments. Using whole genome sequencing (WGS) and phenotype screening assays for sanitizer tolerance, we characterized 48 L. monocytogenes isolates previously recovered from environmental samples in five produce handling facilities. Within the studied population there were 10 sequence types (STs) and 16 cgMLST types (CTs). Pairwise single nucleotide polymorphisms (SNPs) ranged from 0 to 3047 SNPs within a CT, revealing closely and distantly related isolates indicative of both sporadic and continuous contamination events within the facility. Within Facility 1, we identified a closely related cluster (0-2 SNPs) of isolates belonging to clonal complex 37 (CC37; CT9492), with isolates recovered during sampling events 1-year apart and in various locations inside and outside the facility. The accessory genome of these CC37 isolates varied from 94 to 210 genes. Notable genetic elements and mutations amongst the isolates included the bcrABC cassette (2/48), associated with QAC tolerance; mutations in the actA gene on the Listeria pathogenicity island (LIPI) 1 (20/48); presence of LIPI-3 (21/48) and LIPI-4 (23/48). This work highlights the potential use of WGS in tracing the pathogen within a facility and understanding properties of L. monocytogenes in produce settings.

Low-energy X-ray inactivation of Listeria monocytogenes in mono-/co-culture biofilms with Pseudomonas fluorescens on food contact surfaces

This study assessed the inactivation kinetics of 150 keV low-energy X-ray on mono-/co-culture biofilms of Listeria monocytogenes and Pseudomonas fluorescens on three different food-contact-surfaces (polyethylene, acrylic, and stainless steel). The results indicated that the level of biofilm formation of mono-/co-cultures of L. monocytogenes and P. fluorescens was the highest on acrylic. The mono-culture L. monocytogenes biofilm cells exhibited higher resistance to the low-energy X-rays than the corresponding mono-culture P. fluorescens biofilm cells, regardless of surface types. Furthermore, co-culture had enhanced the resistance of both P. fluorescens and L. monocytogenes biofilm cells to the low-energy X-ray. Two kinetic models for the inactivation process were investigated, including (i) Linear model and (ii) Weibull model. Based on R², RMSE and AIC analysis, the Weibull model was superior in fitting the inactivation curves of low-energy X-ray on L. monocytogenes in mono-/co-culture biofilms with P. fluorescens. For mono-culture biofilms, the irradiation achieved the t_R1 value (derived from the Weibull model, i.e., the dose required for the first 1-log reduction) of 46.36-50.81 Gy for L. monocytogenes and the t_R1 value of 25.61-31.33 Gy for P. fluorescens. For co-culture biofilms, higher t_R1 values for L. monocytogenes (59.54-70.77 Gy) and P. fluorescens (32.73-45.13 Gy) were yielded than those for their individual counterparts in mono-culture biofilm.

Influence of the design of fresh-cut food washing tanks on the growth kinetics of Pseudomonas fluorescens biofilms

Mitigation of cross-contamination of fresh-cut food products at the washing step was studied by investigating how the vat design would affect the biofilm contamination surfaces. Hygienic design features such as no horizontal surfaces and only open angles exceeding 100° were proposed. The flow organization (velocity streamlines, wall shear stresses, and dynamics of the flow) was identified by means of computational fluid dynamics (CFD) calculation. Pseudomonas fluorescens PF1 biofilm growth kinetics were then mapped. The change in some geometrical features induced a better flow organization reducing “dead zones”. This significantly changed the biofilm growth kinetics, delaying the detection of biofilms from 20 hr to 24 hr. Critical areas such as welds, corners, and interfaces appeared far less prone to strong bacterial development. This would mean milder or less chemicals required at the washing step and faster and easier cleaning.

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Pseudomonas fluorescens biofilm growth kinetics strongly related to the vat design

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Improved design lead to 24hr lag time before the biofilm exponential growth phase

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Corners, welds, and wetting front areas contamination could be largely mitigated

Recent developments in antimicrobial and antifouling coatings to reduce or prevent contamination and cross-contamination of food contact surfaces by bacteria

Illness as the result of ingesting bacterially contaminated foodstuffs represents a significant annual loss of human quality of life and economic impact globally. Significant research investment has recently been made in developing new materials that can be used to construct food contacting tools and surfaces that might minimize the risk of cross-contamination of bacteria from one food item to another. This is done to mitigate the spread of bacterial contamination and resultant foodborne illness. Internet-based literature search tools such as Web of Science, Google Scholar, and Scopus were utilized to investigate publishing trends within the last 10 years related to the development of antimicrobial and antifouling surfaces with potential use in food processing applications. Technologies investigated were categorized into four major groups: antimicrobial agent-releasing coatings, contact-based antimicrobial coatings, superhydrophobic antifouling coatings, and repulsion-based antifouling coatings. The advantages for each group and technical challenges remaining before wide-scale implementation were compared. A diverse array of emerging antimicrobial and antifouling technologies were identified, designed to suit a wide range of food contact applications. Although each poses distinct and promising advantages, significant further research investment will likely be required to reliably produce effective materials economically and safely enough to equip large-scale operations such as farms, food processing facilities, and kitchens.

Listeria monocytogenes: review of pathogenesis and virulence determinants-targeted immunological assays

Listeria monocytogenes is one of the most invasive foodborne pathogens and is responsible for numerous outbreaks worldwide. Most of the methods to detect this bacterium in food require selective enrichment using traditional bacterial culture techniques that can be time-consuming and labour-intensive. Moreover, molecular methods are expensive and need specific technical knowledge. In contrast, immunological approaches are faster, simpler, and user-friendly alternatives and have been developed for the detection of L. monocytogenes in food, environmental, and clinical samples. These techniques are dependent on the constitutive expression of L. monocytogenes antigens and the specificity of the antibodies used. Here, updated knowledge on pathogenesis and the key immunogenic virulence determinants of L. monocytogenes that are used for the generation of monoclonal and polyclonal antibodies for the serological assay development are summarised. In addition, immunological approaches based on enzyme-linked immunosorbent assay, immunofluorescence, lateral flow immunochromatographic assays, and immunosensors with relevant improvements are highlighted. Though the sensitivity and specificity of the assays were improved significantly, methods still face many challenges that require further validation before use.

Disinfectant resistance in bacteria: Mechanisms, spread, and resolution strategies

Disinfectants are widely acknowledged for removing microorganisms from the surface of the objects and transmission media. However, the emergence of disinfectant resistance has become a severe threat to the safety of life and health and the rational allocation of resources due to the reduced disinfectant effectiveness. The horizontal gene transfer (HGT) of disinfectant resistance genes has also expanded the resistant flora, making the situation worse. This review focused on the resistance mechanisms of disinfectant resistant bacteria on biofilms, cell membrane permeability, efflux pumps, degradable enzymes, and disinfectant targets. Efflux can be the fastest and most effective resistance mechanism for bacteria to respond to stress. The qac genes, located on some plasmids which can transmit resistance through conjugative transfer, are the most commonly reported in the study of disinfectant resistance genes. Whether the qac genes can be transferred through transformation or transduction is still unclear. Studying the factors affecting the resistance of bacteria to disinfectants can find breakthrough methods to more adequately deal with the problem of reduced disinfectant effectiveness. It has been confirmed that the interaction of probiotics and bacteria or the addition of 4-oxazolidinone can inhibit the formation of biofilms. Chemicals such as eugenol and indole derivatives can increase bacterial sensitivity by reducing the expression of efflux pumps. The role of these findings in anti-disinfectant resistance has proved invaluable.

Antimicrobial activity of 405 nm light-emitting diode (LED) in the presence of riboflavin against Listeria monocytogenes on the surface of smoked salmon

This study investigated the antimicrobial activity of 405 nm light-emitting diode (LED) with and without riboflavin against Listeria monocytogenes in phosphate buffered saline (PBS) and on smoked salmon at different storage temperatures and evaluated its impact on food quality. The results show that riboflavin-mediated LED illumination in PBS 25 °C significantly inactivated L. monocytogenes cells by 6.2 log CFU/mL at 19.2 J/cm², while illumination alone reduced 1.9 log CFU/mL of L. monocytogenes populations at 57.6 J/cm². L. monocytogenes populations on illuminated smoked salmon decreased by 1.0-2.2 log CFU/cm² at 1.27-2.76 kJ/cm² at 4, 12, and 25 °C, regardless of the presence of riboflavin. Although illumination with and without riboflavin caused the lipid peroxidation and color change in smoked salmon, this study demonstrates the potential of a 405 nm LED to preserve the smoked salmon products, reducing the risk of listeriosis.

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.

Thermal and non-thermal treatment effects on Staphylococcus aureus biofilms formed at different temperatures and maturation periods

The objective of this study was to investigate the effect of temperature and maturation period on the resistance of Staphylococcus aureus biofilms to thermal and non-thermal treatments. First, biofilm development was compared at three different temperatures (15, 25, and 37°C) for 5 days. The cell population at 15 and 25°C remained relatively consistent approximately at 6.3 log CFU/cm², whereas 37°C resulted in the highest cell population on day 1 (7.6 log CFU/cm²) followed by a continual decline. Then, biofilm resistance to steam and sodium hypochlorite (NaOCl) treatments was evaluated. Obtained results highlighted that biofilms had different resistance to both treatments depending on development conditions. Specifically, steam treatment of 10 s eliminated 4.1 log CFU/cm² of the biofilm formed at 25°C for 5 days. The same treatment inactivated over 5 log population of biofilms developed in other temperature and maturation period conditions. Treatment with NaOCl reduced approximately 1 log CFU/cm² of biofilm cells developed at 25°C for 5 days. However, inactivation was found to be over 2 log CFU/cm² under other development conditions. An extracellular polymeric substances (EPS) quantification using 96-well plates and stainless steel coupons was conducted. In the 96-well plate experiment, it was found that the highest amount of polysaccharide was secreted at 25°C (p < 0.05), while total biomass and protein contents were greatest at 37°C (p < 0.05). No significant difference in EPS content was observed for stainless steel, but the results displayed a similar trend to the 96-well plate. In particular, biofilms developed at 25°C tended to secret the highest amount of polysaccharide, which aligned with the current literature. This finding indicated that polysaccharide was the main contribution to the enhanced resistance of S. aureus biofilms. Overall, it was shown that biofilms formed at 25°C for 5 days exhibited the greatest resistance to thermal and nonthermal treatments due to the elevated exopolysaccharide secretion. This study demonstrates that temperature and maturation period significantly affect the resistance of S. aureus biofilms to thermal and non-thermal treatments.

Genetic diversity, biofilm and virulence characteristics of Listeria monocytogenes in salmon sushi

Sushi is a ready-to-eat (RTE) food prepared from raw or cooked fish that is widely consumed worldwide. Listeria monocytogenes is the foodborne pathogen most commonly associated with RTE and fish products. The aim of the present study was to evaluate the presence of L. monocytogenes in salmon sushi commercialized in Pelotas city, Brazil, and to evaluate the genetic diversity, biofilm-forming ability in stainless steel, and virulence characteristics of the isolates. Four sampling events were carried out in seven specialized sushi establishments totaling 28 sushi pools. Listeria monocytogenes was detected in six samples (21.4%) from two establishments (28.6%). All isolates belonged to serotype 4b and carried the prfA, plcA, plcB, hlyA, mpl, actA, inlA, inlC, inlJ, and iap genes. The inlB gene was not detected in two isolates. The PFGE analysis grouped the isolates into four pulsotypes. All isolates had the ability to form biofilm on stainless steel and the average of biofilm formation counts varied between 6.4 and 7.2 log CFU.cm^-2. The isolates harbored the biofilm-related genes agrA, agrB, agrC, agrD, and prfA, with the exception of two isolates that did not harbor the agrD gene. The presence of L. monocytogenes in RTE sushi is a concern, demonstrating that sushi consumption may be a risk of human listeriosis. Furthermore, it was possible to identify the persistence of this pathogen for at least one month (pulsotypes III and IV), in two establishments (A and G), highlighting the need for improving the cleaning and sanitation procedures in establishments that commercialize RTE sushi.

The use of a CDC biofilm reactor to grow multi-strain Listeria monocytogenes biofilm

Listeria monocytogenes is one of the most concerning pathogens for the food industry due to its ability to form biofilms, particularly in difficult-to-clean sites of processing facilities. There is a current industry-wide lack of data to refer to when selecting a strategy to control L. monocytogenes biofilms in the food premises. Many strategies have been developed to study biofilm formation of bacteria; however, few have targeted L. monocytogenes biofilms under dynamic conditions. This study addresses the biofilm formation ability of L. monocytogenes on stainless steel and polycarbonate under dynamic conditions using TSBYE or BHI as media culture at 30 °C or 37 °C. Higher cell counts were recovered at 30 °C in TSBYE on polycarbonate while lower counts were obtained at 37 °C in BHI on stainless steel (P < 0.05). Nonetheless, all factors (temperature, media and material) were statistically significant (P < 0.05) and an interaction between temperature and media was observed (P < 0.05). To our knowledge, this work represents an initial framework to develop L. monocytogenes biofilms under different dynamic conditions. The use of CDC Biofilm Reactor is not widely used yet in the food industry and represent a novel approach to help sanitary control strategies implementation.

Interspecies Interactions in Dual-Species Biofilms Formed by Psychrotrophic Bacteria and the Tolerance of Sessile Communities to Disinfectants

ABSTRACT

Biofilms on the surface of food processing equipment act as potential reservoirs of microbial contamination. Bacterial interactions are believed to play key roles in both biofilm formation and antimicrobial tolerance. In this study, Aeromonas hydrophila, Chryseobacterium oncorhynchi, and Pseudomonas libanensis, which were previously isolated from Chinese raw milk samples, were selected to establish two dual-species biofilm models (P. libanensis plus A. hydrophila and P. libanensis plus C. oncorhynchi) on stainless steel at 7°C. Subsequently, three disinfectants, hydrogen peroxide (100 ppm), peracetic acid (100 ppm), and sodium hypochlorite (100 ppm), were used to treat the developed sessile communities for 10 min. Structural changes after exposure to disinfectants were analyzed with confocal laser scanning microscopy. The cell numbers of both A. hydrophila and C. oncorhynchi recovered from surfaces increased when grown as dual species biofilms with P. libanensis. Dual-species biofilms were more tolerant of disinfectants than were each single-species biofilm. Peracetic acid was the most effective disinfectant for removing biofilms, followed by hydrogen peroxide and sodium hypochlorite. The results expand the knowledge of mixed-species biofilms formed by psychrotrophic bacteria and will be helpful for developing effective strategies to eliminate bacterial mixed-species biofilms.

HIGHLIGHTS

Fighting Mixed-Species Microbial Biofilms With Cold Atmospheric Plasma

Most biofilms in nature are formed by multiple microbial species, and such mixed-species biofilms represent the actual lifestyles of microbes, including bacteria, fungi, viruses (phages), and/or protozoa. Microorganisms cooperate and compete in mixed-species biofilms. Mixed-species biofilm formation and environmental resistance are major threats to water supply, food industry, and human health. The methods commonly used for microbial eradication, such as antibiotic or disinfectant treatments, are often ineffective for mixed-species biofilm consortia due to their physical matrix barrier and physiological interactions. For the last decade, an increasing number of investigations have been devoted to the usage of cold atmospheric plasma (CAP), which is produced by dielectric barrier discharges or plasma jets to prevent or eliminate microbial biofilms. Here, we summarized the production of CAP, the inactivation of microorganisms upon CAP treatment, and the microbial factors affecting the efficacy of CAP procedure. The applications of CAP as antibiotic alternative strategies for fighting mixed-species biofilms were also addressed.

Architecture and Viability of the Biofilms Formed by Nine Listeria Strains on Various Hydrophobic and Hydrophilic Materials

Biofilms are a key factor in the persistence of Listeria in food processing plants, representing a potential source of foodstuff contamination. Nine Listeria strains (eight Listeria monocytogenes and one Listeria ivanovii) were studied by confocal laser scanning microscopy (CLSM) for their ability to form biofilm on glass, polystyrene, graphene and resin after 120 h of incubation at 12 °C. The relationship between cell surface hydrophobicity and biofilm formation was also investigated. On comparing the data for all the strains, similar (P > 0.05) biovolume values were obtained on glass (average 3.39 ± 1.69 µm3/µm2) and graphene (2.93 ± 1.14 µm3/µm2), while higher (P < 0.05) values were observed for polystyrene (4.39 ± 4.14 µm3/µm2). The highest (P < 0.01) biovolume levels were found in the biofilms formed on resin (7.35 ± 1.45 µm3/µm2), which also had the smallest biomass of inactivated cells (0.38 ± 0.37 µm3/µm2 vs. 1.20 ± 1.12 µm3/µm2 on the remaining surfaces; P < 0.001). No relationship was noted between cell surface hydrophobicity and biofilm-forming ability.

Isolation, Characterization, and Inactivation of Stenotrophomonas maltophilia From Leafy Green Vegetables and Urban Agriculture Systems

Stenotrophomonas maltophilia is an emerging opportunistic pathogen that, on the one hand, causes severe nosocomial infection in immunocompromised populations with a high mortality rate and, on the other hand, is present ubiquitously in the environment. This study, for the first time to the best of our knowledge, isolated and characterized S. maltophilia from leafy green vegetables produced by hydroponic farms and from a hydroponic farming facility in Singapore. Eleven S. maltophilia isolates were obtained from three types of leafy green vegetables (sweet basil, kale, and parsley) and from the nutrient solution used by a hydroponic farm. The antimicrobial resistance (AMR), biofilm-forming ability, and resistance to UV and quaternary ammonium compound (QAC) treatments were investigated, as was the fate of S. maltophilia in a simulated leafy green vegetable environment during a storage period of 6 days at different temperatures. The results showed that high population levels of S. maltophilia could be reached on leafy green vegetables, especially after being stored at abused temperatures (>8-log CFU/ml in basil juice after 6 days storage at 20°C) and on hydroponic farming facilities, probably due to biofilm formation (8 to 9-log CFU/well in biofilms). At 4°C, S. maltophilia was able to survive, but no growth was observed during storage in either bacteria culture media or basil juice for a period of 6 days. UV treatment, which induced substantial reductions in S. maltophilia in both single-species and dual-species biofilms mixed with Salmonella enterica serovar Typhimurium reference strain (ATCC 14028) or self-isolated Pseudomonas fluorescens (>4-log reductions by 250 mJ/cm² UV), is recommended for employment by hydroponic farms to treat their nutrient solutions and farming facilities so as to enhance microbial safety.