Electroactive biofilms of sulphate reducing bacteria

Biofilms formed from a pure strain of Desulfovibrio desulfuricans 27774 on stainless steel and graphite polarised surfaces were studied. The polarisation conditions applied were −0.4 V vs. SCE for different times. A cathodic current related with the biofilms growth was observed with a maximum intensity of −270 mA m⁻² that remained stable for several days using graphite electrodes. These sulphate reducing bacteria biofilms present electrocatalytic activity towards hydrogen and oxygen reduction reactions. Electrode polarisation has a selective effect on the catalytic activity. The biofilms were also observed by scanning electronic microscopy revealing the formation of homogeneous films on the surfaces.     

Methodological aspects of the induction of biofilms for remediation of the visual impact generated by quartz mining

The opening up of mine faces causes severe alteration to the environment that must be remedied. In the case of quartz mines, the bright white colour of the mineral gives rise to a significant impact that is visible for long distances. New techniques to correct this visual impact, based on the induction of biofilms on the faces, have recently been developed in an attempt to imitate natural effects that mask the impact. Although techniques have been applied successfully in the laboratory, they must be optimized to ensure their success in the field. In the present study, the results of a randomized factorial experiment indicated that although the biofilms adhere to the faces naturally, without the use of adhesives, continued development of the film depends to a large extent on the input of nutrients. Water must also be provided with nutritional supplement to maintain the required level of moisture.     

冷却猪肉及托盘表面细菌生物被膜分析和肉源荧光假单胞菌的鉴定与被膜研究

为研究冷却猪肉及其接触面中细菌生物被膜能力并探讨肉类特定腐败菌的生物被膜特征,分析了腐败的冷却肉和销售托盘表面的细菌生物被膜形成能力;利用形态学观察、16S r DNA分析和VITEK2微生物鉴定系统鉴定了冷却肉中一株生物被膜能力较强的荧光假单胞菌(Pseudomonas fluorescens);测定了该菌在培养基和猪肉浸提液中的生物被膜能力;用激光共聚焦和扫描电子显微镜观测了其生物被膜的结构特征。结果发现:冷却肉中及其销售托盘表面的细菌能够形成生物被膜的比例较高,37%和45%的细菌具有较强生物被膜形成能力。肉源性荧光假单胞菌能够在培养基和肉液中形成大量生物被膜,具有较强的生物被膜能力。显微成像结果表明荧光假单胞菌在培养6 h后有明显黏附,18 h后多糖分泌增多、菌体堆积并形成具有生物功能的立体生物被膜。这些特点可能有利于荧光假单胞菌在冷却肉表面黏附并成为优势腐败菌。     

Bacterial biofilms in food processing environments: a review of recent developments in chemical and biological control

Biofilms are immobile communities of micro‐organisms attached to any surface, such as stainless steel or a food matrix surface or on packaging material. They may be composed of a single species, but more generally, in the natural environment, they consist of mixed species together with an extracellular matrix. Biofilms provide a common mechanism of persistence for a number of bacterial species especially in food processing environments, and therefore, prevention of biofilm formation and the removal of preformed biofilms are an important issue for the food industry. This article reviews the current understanding on the formation of biofilms and recent developments in biological and chemical methods for prevention and removal.     

Technological and safety properties of lactic acid bacteria isolated from Spanish dry-cured sausages

Technological and safety-related properties were analyzed in lactic acid bacteria isolated from Spanish dry-cured sausages in order to select them as starter cultures. In relation to technological properties, all the strains showed significative nitrate reductase activity; Lactobacillus plantarum, Lactobacillus paracasei and 52% of the Enterococcus faecium strains showed lipolytic activity and only Lactobacillus sakei strains (43%) were able to form biofilms. Related to safety aspects, E. faecium strains were the most resistant to antibiotics, whereas, L. sakei strains were the most sensitive. In relation to virulence factors, in the E. faecium strains analyzed, only the presence of efaA gene was detected. The analysis of biogenic amine production showed that most E. faecium strains and L. sakei Al-142 produced tyramine. In conclusion, L. paracasei Al-128 and L. sakei Al-143 strains possess the best properties to be selected as adequate and safe meat starter cultures.     

Drying Kinetics of Biofilms Obtained from Chestnut Starch and Carrageenan with and without Glycerol

Drying kinetics of biofilms from chestnut starch and hybrid carrageenan were experimentally determined and modelled. The biofilms were obtained by mixing the respective solutions (4.0% w/w) of both substances in the proportion 80/20 of chestnut starch/hybrid carrageenan. Glycerol (at 5.0% w/w) was added to some biofilms. Biofilms were prepared using a casting method. Drying kinetics were performed at 30, 40, and 50°C for forced air convection (1.8 ± 0.1 m/s) and 30 and 50°C for natural convection. Constant drying rate period, above critical moisture content (5.4 and 4.2 kg water/kg d.b. without and with glycerol, respectively) allowed the evaluation of the heat transfer coefficients. The water diffusion coefficients were determined in the falling rate period assuming a semi-infinite slab with variable thickness. Water sorption isotherms of biofilms at 25°C were determined and fitted with the GAB model. Mechanical properties of different biofilms showed no significant differences for Young's modulus values (44.0 ± 4.6 MPa). Nevertheless, films prepared at 30°C under natural drying showed the lowest values of tensile strength and elongation (32.2 ± 4.9 MPa and 0.85 ± 0.11%).     

Investigation and Modeling of Growth,Structure and Oxygen Penetration in Particle Supported Biofilms

Particle supported biofilms have been investigated with respect to biofilm formation, substrate transport and utilization. The investigated autotrophic and heterotrophic biofilms were cultivated in airlift suspension reactors. CLSM was used to describe the biofilm structure by recording volumes of bacteria and EPS glycoconjugates. Additionally, the microelectrode technique was used to measure transport and substrate utilization in the biofilm system. The experimental results on the microscopic scale were used to improve a mathematical model for biofilm growth. The oxygen profiles measured in the particle supported biofilms and the data from CLSM were used to optimize the model parameters.     

Effect of salinity and inorganic nitrogen concentrations on nitrification and denitrification rates in intertidal sediments and rocky biofilms of the Douro River estuary, Portugal

The regulatory effects of salinity and inorganic nitrogen compounds on nitrification and denitrification were studied in intertidal sandy sediments and rocky biofilms in the Douro River estuary, Portugal, over a 12-month period. Nitrification and denitrification rates were measured in slurries of field samples and enrichment experiments using the difluoromethane and the acetylene inhibition techniques, respectively. Salinity did not regulate denitrification in either environment, suggesting that halotolerant bacteria dominated the denitrifier communities. However, nitrification rates were stimulated when salinity increased from 0 to 15 practical salinity units. NO3- addition experiments revealed that NO3- availability stimulates denitrification rates in sandy sediments, but not in rocky biofilms; however, in rocky biofilms a positive and linear relationship was observed between denitrification rates and water column NO3- concentrations (r=0.92) during the monthly surveys. The N2O:N2 ratios increased rapidly when NO3- increased from 63 to 363 microM; however, results from monthly surveys showed that environmental parameters other than NO3- availability may be important in controlling the variation in N2O production via denitrification. Ammonium additions to sandy sediments stimulated nitrification rates by 35% for the 20 microM NH4+ addition, but NH4+ appeared to inhibit nitrification at high concentration addition (200 microM NH4+). In contrast, rocky biofilm nitrification was stimulated by 65% when 200 microM NH4+ was added.     

Effect of neutral electrolyzed water on lux-tagged Listeria monocytogenes EGDe biofilms adhered to stainless steel and visualization with destructive and non-destructive microscopy techniques

The presence of Listeria monocytogenes in food processing environment is a risk of food contamination by persistent cells due to their ability to attach to stainless steel and other surfaces. We aimed to study biofilms formation of lux-tagged L. monocytogenes EGDe on stainless steel surfaces and their control using neutral electrolyzed water (NEW), where biofilms development was monitored using destructive and non-destructive microscopy techniques. The development of biofilms was monitored for 5 days on stainless steel chips. We used two sources of NEW, commercial (NEW-1) and from a prototype (NEW-2) for treatments of free and biofilm L. monocytogenes EGDe cells. Complete inhibition of L. monocytogenes EGDe free cells was observed after 1 min contact time for both NEW sources, but NEW-1 concentration used (9 mg/L total available chlorine, TAC) was 1.8 times higher. Cells within biofilms were more resistant to NEW compared to planktonic cells. Same concentration of both NEW sources (70 mg/L TAC) exhibited complete inhibition of biofilm cells after 3 min contact time. However, using a sub-lethal dose of 40 mg/L TAC, NEW-2 reduced about 2 log CFU/cm² biofilm cells while NEW-1 inhibited 0.3 log CFU/cm² only. Biofilms formation and antagonistic effect of NEW could be visualized by epifluorescence and scanning electron microscopy, revealing significant biofilms structure. The disinfectant effect of NEW may be attributed to the combined antimicrobial effect of available chlorine and high ORP exhibited by its oxidizing compounds. NEW does not promote metal equipment corrosion due to its neutral pH, and is also environmentally friendly.     

Evaluation of optical microscopy efficacy in evaluating cassava starch biofilms microstructure

With the aim to evaluate optical microscopy efficacy as an imaging tool for evaluating bio-based films microstructure, plasticizing sugars (sucrose and inverted sugar), other additives (gelatin, soybean oil, sodium phosphate, and propylene glycol), and pH variation effect on cassava starch biofilms crystallization and phase separation during storage were investigated. Material crystallization, when using sucrose as additive, was observed through non-polarized and confirmed with polarized light microscopy. Scanning electron microscopy (SEM) and X-ray diffraction analysis have reinforced the observed results. Light microscopy also has indicated phase separation when soybean oil was added. Light microscopy analysis may be a fast and simple image tool to indicate crystallization and phase separation during bio-based materials storage.