Synergistic effects of sodium chloride, glucose, and temperature on biofilm formation by Listeria monocytogenes serotype 1/2a and 4b strains

Elucidating the biofilm formation process, microstructure and functional gene expression of Listeria monocytogenes in beef juice

Listeria monocytogenes biofilm is recognized as a frequent cross-contamination source in the food industry, with raw beef and beef products as common food reservoirs. L. monocytogenes sequence types 9 (ST9) and ST8 are frequently isolated in meats and meat processing environment. In this study, beef juice was selected and compared to a laboratory medium (tryptone soy broth with 0.6 % yeast extract, TSB-YE). The purpose of this work was to investigate the effect of beef juice on the biofilm formation of ST9 and ST8 strains, including biofilm microstructure and modelling the biofilm formation process. Then the expression of biofilm functional genes in two culture media was also investigated. L. monocytogenes ST9 and ST8 can form a dense three-dimensional structure biofilm with multilayers of cells in beef juice after 48 h of incubation, but both strains formed a monolayer biofilm structure in TSB-YE. The ST9 strain developed more sessile cells on the stainless-steel surfaces than the ST8 strain under the same culture conditions. The Logistic model showed a good fit for with the biofilm formation process, and the estimated model parameters in beef juice and TSB-YE were considerably different. Under the same conditions, the maximum specific biofilm formation rate (μmax) in beef juice was higher than that in TSB-YE. This indicated that beef juice can facilitate the biofilm formation of L. monocytogenes, suggesting that the particles in beef juice act as a surface conditioner to support attachment. However, the maximum counts of L. monocytogenes biofilm formed on stainless steel coupon (Ymax) in beef juice was smaller than that in TSB-YE. The ST9 strain exhibited a stronger biofilm formation ability than the ST8 strain, and this was consistent with the scanning electron microscopy images. In the corresponding culture suspensions, the number of adherent cells increases with the number of planktonic cells. Moreover, the expression of biofilm functional genes was significantly different in the two culture media. Compared to biofilm cultured in TSB-YE, the expression of the agrA gene of biofilm in beef juice was significantly down-regulated for both the ST9 and the ST8 strains, and the expression of the inlB and the actA genes were dramatically up-regulated for the ST8 strain. Our results suggested that beef juice promotes biofilm formation of L. monocytogenes in meat processing and provide new insights into controlling biofilm.

Recent knowledge in phages, phage-encoded endolysin, and phage encapsulation against foodborne pathogens

The use of antibiotics had reached a plateau due to antibiotic resistance, overuse, and residue. Bacteriophages have recently attracted considerable attention as alternative biocontrol agents. Here, we provide an up-to-date overview of phage applications in the food industry. We reviewed recently reported phages against ten typical foodborne pathogens, studies of competitive phage-encoded endolysins, and the primary outcomes of phage encapsulation in food packaging and pathogen detection. Furthermore, we identified existing barriers that still need to be addressed and proposed potential solutions to overcome these obstacles in the future.

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

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

IMPORTANCE

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

Comparative Analysis of Growth, Survival, and Virulence Characteristics of Listeria monocytogenes Isolated from Imported Meat

Listeria monocytogenes is an important foodborne pathogen with worldwide prevalence. Understanding the variability in the potential pathogenicity among strains of different subtypes is crucial for risk assessment. In this study, the growth, survival, and virulence characteristics of 16 L. monocytogenes strains isolated from imported meat in China (2018-2020) were investigated. The maximum specific growth rate (μmax) and lag phase (λ) were evaluated using the time-to-detection (TTD) method and the Baranyi model at different temperatures (25, 30, and 37 °C). Survival characteristics were determined by D-values and population reduction after exposure to heat (60, 62.5, and 65 °C) and acid (HCl, pH = 2.5, 3.5, and 4.5). The potential virulence was evaluated via adhesion and invasion to Caco-2 cells, motility, and lethality to Galleria mellonella. The potential pathogenicity was compared among strains of different lineages and subtypes. The results indicate that the lineage I strains exhibited a higher growth rate than the lineage II strains at three growth temperatures, particularly serotype 4b within lineage I. At all temperatures tested, serotypes 1/2a and 1/2b consistently demonstrated higher heat resistance than the other subtypes. No significant differences in the log reduction were observed between the lineage I and lineage II strains at pH 2.5, 3.5, and 4.5. However, the serotype 1/2c strains exhibited significantly low acid resistance at pH 2.5. In terms of virulence, the lineage I strains outperformed the lineage II strains. The invasion rate to Caco-2 cells and lethality to G. mellonella exhibited by the serotype 4b strains were higher than those observed in the other serotypes. This study provides meaningful insights into the growth, survival, and virulence of L. monocytogenes, offering valuable information for understanding the correlation between the pathogenicity and subtypes of L. monocytogenes.

Studying the metabolic factors that may impact the growth of co-cultured Listeria monocytogenes strains at low temperature

The simultaneous presence of more than one strains of Listeria monocytogenes in the same food product may affect the growth capacity of each strain. The present study evaluated the metabolites composition that may potentially influence the growth of individual L. monocytogenes strains in a dual strain composite. Based on previous studies, L. monocytogenes strains, C5 (4b) and 6179 (1/2a) were selected due to the remarkable interaction, which was observed during their co-culture. The selected strains were inoculated (2.0 - 3.0 log CFU/mL) in Tryptic Soy Broth with 0.6% Yeast Extract (TSB-YE) in single and two-strain cultures (1:1 strain ratio). Bacterial growth was assessed during storage at 7 °C, under aerobic conditions (AC). Their resistance to different antibiotics enabled the selective enumeration of each strain in the co-culture. After reaching stationary phase, single and dual cultures were centrifuged and filtered. The cell-free spent medium (CFSM) was either characterized by Fourier transform infrared (FTIR-ATR) spectrometry or re-inoculated, after the addition of concentrated TSB-YE (for nutrient replenishment), with single and two-strain cultures for the evaluation of growth under the influence of metabolites produced from the same singly and co-cultured strains in the different combinations of strains and CFSM origin (7 °C/AC) (n = 2x3). By the end of storage, singly-cultured C5 and 6179 had reached 9.1 log CFU/mL, while in dual culture, 6179 was affected by the presence of C5 attaining only 6.4 ± 0.8 log CFU/mL. FTIR-ATR spectra of CFSM produced by singly-cultured 6179 and the co-culture were almost identical. Characteristic peaks in FTIR-ATR spectrum of CFSM of singly-cultured C5 at 1741, 1645 and 1223 cm^-1 represent functional groups which were not present in the CFSM of the co-culture. These molecules may be located intracellularly or mounted on bacterial cell surface and removed from the supernatant during cell filtration of the co-culture. Both singly- and co-cultured 6179 managed to grow similarly regardless of CFSM origin. Contrarily, both singly- and co-cultured C5 managed to outgrow 6179 in CFSM which contained high concentration of C5 metabolites, while in CFSM produced by singly-cultured 6179, C5 did not grow, suggesting that the produced metabolites of strain 6179 appears to be harmful to strain C5. However, during co-culture, C5 may produce molecules that counteract the inhibitory effect of 6179. The findings shed more light on the mechanism behind the inter-strain interactions of L. monocytogenes indicating that both contact of cells and extracellular metabolites may influence the behavior of the different co-existing strains.

Sugar Modification of Wall Teichoic Acids Determines Serotype-Dependent Strong Biofilm Production in Listeria monocytogenes

Biofilm production is responsible for persistent food contamination by Listeria monocytogenes, threatening food safety and public health. Human infection and food contamination with L. monocytogenes are caused primarily by serotypes 1/2a, 1/2b, and 4b. However, the association of biofilm production with phylogenic lineage and serotype has not yet been fully understood. In this study, we measured the levels of biofilm production in 98 clinical strains of L. monocytogenes at 37°C, 25°C, and 4°C. The phylogenetic clusters grouped by core genome multilocus sequence typing (cgMLST) exhibited association between biofilm production and phylogenetic lineage and serotype. Whereas clusters 1 and 3 consisting of serotype 4b strains exhibited weak biofilm production, clusters 2 (serotype 1/2b) and 4 (serotype 1/2a) were composed of strong biofilm formers. Particularly, cluster 2 (serotype 1/2b) strains exhibited the highest levels of biofilm production at 37°C, and the levels of biofilm production of cluster 4 (serotype 1/2a) strains were significantly elevated at all tested temperatures. Pan-genome analysis identified 22 genes unique to strong biofilm producers, most of which are related to the synthesis and modification of teichoic acids. Notably, a knockout mutation of the rml genes related to the modification of wall teichoic acids with l-rhamnose, which is specific to serogroup 1/2, significantly reduced the level of biofilm production by preventing biofilm maturation. Here, the results of our study show that biofilm production in L. monocytogenes is related to phylogeny and serotype and that the modification of wall teichoic acids with l-rhamnose is responsible for serotype-specific strong biofilm formation in L. monocytogenes. IMPORTANCE Biofilm formation on the surface of foods or food-processing facilities by L. monocytogenes is a serious food safety concern. Here, our data demonstrate that the level of biofilm production differs among serotypes 1/2a, 1/2b, and 4b depending on the temperature. Furthermore, sugar decoration of bacterial cell walls with l-rhamnose is responsible for strong biofilm production in serotypes 1/2a and 1/2b, commonly isolated from foods and listeriosis cases. The findings in this study improve our understanding of the association of biofilm production with phylogenetic lineage and serotype in L. monocytogenes.

Effect of nutritional and environmental conditions on biofilm formation of avian pathogenic Escherichia coli

AIMS

To study the effects of environmental stress and nutrient conditions on biofilm formation of avian pathogenic Escherichia coli (APEC).

METHODS AND RESULTS

The APEC strain DE17 was used to study biofilm formation under various conditions of environmental stress (including different temperatures, pH, metal ions, and antibiotics) and nutrient conditions (LB and M9 media, with the addition of different carbohydrates, if necessary). The DE17 biofilm formation ability was strongest at 25°C in LB medium. Compared to incubation at 37°C, three biofilm-related genes (csgD, dgcC, and pfs) were significantly upregulated and two genes (flhC and flhD) were downregulated at 25°C, which resulted in decreased motility. However, biofilm formation was strongest in M9 medium supplemented with glucose at 37°C, and the number of live bacteria was the highest as determined by confocal laser scanning microscopy (CLSM). The bacteria in the biofilm were surrounded by a thick extracellular matrix, and honeycomb-like or rough surfaces were observed by scanning electron microscopy (SEM). Moreover, biofilm formation of the DE17 strain was remarkably inhibited under acidic conditions, whereas neutral and alkaline conditions were more suitable for biofilm formation. Biofilm formation was also inhibited at specific concentrations of cations (Na⁺ , K⁺ , Ca²⁺ , and Mg²⁺ ) and antibiotics (ampicillin, chloramphenicol, kanamycin, and spectinomycin). The qRT-PCR showed that the transcription levels of biofilm-related genes change under different environmental conditions.

CONCLUSIONS

Nutritional and environmental factors played an important role in DE17 biofilm development. The transcription levels of biofilm-related genes changed under different environmental and nutrient conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

The findings suggest that nutritional and environmental factors play an important role in APEC biofilm development. Depending on the different conditions involved in this study, it can serve as a guide to treating biofilm-related infections and to eliminating biofilms from the environment.

Innovative Perspectives on Biofilm Interactions in Poultry Drinking Water Systems and Veterinary Antibiotics Used Worldwide

Prudent use of antibiotics in livestock is widely considered to be important to prevent antibiotic resistance. This study aimed to evaluate the interactions between biofilms and veterinary antibiotics in therapeutic concentrations administrated via drinking water through a standardized experimental setup. In this context, two biofilms formed by pseudomonads (Pseudomonas (P.) aeruginosa or P. fluorescens) and a susceptible Escherichia (E.) coli strain were developed in a nutrient-poor medium on the inner surface of polyvinyl chloride pipe pieces. Subsequently, developing biofilms were exposed to sulfadiazine/trimethoprim (SDZ/TMP) or tylosin A (TYL A) in dosages recommended for application in drinking water for 5 or 7 days, respectively. Various interactions were detected between biofilms and antibiotics. Microbiological examinations revealed that only TYL A reduced the number of bacteria on the surface of the pipes. Additionally, susceptible E. coli survived both antibiotic treatments without observable changes in the minimum inhibitory concentration to 13 relevant antibiotics. Furthermore, as demonstrated by HPLC-UV, the dynamics of SDZ/TMP and TYL A in liquid media differed between the biofilms of both pseudomonads over the exposure period. We conclude that this approach represents an innovative step toward the effective evaluation of safe veterinary antibiotic use.

Isolation of Shewanella putrefaciens GRD 03 from Fish and Explication of Biofilm Adherence Potency on Different Substrates

Foodborne pathogens are the main threat and cause of food poisoning. The majority of food infections have been related to the biofilm formation of foodborne pathogens in the food industry. Shewanella putrefaciens (KX355803, GRD 03), a Gram-negative pathogen isolated from mackerel fish, was identified and recognized as a food spoilage bacterium and a strong biofilm producer. The adhesion or attachment ability of Shewanella putrefaciens was determined on steel, plastic, glass, PVC and wood. NB (Nutrient broth), LB (Luria-Bertani broth), TSB (Tryptic soy broth) and BHI (Brain heart infusion broth) were enriched with glucose and shows optimum for bacterial adhesion. In the microtiter plate method (MTP), the strong attachment was observed at 48 and 72 hours of incubation and significant differences were obtained at p < 0.05. As the incubation period increases, the OD value (Optical density) of samples also increase. Biofilm formation is the major cause cross-contamination, and shows resistance to certain disinfectants, which leads to environmental stress tolerance. This study suggested with optimum biofilm production of isolate from fish by using glucose enriched media on different substrates, also comparing different growth media provide a detailed idea about biofilm-forming ability at different incubation time intervals.

Effects of environmental conditions (temperature, pH, and glucose) on biofilm formation of Salmonella enterica serotype Kentucky and virulence gene expression

Salmonella is a foodborne pathogen and an emerging zoonotic bacterial threat in the food industry. The aim of this study was to evaluate the biofilm formation by a cocktail culture of 3 wild isolates of Salmonella enterica serotype Kentucky on plastic (PLA), silicon rubber (SR), and chicken skin surfaces under various temperatures (4, 10, 25, 37, and 42°C) and pH values (4.0, 5.0, 6.0, 7.0, and 8.0). Then, at the optimum temperature and pH, the effects of supplementation with glucose (0, 0.025, 0.05, and 0.4% w/v) on biofilm formation were assessed on each of the surfaces. The results indicated that higher temperatures (25 to 42°C) and pH values (7.0 and 8.0) led to more robust biofilm formation than lower temperatures (4 and 10°C) and lower pH levels (4.0 to 6.0). Moreover, biofilm formation was induced by 0.025% glucose during incubation at the optimum temperature (37°C) and pH (7.0) but inhibited by 0.4% glucose. Consistent with this finding, virulence related gene (rpoS, rpoH, hilA, and avrA) expression was increased at 0.025% glucose and significantly reduced at 0.4% glucose. This results also confirmed by field emission scanning electron microscope, confocal laser scanning microscopy, and autoinducer-2 determination. This study concluded that optimum environmental conditions (temperature 37°C, pH 7.0, and 0.25% glucose) exhibited strong biofilm formation on food and food contract surfaces as well as increased the virulence gene expression levels, indicating that these environmental conditions might be threating conditions for food safety.

Effect of quorum-sensing molecule 2-phenylethanol and ARO genes on Saccharomyces cerevisiae biofilm

Biofilms are a form of microbial community that can be beneficial for industrial fermentation because of their remarkable environmental resistance. However, the mechanism of biofilm formation in Saccharomyces cerevisiae remains to be fully explored, and this may enable improved industrial applications for this organism. Although quorum-sensing (QS) molecules are known to be involved in bacteria biofilm formation, few studies have been undertaken with these in fungi. 2-phenylethanol (2-PE) is considered a QS molecule in S. cerevisiae. Here, we found that exogenous 2-PE could stimulate biofilm formation at low cell concentrations. ARO8p and ARO9p are responsible for the synthesis of 2-PE and were crucial to the formation of biofilm. Deletion of the ARO8 and ARO9 genes reduced the content of 2-PE in the early stage of fermentation, reduced ethanol yield and decreased biofilm formation. The expression of FLOp, which is involved in cell adhesion, and the content of extracellular polysaccharides of mutant strains ΔARO8 and ΔARO9 were also significantly reduced. These findings indicate that the production of 2-PE had a positive effect on biofilm formation in S. cerevisiae, thereby providing further key details for studying the formation of biofilm mechanism in the future. KEY POINTS: • Quorum-sensing molecule 2-PE positively affects biofilm formation in S. cerevisiae. • 2-PE synthetic genes ARO8 and ARO9 deletion reduced extracellular polysaccharide. • ARO8 and ARO9 deletion reduced the gene expression of the FLO family.

Genomic diversity and characterization of Listeria monocytogenes from dry-cured ham processing plants

Genomic diversity of Listeria monocytogenes isolates from the deboning and slicing areas of three dry-cured ham processing plants was analysed. L. monocytogenes was detected in 58 out of 491 samples from the environment and equipment surfaces, all from the deboning area, with differences in prevalence among facilities. The most frequent PCR-serogroup was IIa (74.1%) followed by IIb and IIc, and only one isolate was serogroup IVb. Twenty different pulsotypes and 11 sequence types (STs) grouped into 10 clonal complexes (CCs) were determined. ST121 (CC121) and ST9 (CC9) were the most abundant. Premature stop codons (PMSC6 and PMSC19) associated with attenuated virulence were found in the inlA sequence in 7 out of 12 selected strains. CC121 strains were strong biofilm formers and some harboured the transposon Tn6188, related with increased tolerance to quaternary ammonium compounds. L. monocytogenes clones considered hypovirulent resulted predominant in the deboning areas. The clonal structure and potential virulence of the isolates could help to establish adequate control measures and cleaning protocols for the comprehensive elimination of the pathogen in dry-cured ham processing environment.

Plant Growth-Promoting Rhizobacteria Isolated from Degraded Habitat Enhance Drought Tolerance of Acacia (Acacia abyssinica Hochst. ex Benth.) Seedlings

Drought stress (DS) is the most impacting global phenomenon affecting the ecological balance of a particular habitat. The search for potential plant growth-promoting rhizobacteria (PGPR) capable of enhancing plant tolerance to drought stress is needed. Thus, this study was initiated to evaluate the effect of inoculating Acacia abyssinica seedlings with PGPR isolated from rhizosphere soil of Ethiopia to enhance DS tolerance. The strains were selected based on in vitro assays associated with tolerance to drought and other beneficial traits such as salinity, acidity, temperature, heavy metal tolerances, biofilm formation, and exopolysaccharide (EPS) production. The strains with the best DS tolerance ability were selected for the greenhouse trials with acacia plants. The results indicate that out of 73 strains, 10 (14%) were completely tolerant to 40% polyethylene glycol. Moreover, 37% of the strains were strong biofilm producers, while 66 (90.41%) were EPS producers with a better production in the medium containing sucrose at 28 ± 2°C and pH 7 ± 0.2. Strains PS-16 and RS-79 showed tolerance to 11% NaCl. All the strains were able to grow in wider ranges of pH (4–10) and temperature (15–45°C) and had high tolerance to heavy metals. The inoculated bacterial strains significantly (p ≤ 0.05) increased root and shoot length and dry biomass of acacia plants. One of the strains identified as P. fluorescens strain FB-49 was outstanding in enhancing DS tolerance compared to the single inoculants and comparable to consortia. Stress-tolerant PGPR could be used to enhance acacia DS tolerance after testing other phytobeneficial traits.

Differential Modulation of Listeria monocytogenes Fitness, In Vitro Virulence, and Transcription of Virulence-Associated Genes in Response to the Presence of Different Microorganisms

Interactions between Listeria monocytogenes and food-associated or environmental bacteria are critical not only for the growth but also for a number of key biological processes of the microorganism. In this regard, limited information exists on the impact of other microorganisms on the virulence of L. monocytogenes In this study, the growth of L. monocytogenes was evaluated in a single culture or in coculture with L. innocua, Bacillus subtilis, Lactobacillus plantarum, or Pseudomonas aeruginosa in tryptic soy broth (10°C/10 days and 37°C/24 h). Transcriptional levels of 9 key virulence genes (inlA, inlB, inlC, inlJ, sigB, prfA, hly, plcA, and plcB) and invasion efficiency and intracellular growth in Caco-2 cells were determined for L. monocytogenes following growth in mono- or coculture for 3 days at 10°C or 9 h at 37°C. The growth of L. monocytogenes was negatively affected by the presence of L. innocua and B. subtilis, while the effect of cell-to-cell contact on L. monocytogenes growth was dependent on the competing microorganism. Cocultivation affected the in vitro virulence properties of L. monocytogenes in a microorganism-specific manner, with L. innocua mainly enhancing and B. subtilis reducing the invasion of the pathogen in Caco-2 cells. Assessment of the mRNA levels of L. monocytogenes virulence genes in the presence of the four tested bacteria revealed a complex pattern in which the observed up- or downregulation was only partially correlated with growth or in vitro virulence and mainly suggested that L. monocytogenes may display a microorganism-specific transcriptional response.IMPORTANCE Listeria monocytogenes is the etiological agent of the severe foodborne disease listeriosis. Important insight regarding the physiology and the infection biology of this microorganism has been acquired in the past 20 years. However, despite the fact that L. monocytogenes coexists with various microorganisms throughout its life cycle and during transmission from the environment to foods and then to the host, there is still limited knowledge related to the impact of surrounding microorganisms on L. monocytogenes' biological functions. In this study, we showed that L. monocytogenes modulates specific biological activities (i.e., growth and virulence potential) as a response to coexisting microorganisms and differentially alters the expression of virulence-associated genes when confronted with different bacterial genera and species. Our work suggests that the interaction with different bacteria plays a key role in the survival strategies of L. monocytogenes and supports the need to incorporate biotic factors into the research conducted to identify mechanisms deployed by this organism for establishment in different environments.

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.

Calcineurin signaling pathway influences Aspergillus niger biofilm formation by affecting hydrophobicity and cell wall integrity

BACKGROUND

Biofilms, as a kind of fixed-cell community, can greatly improve industrial fermentation efficiency in immobilized fermentation, but the regulation process is still unclear, which restricts their application. Ca2+ was reported to be a key factor affecting biofilm formation. However, the effect of Ca2+ on biofilm structure and microbiology was yet only studied in bacteria. How Ca2+-mediated calcineurin signaling pathway (CSP) alters biofilm formation in bacteria and fungi has rarely been reported. On this basis, we investigated the regulation of CSP on the formation of biofilm in Aspergillus niger.

RESULTS

Deletion of the key genes MidA, CchA, CrzA or CnaA in the CSP lowered the Ca2+ concentration in the mycelium to a different extent, inhibited the formation of A. niger biofilm, reduced the hydrophobicity and adhesion of spores, destroyed the cell wall integrity of hyphae, and reduced the flocculation ability of hyphae. qRT-PCR results showed that the expression of spore hydrophobic protein RodA, galactosaminogalactan (GAG) biosynthesis genes (uge3, uge5, agd3, gtb3), and α-1,3-glucan biosynthesis genes (ags1, ags3) in the ∆MidA, ∆CchA, ∆CrzA, ∆CnaA strains were significantly down-regulated compared with those of the wild type (WT). In addition, the transcription levels of the chitin synthesis gene (chsB, chsD) and β-1,3-glucan synthesis gene (FksA) were consistent with the change in chitin and β-1,3-glucan contents in mutant strains.

CONCLUSION

These results indicated that CSP affected the hydrophobicity and adhesion of spores, the integrity of mycelial cell walls and flocculation by affecting Ca2+ levels in mycelium, which in turn affected biofilm formation. This work provides a possible explanation for how CSP changes the formation of A. niger biofilm, and reveals a pathway for controlling biofilm formation in industrial immobilized fermentation.

Biofilm Formation of Listeria monocytogenes Strains Under Food Processing Environments and Pan-Genome-Wide Association Study

Concerns about food contamination by Listeria monocytogenes are on the rise with increasing consumption of ready-to-eat foods. Biofilm production of L. monocytogenes is presumed to be one of the ways that confer its increased resistance and persistence in the food chain. In this study, a collection of isolates from foods and food processing environments (FPEs) representing persistent, prevalent, and rarely detected genotypes was evaluated for biofilm forming capacities including adhesion and sessile biomass production under diverse environmental conditions. The quantity of sessile biomass varied according to growth conditions, lineage, serotype as well as genotype but association of clonal complex (CC) 26 genotype with biofilm production was evidenced under cold temperature. In general, relative biofilm productivity of each strain varied inconsistently across growth conditions. Under our experimental conditions, there were no clear associations between biofilm formation efficiency and persistent or prevalent genotypes. Distinct extrinsic factors affected specific steps of biofilm formation. Sudden nutrient deprivation enhanced cellular adhesion while a prolonged nutrient deficiency impeded biofilm maturation. Salt addition increased biofilm production, moreover, nutrient limitation supplemented by salt significantly stimulated biofilm formation. Pan-genome-wide association study (Pan-GWAS) assessed genetic composition with regard to biofilm phenotypes for the first time. The number of reported genes differed depending on the growth conditions and the number of common genes was low. However, a broad overview of the ontology contents revealed similar patterns regardless of the conditions. Functional analysis showed that functions related to transformation/competence and surface proteins including Internalins were highly enriched.

Persistent Listeria monocytogenes Isolates from a Poultry-Processing Facility Form more Biofilm but Do Not Have a Greater Resistance to Disinfectants Than Sporadic Strains

Some strains of Listeria monocytogenes can persist in food-processing environments, increasing the likelihood of the contamination of foodstuffs. To identify traits that contribute to bacterial persistence, a selection of persistent and sporadic L. monocytogenes isolates from a poultry-processing facility was investigated for biofilm-forming ability (crystal violet assay). The susceptibility of sessile cells to treatments (five minutes) with sodium hypochlorite having 10% active chlorine (SHY: 10,000 ppm, 25,000 ppm, and 50,000 ppm) and benzalkonium chloride (BZK: 2500 ppm, 10,000 ppm, and 25,000 ppm) was also studied. All isolates exhibited biofilm formation on polystyrene. Persistent strains showed larger (p < 0.001) biofilm formation (OD580 = 0.301 ± 0.097) than sporadic strains (OD580 = 0.188 ± 0.082). A greater susceptibility to disinfectants was observed for biofilms of persistent strains than for those of sporadic strains. The application of SHY reduced biofilms only for persistent strains. BZK increased OD580 in persistent strains (2500 ppm) and in sporadic strains (all concentrations). These results indicate that the use of BZK at the concentrations tested could represent a public health risk. Findings in this work suggest a link between persistence and biofilm formation, but do not support a relationship between persistence and the resistance of sessile cells to disinfectants.

Prevalence, Molecular Typing, and Determination of the Biofilm-Forming Ability of Listeria monocytogenes Serotypes from Poultry Meat and Poultry Preparations in Spain

A study was undertaken of the presence of Listeria monocytogenes in 260 samples of poultry meat obtained from retail outlets in northwestern Spain. L. monocytogenes was detected in 20 samples (7.7%). Twenty strains (one strain per positive sample) were characterized. The strains belonged to 10 serotypes: 1/2a (2 strains), 1/2b (2), 1/2c (2), 3a (1), 3b (2), 3c (2), 4a (2), 4b (4), 4c (1), and 4d (2). Cluster analysis (ribotyping; EcoRI) showed a strong genetic relationship between strains isolated from samples coming from different outlets. Ribotyping permitted some isolates of the same serotype to be differentiated, which points to the possible usefulness of this technique in the epidemiological surveillance of L. monocytogenes. All strains formed biofilm on polystyrene, as shown by confocal laser scanning microscopy. The biovolume (between 621.7 ± 36.0 µm3 and 62,984.0 ± 14,888.2 µm3 in the observational field of 14,161 μm2), percentage of surface coverage (from 2.17 ± 0.84% to 94.43 ± 3.97%), roughness (between 0.399 ± 0.052 and 0.830 ± 0.022), and maximum thickness (between 9.00 ± 0.00 µm and 24.00 ± 14.93 µm) of biofilms varied between strains (p < 0.05). These results expand knowledge of the characteristics of L. monocytogenes isolates from poultry.

Characterization of Listeria Monocytogenes Originating from the Spanish Meat-Processing Chain

Using agglutination techniques, 118 Listeria monocytogenes isolates from red meat and poultry were serotyped. Strains were ascribed to the serotypes 4b/4e (44.1% of the strains), 1/2 (a, b or c; 28.0%), 4c (6.8%), 4d/4e (5.9%) and 3 (a, b or c; 2.5%). Among these are the serotypes most frequently involved in cases of human listeriosis. The susceptibility of 72 strains to 26 antibiotics of clinical importance was determined by disc diffusion (Clinical and Laboratory Standards Institute; CLSI). High levels of resistance were observed to cefoxitin (77.8% of the strains showed resistance), cefotaxime (62.5%), cefepime (73.6%), and nalidixic acid (97.2%), nitrofurantoin (51.4%) and oxacillin (93.1%). Less than 3% of the strains showed resistance to the antibiotic classes used in human listeriosis therapy (i.e., ampicillin, gentamicin, rifampicin, chloramphenicol, enrofloxacin, vancomycin, trimethoprim-sulfamethoxazole, erythromycin, and tetracycline). The influence of species and serotype on the growth kinetics (modified Gompertz equation) and on the adhesion ability (crystal violet staining) of nine isolates of L. monocytogenes (serotypes 1/2a, 1/2b, 1/2c, 3a, 3b, 3c, 4a, 4b, and 4d), and one strain of Listeria ivanovii were investigated. The maximum growth rate (ΔOD420-580/h) varied between 0.073 ± 0.018 (L. monocytogenes 1/2a) and 0.396 ± 0.026 (L. monocytogenes 4b). The isolates of L. monocytogenes belonging to serotypes 3a and 4a, as well as L. ivanovii, showed a greater (p < 0.05) biofilm-forming ability than did the remaining strains, including those that belong to the serotypes commonly implied in human listeriosis (1/2a, 1/2b, 1/2c and 4b). The need for training in good hygiene practices during the handling of meat and poultry is highlighted to reduce the risk of human listeriosis.

Effects of exogenous glucose on Pseudomonas aeruginosa biofilm formation and antibiotic resistance

Pseudomonas aeruginosa is commonly found in nosocomial and life‐threatening infections in patients. Biofilms formed by P. aeruginosa exhibit much greater resistance to antibiotics than the planktonic form of the bacteria. Few groups have studied the effects of glucose, a major carbon source, and metabolite, on P. aeruginosa biofilm formation and on its metabolic pathways. In this study, we investigated the effect of glucose on the biofilm formation ability of P. aeruginosa and carried out a metabolomic analysis to identify whether glucose alters the metabolic activity of P. aeruginosa in biofilms. We found that glucose efficiently promoted P. aeruginosa biofilm formation by upregulating the expression of the extracellular polysaccharide‐related gene pslA. Treatment with glucose caused an increase in 7 metabolites (including 3‐hydroxypropionic acid, glucose‐6‐phosphate, and 2,3‐dimethylsuccinic acid) and a decrease in 18 metabolites (including myo‐inositol, glutamine, and methoxamedrine) in the biofilm. In addition, there was a synergistic effect between glucose and horse serum on biofilm formation when the two were added in combination, which also increased the resistance of biofilm to levofloxacin therapy. Thus, our work sheds light on the underlying mechanisms by which glucose may enhance biofilm formation and identifies novel targets for developing strategies to counteract biofilm formation.

Disinfectant Susceptibility of Biofilm Formed by Listeria monocytogenes under Selected Environmental Conditions

Listeria monocytogenes is a one of the most important food-borne pathogens. Its ability to form biofilm contributes to increased resistance to disinfectants and inefficient disinfection, posing a serious threat for the food industry, and in the end the consumer. The aim of this study was the comparison of the biofilm formation ability of L. monocytogenes strains on stainless steel, under different environmental conditions (temperature, pH, NaCl concentration, nutrients availability), and the assessment of biofilm susceptibility to disinfectants. The bactericidal activity of four disinfectants in two concentrations (100% and 50% of working solution) against biofilm was conducted on four clinical strains, four strains isolated from food and one reference strain ATCC 19111. It was found that biofilm susceptibility to disinfectants was influenced by environmental conditions. Biofilm susceptibility correlated with the decrease of temperature, pH, nutrients availability and salinity of the environment. The least sensitive to disinfectants was biofilm produced at pH = 4 (the bacterial number ranged from 0.25 log CFU × cm-2 to 1.72 log CFU × cm-2) whereas the most sensitive was biofilm produced at pH = 9 (5.16 log CFU × cm-2 to 7.84 log CFU × cm-2). Quatosept was the most effective disinfectant, regardless of the conditions. In conclusion, biofilm susceptibility to disinfectants is strain-dependent and is affected by environmental conditions.

Hypervirulent Listeria monocytogenes clones' adaption to mammalian gut accounts for their association with dairy products

Listeria monocytogenes (Lm) is a major human and animal foodborne pathogen. Here we show that hypervirulent Lm clones, particularly CC1, are strongly associated with dairy products, whereas hypovirulent clones, CC9 and CC121, are associated with meat products. Clone adaptation to distinct ecological niches and/or different food products contamination routes may account for this uneven distribution. Indeed, hypervirulent clones colonize better the intestinal lumen and invade more intestinal tissues than hypovirulent ones, reflecting their adaption to host environment. Conversely, hypovirulent clones are adapted to food processing environments, with a higher prevalence of stress resistance and benzalkonium chloride tolerance genes and a higher survival and biofilm formation capacity in presence of sub-lethal benzalkonium chloride concentrations. Lm virulence heterogeneity therefore reflects the diversity of the ecological niches in which it evolves. These results also have important public health implications and may help in reducing food contamination and improving food consumption recommendations to at-risk populations.

Nitric oxide increases biofilm formation in Saccharomyces cerevisiae by activating the transcriptional factor Mac1p and thereby regulating the transmembrane protein Ctr1

BACKGROUND

Biofilms with immobilized cells encased in extracellular polymeric substance are beneficial for industrial fermentation. Their formation is regulated by various factors, including nitric oxide (NO), which is recognized as a quorum-sensing and signal molecule. The mechanisms by which NO regulates bacterial biofilms have been studied extensively and deeply, but were rarely studied in fungi. In this study, we observed the effects of low concentrations of NO on biofilm formation in Saccharomyces cerevisiae. Transcriptional and proteomic analyses were applied to study the mechanism of this regulation.

RESULTS

Adding low concentrations of NO donors (SNP and NOC-18) enhanced biofilm formation of S. cerevisiae in immobilized carriers and plastics. Transcriptional and proteomic analyses revealed that expression levels of genes regulated by the transcription factor Mac1p was upregulated in biofilm cells under NO treatment. MAC1 promoted yeast biofilm formation which was independent of flocculation gene FLO11. Increased copper and iron contents, both of which were controlled by Mac1p in the NO-treated and MAC1-overexpressing cells, were not responsible for the increased biofilm formation. CTR1, one out of six genes regulated by MAC1, plays an important role in biofilm formation. Moreover, MAC1 and CTR1 contributed to the cells' resistance to ethanol by enhanced biofilm formation.

CONCLUSIONS

These findings suggest that a mechanism for NO-mediated biofilm formation, which involves the regulation of CTR1 expression levels by activating its transcription factor Mac1p, leads to enhanced biofilm formation. The role of CTR1 protein in yeast biofilm formation may be due to the hydrophobic residues in its N-terminal extracellular domain, and further research is needed. This work offers a possible explanation for yeast biofilm formation regulated by NO and provides approaches controlling biofilm formation in industrial immobilized fermentation by manipulating expression of genes involved in biofilm formation.

Risk Factors and Level of Listeria monocytogenes Contamination of Raw Pork in Retail Markets in China

Listeria monocytogenes can contaminate various foods via food processing environments and contamination of raw materials. There is a limited understanding of L. monocytogenes transmission in retail market and the role of insects in L. monocytogenes transmission in the retail environments. To better understand the risk factors of raw pork contamination, the prevalence of L. monocytogenes was examined in raw pork, retail environments and insects in a retail market over a 6-month period from March to August in 2016 in Beijing, China. A total of 2,789 samples were collected, including 356 raw pork samples, 1,392 meat contact surface swabs (MCS), 712 non-meat contact surface swabs (NMCS) and 329 insect samples. Overall, 424 (15.20%) of the samples were found to be contaminated by L. monocytogenes. Analyzed by serotyping, multilocus sequence typing and pulsed-field gel electrophoresis, the 424 L. monocytogenes isolates were divided into three serotypes (1/2c, 1/2a and 3a), 15 pulsotypes (PTs) and nine sequence types (STs), 1/2c/PT4/ST9 (244/424, 58%) was the most prevalent type of L. monocytogenes strains. The raw pork, MCS of the environments and insects were contaminated with higher levels of L. monocytogenes than NMCS of the environments, which suggested that cross contamination of L. monocytogenes between raw pork and the environment existed in the retail market, and long-term contaminated surfaces and vector insects would act as high risk factors to transmit L. monocytogenes to raw pork. Thus more effective strategies are needed to reduce the risk of retail pork meat contamination by L. monocytogenes and prevent foodborne human listeriosis.

Control of Listeria innocua Biofilms on Food Contact Surfaces with Slightly Acidic Electrolyzed Water and the Risk of Biofilm Cells Transfer to Duck Meat

Biofilm formation on food contact surfaces is a potential hazard leading to cross-contamination during food processing. We investigated Listeria innocua biofilm formation on various food contact surfaces and compared the washing effect of slightly acidic electrolyzed water (SAEW) at 30, 50, 70, and 120 ppm with that of 200 ppm of sodium hypochlorite (NaClO) on biofilm cells. The risk of L. innocua biofilm transfer and growth on food at retail markets was also investigated. The viability of biofilms that formed on food contact surfaces and then transferred cells to duck meat was confirmed by fluorescence microscopy. L. innocua biofilm formation was greatest on rubber, followed by polypropylene, glass, and stainless steel. Regardless of sanitizer type, washing removed biofilms from polypropylene and stainless steel better than from rubber and glass. Among the various SAEW concentrations, washing with 70 ppm of SAEW for 5 min significantly reduced L. innocua biofilms on food contact surfaces during food processing. Efficiency of transfer of L. innocua biofilm cells was the highest on polypropylene and lowest on stainless steel. The transferred biofilm cells grew to the maximum population density, and the lag time of transferred biofilm cells was longer than that of planktonic cells. The biofilm cells that transferred to duck meat coexisted with live, injured, and dead cells, which indicates that effective washing is essential to remove biofilm on food contact surfaces during food processing to reduce the risk of foodborne disease outbreaks.

Sodium Chloride Does Not Ensure Microbiological Safety of Foods: Cases and Solutions

Addition of salt or salt-containing water to food is one of the oldest and most effective preservation methods in history; indeed, salt-cured foods are generally recognized as microbiologically safe due to their high salinity. However, a number of microbiological risks remain. The microbiological hazards and risks associated with salt-cured foods must be addressed more in-depth as they are likely to be underestimated by previous studies. This review examined a number of scientific reports and articles about the microbiological safety of salt-cured foods, which included salted, brined, pickled, and/or marinated vegetables, meat, and seafood. The following subjects are covered in order: (1) clinical cases and outbreaks attributed to salt-cured foods; (2) the prevalence of foodborne pathogens in such foods; (3) the molecular, physiological, and virulent responses of the pathogens to the presence of NaCl in both laboratory media and food matrices; (4) the survival and fate of microorganisms in salt-cured foods (in the presence/absence of additional processes); and (5) the interaction between NaCl and other stressors in food processes (e.g., acidification, antimicrobials, drying, and heating). The review provides a comprehensive overview of potentially hazardous pathogens associated with salt-cured foods and suggests further research into effective intervention techniques that will reduce their levels in the food chain.

Listeria monocytogenes in Food-Processing Facilities, Food Contamination, and Human Listeriosis: The Brazilian Scenario

Listeria monocytogenes is a foodborne pathogen that contaminates food-processing environments and persists within biofilms on equipment, utensils, floors, and drains, ultimately reaching final products by cross-contamination. This pathogen grows even under high salt conditions or refrigeration temperatures, remaining viable in various food products until the end of their shelf life. While the estimated incidence of listeriosis is lower than other enteric illnesses, infections caused by L. monocytogenes are more likely to lead to hospitalizations and fatalities. Despite the description of L. monocytogenes occurrence in Brazilian food-processing facilities and foods, there is a lack of consistent data regarding listeriosis cases and outbreaks directly associated with food consumption. Listeriosis requires rapid treatment with antibiotics and most drugs suitable for Gram-positive bacteria are effective against L. monocytogenes. Only a minority of clinical antibiotic-resistant L. monocytogenes strains have been described so far; whereas many strains recovered from food-processing facilities and foods exhibited resistance to antimicrobials not suitable against listeriosis. L. monocytogenes control in food industries is a challenge, demanding proper cleaning and application of sanitization procedures to eliminate this foodborne pathogen from the food-processing environment and ensure food safety. This review focuses on presenting the L. monocytogenes distribution in food-processing environment, food contamination, and control in the food industry, as well as the consequences of listeriosis to human health, providing a comparison of the current Brazilian situation with the international scenario.

Analysis of Shigella flexneri Resistance, Biofilm Formation, and Transcriptional Profile in Response to Bile Salts

The Shigella species cause millions of cases of watery or bloody diarrhea each year, mostly in children in developing countries. While many aspects of Shigella colonic cell invasion are known, crucial gaps in knowledge regarding how the bacteria survive, transit, and regulate gene expression prior to infection remain. In this study, we define mechanisms of resistance to bile salts and build on previous research highlighting induced virulence in Shigella flexneri strain 2457T following exposure to bile salts. Typical growth patterns were observed within the physiological range of bile salts; however, growth was inhibited at higher concentrations. Interestingly, extended periods of exposure to bile salts led to biofilm formation, a conserved phenotype that we observed among members of the Enterobacteriaceae Characterization of S. flexneri 2457T biofilms determined that both bile salts and glucose were required for formation, dispersion was dependent upon bile salts depletion, and recovered bacteria displayed induced adherence to HT-29 cells. RNA-sequencing analysis verified an important bile salt transcriptional profile in S. flexneri 2457T, including induced drug resistance and virulence gene expression. Finally, functional mutagenesis identified the importance of the AcrAB efflux pump and lipopolysaccharide O-antigen synthesis for bile salt resistance. Our data demonstrate that S. flexneri 2457T employs multiple mechanisms to survive exposure to bile salts, which may have important implications for multidrug resistance. Furthermore, our work confirms that bile salts are important physiological signals to activate S. flexneri 2457T virulence. This work provides insights into how exposure to bile likely regulates Shigella survival and virulence during host transit and subsequent colonic infection.

Assessment of biofilm removal capacity of a broad host range bacteriophage JHP against Pseudomonas aeruginosa

Pseudomonas aeruginosa is an efficient biofilm-dwelling microbial pathogen, associated with nosocomial infections. These biofilm-associated infections are resistant to antibiotics and immune defenses, therefore pose major problem against their treatment. This scenario demands alternative therapeutic regimens, and bacteriophage therapy is one among potential strategies for clinical management of multiple drug resistance. In this investigation, the efficacy of a bacteriophage, JHP, is evaluated to eradicate P. aeruginosa biofilms. Growth kinetics of P. aeruginosa biofilm revealed that the highest cell density biofilm (1.5 × 10¹⁶ CFU/mL) was established within the polystyrene microtiter plate at 72 h post inoculation. Pseudomonas aeruginosa biofilms of different ages, treated with JHP (0.6 MOI) for different post-infection durations, reduced biomass from 2 to 4.5 logs (60-90%). JHP treatment before biofilm development reduced the bacterial load up to 9 logs (>95% bacterial load reduction) as compared with untreated control, which highlights its potential to prevent biofilm formation in indwelling medical devices. Combinations of JHP with other phages or antibiotics could be an efficient alternative for P. aeruginosa biofilm removal in clinical and industrial settings.

Biofilm Formation by Clostridium ljungdahlii Is Induced by Sodium Chloride Stress: Experimental Evaluation and Transcriptome Analysis

The acetogen Clostridium ljungdahlii is capable of syngas fermentation and microbial electrosynthesis. Biofilm formation could benefit both these applications, but was not yet reported for C. ljungdahlii. Biofilm formation does not occur under standard growth conditions, but attachment or aggregation could be induced by different stresses. The strongest biofilm formation was observed with the addition of sodium chloride. After 3 days of incubation, the biomass volume attached to a plastic surface was 20 times higher with than without the addition of 200 mM NaCl to the medium. The addition of NaCl also resulted in biofilm formation on glass, graphite and glassy carbon, the latter two being often used electrode materials for microbial electrosynthesis. Biofilms were composed of extracellular proteins, polysaccharides, as well as DNA, while pilus-like appendages were observed with, but not without, the addition of NaCl. A transcriptome analysis comparing planktonic (no NaCl) and biofilm (NaCl addition) cells showed that C. ljungdahlii coped with the salt stress by the upregulation of the general stress response, Na⁺ export and osmoprotectant accumulation. A potential role for poly-N-acetylglucosamines and D-alanine in biofilm formation was found. Flagellar motility was downregulated, while putative type IV pili biosynthesis genes were not expressed. Moreover, the gene expression analysis suggested the involvement of the transcriptional regulators LexA, Spo0A and CcpA in stress response and biofilm formation. This study showed that NaCl addition might be a valuable strategy to induce biofilm formation by C. ljungdahlii, which can improve the efficacy of syngas fermentation and microbial electrosynthesis applications.

Standardization and Classification of In vitro Biofilm Formation by Clinical Isolates of Staphylococcus aureus

BACKGROUND

Staphylococcus aureus is Gram-positive bacterium commonly associated with nosocomial infections. The development of biofilm exhibiting drug resistance especially in foreign body associated infections has enabled the bacterium to draw considerable attention. However, till date, consensus guidelines for in vitro biofilm quantitation and categorization criterion for the bacterial isolates based on biofilm-forming capacity are lacking. Therefore, it was intended to standardize in vitro biofilm formation by clinical isolates of S. aureus and then to classify them on the basis of their biofilm-forming capacity.

MATERIALS AND METHODS

A study was conducted for biofilm quantitation by tissue culture plate (TCP) assay employing 61 strains of S. aureus isolated from clinical samples during May 2015- December 2015 wherein several factors influencing the biofilm formation were optimized. Therefore, it was intended to propose a biofilm classification criteria based on the standard deviation multiples of the control differentiating them into non, low, medium, and high biofilm formers.

RESULTS

Brain-heart infusion broth was found to be more effective in biofilm formation compared to trypticase soy broth. Heat fixation was more effective than chemical fixation. Although, individually, glucose, sucrose, and sodium chloride (NaCl) had no significant effect on biofilm formation, a statistically significant increase in absorbance was observed after using the supplement mix consisting of 222.2 mM glucose, 116.9 mM sucrose, and 1000 mM NaCl (P= 0.037).

CONCLUSIONS

The present study puts forth a standardized in vitro TCP assay for biofilm biomass quantitation and categorization criteria for clinical isolates of S. aureus based on their biofilm-forming capacity. The proposed in vitro technique may be further evaluated for its usefulness in the management of persistent infections caused by the bacterium.

Development of an in vitro Assay, Based on the BioFilm Ring Test®, for Rapid Profiling of Biofilm-Growing Bacteria

Microbial biofilm represents a major virulence factor associated with chronic and recurrent infections. Pathogenic bacteria embedded in biofilms are highly resistant to environmental and chemical agents, including antibiotics and therefore difficult to eradicate. Thus, reliable tests to assess biofilm formation by bacterial strains as well as the impact of chemicals or antibiotics on biofilm formation represent desirable tools for a most effective therapeutic management and microbiological risk control. Current methods to evaluate biofilm formation are usually time-consuming, costly, and hardly applicable in the clinical setting. The aim of the present study was to develop and assess a simple and reliable in vitro procedure for the characterization of biofilm-producing bacterial strains for future clinical applications based on the BioFilm Ring Test® (BRT) technology. The procedure developed for clinical testing (cBRT) can provide an accurate and timely (5 h) measurement of biofilm formation for the most common pathogenic bacteria seen in clinical practice. The results gathered by the cBRT assay were in agreement with the traditional crystal violet (CV) staining test, according to the κ coefficient test (κ = 0.623). However, the cBRT assay showed higher levels of specificity (92.2%) and accuracy (88.1%) as compared to CV. The results indicate that this procedure offers an easy, rapid and robust assay to test microbial biofilm and a promising tool for clinical microbiology.

The Effects of Sugars on the Biofilm Formation of Escherichia coli 185p on Stainless Steel and Polyethylene Terephthalate Surfaces in a Laboratory Model

BACKGROUND

Bacteria utilize various methods in order to live in protection from adverse environmental conditions. One such method involves biofilm formation; however, this formation is dependent on many factors. The type and concentration of substances such as sugars that are present in an environment can be effective facilitators of biofilm formation.

METHODS

First, the physico-chemical properties of the bacteria and the target surface were studied via the MATS and contact angle measurement methods. Additionally, adhesion to different surfaces in the presence of various concentrations of sugars was compared in order to evaluate the effect of these factors on the biofilm formation of Escherichia coli, which represents a major food contaminant.

RESULTS

Results showed that the presence of sugars has no effect on the bacterial growth rate; all three concentrations of sugars were hydrophilic and demonstrated a high affinity toward binding to the surfaces.

CONCLUSIONS

The impact of sugars and other factors on biofilm formation can vary depending on the type of bacteria present.

Effect of glucose on Listeria monocytogenes biofilm formation, and assessment of the biofilm's sanitation tolerance

Listeria monocytogenes is an important cause of human foodborne infections and its ability to form biofilms is a serious concern to the food industry. To reveal the effect of glucose conditions on biofilm formation of L. monocytogenes, 20 strains were investigated under three glucose conditions (0.1, 1.0, and 2.0% w v(-1)) by quantifying the number of cells in the biofilm and observing the biofilm structure after incubation for 24, 72, and 168 h. In addition, the biofilms were examined for their sensitivity to sodium hypochlorite. It was found that high concentrations of glucose reduced the number of viable cells in the biofilms and increased extracellular polymeric substance production. Moreover, biofilms formed at a glucose concentration of 1.0 or 2.0% were more resistant to sodium hypochlorite than those formed at a glucose concentration of 0.1%. This knowledge can be used to help design the most appropriate sanitation strategy.

Recombinant Expression of a Putative Amidase Cloned from the Genome of Listeria monocytogenes that Lyses the Bacterium and its Monolayer in Conjunction with a Protease

Listeria monocytogenes is a Gram-positive, non-spore forming, catalase-positive rod that is a major bacterial food-borne disease agent associated with uncooked meats, including poultry, uncooked vegetables, soft cheeses, and unpasteurized milk. The bacterium may be carried by animals without signs of disease, can replicate at refrigeration temperatures, and is frequently associated with biofilms. There is a need to discover innovative pathogen intervention technologies for this bacterium. Consequently, bioinformatic analyses were used to identify genes encoding lytic protein sequences in the genomes of L. monocytogenes isolates. PCR primers were designed that amplified nucleotide sequences of a putative N-acetylmuramoyl-L-alanine amidase gene from L. monocytogenes strain 4b. The resultant amplification product was cloned into an expression vector, propagated in Escherichia coli Rosetta strains, and the recombinant protein was purified to homogeneity. Gene and protein sequencing confirmed that the predicted and chemically determined amino acid sequence of the recombinant protein designated PlyLM was a putative N-acetylmuramoyl-L-alanine amidase. The recombinant lytic protein was capable of lysing both the parental L. monocytogenes strain as well as other strains of the bacterium in spot and MIC/MIB assays, but was not active against other bacteria beyond the genus. A microtiter plate assay was utilized to assay for the ability of the recombinant lysin protein to potentially aid with digestion of a L. monocytogenes biofilm. Protease or lysozyme digestion alone did not significantly reduce the L. monocytogenes biofilm. Although the recombinant protein alone reduced the biofilm by only 20%, complete digestion of the bacterial monolayer was accomplished in conjunction with a protease.

Complete Genome Sequences of Two Listeria monocytogenes Serovars, 1/2a and 4b, Isolated from Dairy Products in Brazil

Listeria monocytogenes is the foodborne pathogen responsible for a bacterial infection called listeriosis. Here, we present the whole-genome sequences of two L. monocytogenes serovars, 1/2a and 4b, which are considered the most prevalent in food processing plants and listeriosis outbreaks, respectively.

Optimization of culture conditions for Gardnerella vaginalis biofilm formation

Bacterial vaginosis is the leading vaginal disorder in women in reproductive age. Although bacterial vaginosis is related with presence of a biofilm composed predominantly by Gardnerella vaginalis, there has not been a detailed information addressing the environmental conditions that influence the biofilm formation of this bacterial species. Here, we evaluated the influence of some common culture conditions on G. vaginalis biofilm formation, namely inoculum concentration, incubation period, feeding conditions and culture medium composition. Our results showed that culture conditions strongly influenced G. vaginalis biofilm formation and that biofilm formation was enhanced when starting the culture with a higher inoculum, using a fed-batch system and supplementing the growth medium with maltose.

Inhibition of pathogenic and spoilage bacteria by a novel biofilm-forming Lactobacillus isolate: a potential host for the expression of heterologous proteins

BACKGROUND

Bacterial biofilms are a preferred mode of growth for many types of microorganisms in their natural environments. The ability of pathogens to integrate within a biofilm is pivotal to their survival. The possibility of biofilm formation in Lactobacillus communities is also important in various industrial and medical settings. Lactobacilli can eliminate the colonization of different pathogenic microorganisms. Alternatively, new opportunities are now arising with the rapidly expanding potential of lactic acid bacteria biofilms as bio-control agents against food-borne pathogens.

RESULTS

A new isolate Lactobacillus plantarum PA21 could form a strong biofilm in pure culture and in combination with several pathogenic and food-spoilage bacteria such as Salmonella enterica, Bacillus cereus, Pseudomonas fluorescens, and Aeromonas hydrophila. Exposure to Lb. plantarum PA21 significantly reduced the number of P. fluorescens, A. hydrophila and B. cereus cells in the biofilm over 2-, 4- and 6-day time periods. However, despite the reduction in S. enterica cells, this pathogen showed greater resistance in the presence of PA21 developed biofilm, either in the planktonic or biofilm phase. Lb. plantarum PA21 was also found to be able to constitutively express GFP when transformed with the expression vector pMG36e which harbors the gfp gene as a reporter demonstrating that the newly isolated strain can be used as host for genetic engineering.

CONCLUSION

In this study, we evaluate the ability of a new Lactobacillus isolate to form strong biofilm, which would provide the inhibitory effect against several spoilage and pathogenic bacteria. This new isolate has the potential to serve as a safe and effective cell factory for recombinant proteins.

Complete Genome Sequences of vB_LmoS_188 and vB_LmoS_293, Two Bacteriophages with Specificity for Listeria monocytogenes Strains of Serotypes 4b and 4e

Listeria monocytogenes is responsible for the rare disease listeriosis, which is associated with the consumption of contaminated food products. We report here the complete genome sequences of vB_LmoS_188 and vB_LmoS_293, phages isolated from environmental sources and that have host specificity for L. monocytogenes strains of the 4b and 4e serotypes.

The influence of subminimal inhibitory concentrations of benzalkonium chloride on biofilm formation by Listeria monocytogenes

Disinfectants, such as benzalkonium chloride (BAC), are commonly used to control Listeria monocytogenes and other pathogens in food processing plants. Prior studies have demonstrated that the resistance to BAC of L. monocytogenes was associated with the prolonged survival of three strains of molecular serotype 1/2a in an Iberian pork processing plant. Because survival in such environments is related to biofilm formation, we hypothesised that the influence of BAC on the biofilm formation potential of L. monocytogenes might differ between BAC-resistant strains (BAC-R, MIC≥10mg/L) and BAC-sensitive strains (BAC-S, MIC≤2.5mg/L). To evaluate this possibility, three BAC-R strains and eight BAC-S strains, which represented all of the molecular serotype 1/2a strains detected in the sampled plant, were compared. Biofilm production was measured using the crystal violet staining method in 96-well microtitre plates. The BAC-R strains produced significantly (p<0.05) less biofilm than the BAC-S in the absence of BAC, independent of the rate of planktonic growth. In contrast, when the biofilm values were measured in the presence of BAC, one BAC-R strain (S10-1) was able to form biofilm at 5mg/L of BAC, which prevented biofilm formation among the rest of the strains. A genetic determinant of BAC resistance recently described in L. monocytogenes (Tn6188) was detected in S10-1. When a BAC-S strain and its spontaneous mutant BAC-R derivative were compared, resistance to BAC led to biofilm formation at 5mg/L of BAC and to a significant (p<0.05) stimulation of biofilm formation at 1.25mg/L of BAC, which significantly (p<0.05) reduced the biofilm level in the parent BAC-S strain. Our results suggest that the effect of subminimal inhibitory concentrations of BAC on biofilm production by L. monocytogenes might differ between strains with different MICs and even between resistant strains with similar MICs but different genetic determinants of BAC resistance. For BAC-R strains similar to S10-1, subminimal inhibitory BAC may represent an advantage, compensating for the weak biofilm formation level that might be associated with resistance. Biofilm formation in the presence of increased subminimal inhibitory concentrations of the disinfectant may represent an important attribute among certain resistant and persistent strains of L. monocytogenes.