Mixed Culture Of Microorganisms Research Articles

Simultaneous electricity generation and sulfide removal via a dual chamber microbial fuel cell

Microbial fuel cells (MFCs) have recently been used to alter different sources of substrates to produce bioelectricity. MFCs can also be used for wastewater treatment and electricity generation simultaneously. Sulfur compounds such as sulfides commonly exist in wastewater and organic waste. In this study a dual chamber MFC was constructed for power production. Sulfide was used as the electron donor in the anaerobic anode compartment. A mixed culture of microorganisms was used as an active biocatalyst to convert the substrate into electricity. The obtained experimental results illustrated that the MFC can successfully alter sulfide to elementary sulfur while generating power. The initial concentration of sulfide in the anode compartment was 0.4 g l-1 and it was completely removed after 3 days of MFC operation. The influence of oxygen was examined in the cathode chamber and the cell voltage gradually increased during aeration, reaching 480 mV after 1200 s. Hexacyanoferrate was added to the cathodic solution in different concentrations and its effects were investigated. The maximum generated voltage, power and current density were 988.9145 mV, 346.746 mW.m-2, 1285.64 mA.m-2, respectively and they were obtained in the presence of 1.4 g l-1 of mediator.

Phenolic Shock Load in a Submerged Ceramic Membrane Bioreactor for the Degradation of Aqueous Phenol

Laboratory-scale experiments were conducted to study the treatment of phenol contaminated wastewater using submerged ceramic membrane bioreactor (SCMBR) with flat-sheet-type ceramic membrane module. The effects of organic loading rate on the phenol degradation were studied using Polyseed bacteria containing a mixed culture of microorganisms with and without acclimatization. The experimental results showed that the phenol removal efficiency was over 75% at phenol concentrations of 400 mg/l with the COD removal efficiency of greater than 80%. For phenol concentrations of 800 mg/l, the removal efficiency dropped to about 40%, however, the system returned to its previous treatment efficiency once the phenol load was removed. As a result, it was observed that the bacteria survived at the high load phenol concentration of 800 mg/l

Improvement of Water Flooding Efficiency Using Mixed Culture of Microorganisms in Heterogeneous Micro-models

One of the possible methods for reducing unwanted problem of reservoir heterogeneity in flooding processes is the application of microbial slugs for bioplugging of high permeable zones. Objective of this research is visualization of potential of mixed-culture microorganisms as bioplugging agents for improving the efficiency of water flooding in two different heterogeneous micro-models: a glass bead and an inventive etched one. In both micro-models, water flooding removed the oil in high permeable zones while it had little effect on the oil in low permeable areas. Injection of a slug, containing a mixed culture of oil degrading microorganisms was done in glass bead micro-model. The etched micro-model was used to investigate the effect of in situ microbial growth and subsequent water flooding. In both cases the microbial treatment improved the oil recovery in comparison with water flooding in low permeable zone considerably. The results of this study showed that mixed-culture microorganisms have an acceptable potential for bioplugging of high permeable layers of heterogeneous porous media.

Biomass Conversion to Hydrocarbon Fuels Using the MixAlcoTMProcess

The MixAlcoTM process converts biomass to hydrocarbons (e.g., gasoline) using the following generic steps: pretreatment, fermentation, descumming, dewatering, thermal ketonization, distillation, hydrogenation, oligomerization and saturation. This study describes the production of bio-gasoline from chicken manure and shredded office paper, both desirable feedstocks that do not require pretreatment. Using a mixed culture of microorganisms derived from marine soil, the biomass was fermented to produce a dilute aqueous solution of carboxylate salts, which were subsequently descummed and dried. The dry salts were thermally converted to raw ketones, which were distilled to remove impurities. Using Raney nickel catalyst, the distilled ketones were hydrogenated to mixed secondary alcohols ranging from C3 to C12. Using zeolite HZSM-5 catalyst, these alcohols were oligomerized to hydrocarbons in a plug -flow reactor. Finally, these unsaturated hydrocarbons were hydrogenated to produce a mixture of hydrocarbons that can be blended into commercial gasoline.

Comparison of three screening methods to select mixed-microbial inoculum for mixed-acid fermentations

Using a mixed culture of microorganisms, the carboxylate platform converts biomass into hydrocarbons and chemicals. To develop a method that identifies the highest performing inoculum for carboxylate fermentations, five bacterial communities were screened and ranked by three fermentation performance tests: (1) 30-day batch screen, (2) 28-day continuum particle distribution model (CPDM), and (3) 5-month continuous countercurrent fermentation trains. To screen numerous inocula sources, these tests were used sequentially in an aseptic environment. For the batch-fermentation screen, Inoculum 1 achieved the highest conversion. For the CPDM evaluation, the operating map for Inoculum 1 had the highest performance. For the continuous countercurrent fermentation, the train resulting from Inoculum 1 was among the best performers. This study suggests that the three screens are a useful and predictive method for choosing optimal inocula sources. The bacterial community with optimal performance in these three screens could be considered for use in commercial-scale fermentations.

Effect of different substrate on performance of microbial fuel cell

Microbial fuel cells are new technology for the production of bioelectricity. The electrons liberated by microorganisms during consumption of organic substrate are transferred from the anode to the cathode linked by an external circuit and produce electricity under anaerobic conditions. In this study, palm oil mill effluent anaerobic sludge was used as active biocatalyst for bioelectricity generation. Several substrates such as glucose, fructose, sucrose and molasses were examined as electron donors by mixed culture of microorganisms in an anaerobic anode compartment. The maximum open circuit voltage achieved was 460 mV and the highest power and current density were 55.25 mW/m 2 and 208.55 mA/m 2 , respectively.

Detection of antibacterial-like activity on a silica surface: fluoroquinolones and their environmental metabolites

Background, scope, and aims Antibacterial fluoroquinolones (FQs) are third-generation antibiotics that are commonly used as therapeutic treatments of respiratory and urinary tract infections. They are used far less in intensively farmed animal production systems, though their use may be permitted in the veterinary treatments of flocks or in medicated feeds. When used, only a fraction of ingested parent FQ actually reaches the in vivo target site of infection, while the remainder is excreted as the parent FQ and its metabolized products. In many species’ metabolism, enrofloxacin (EF) is converted into ciprofloxacin (CF) while both FQs are classified as parent FQs in human treatments. It is therefore likely that both FQs and their metabolic products will contribute to a common pool of metabolites in biological wastes. Wastes from intensive farming practices are either directly applied to agricultural land without treatment or may be temporarily stored prior to disposal. However, human waste is treated in sewage treatment plants (STPs) where it is converted into biosolids. In the storage or treatment process of STPs, FQs and their in vivo metabolites are further converted into other environmental metabolites (FQEMs) by ex vivo physicochemical processes that act and interact to produce complex mixtures of FQEMs, some of which have antibacterial-like activities. Biosolids are then often applied to agricultural land as a fertilizer amendment where FQs and FQEMs can be further converted into additional FQEMs by soil processes. It is therefore likely that FQ-contaminated biowaste-treated soils will contain complex mixtures of FQEMs, some of which may have antibacterial-like activities that may be expressed on bacteria endemic to the receiving agricultural soil environment. Concern has arisen in the scientific and in the general community that repeated use of FQ-contaminated biowaste as fertilizer amendments of nutrient-impoverished agricultural land may create a selective environment in which FQ-resistant bacteria might grow. The likelihood of this happening will depend, to some extent, on whether bioactive FQEMs are first synthesized from the parent FQs by the action and interaction of in vivo and ex vivo processes producing bioactive FQEMs in biowastes and biosolids. The postulated creation of a selective environment will also depend, in part, on whether such bioactive FQEMs are biologically available to bacteria, which may, in turn, be influenced by soil type, amendment regime, and the persistence of the bioactive FQEMs. Additionally, soil bacteria and soil processes may be affected in different ways or extents by bioactive FQEMs that could possibly act additively or synergistically at ecological targets in these non-target bacteria. This is an important consideration, since, while parent FQs have well-defined ecological targets (DNA gyrase and topoisomerase IV) and modes of bactericidal action, the FQEMs and their possible modes of action on the many different species of soil bacteria is less well studied. It is therefore understandable that there is a lack of conclusive evidence directly attributing biosolid usage to any increase in FQ-resistant bacteria detected in biowaste-amended agricultural soil. However, a lack of evidence may simply imply that a causal relationship between biosolid usage programs and any detection of low levels of FQ-resistant bacteria in soils has yet to be established, rather than an assumption of no relationship whatsoever. Based on results presented in this paper, the precautionary principle should be applied in the usage of FQ-contaminated biosolids as fertilizer amendments of agricultural land. The aim of this research was to test whether any bioactive FQEMs of EF could be synthesized by aerobic fermentation processes using Mycobacterium gilvum (American Tissue Culture Collection) and a mixed culture of microorganisms derived from an agricultural soil. High-performance thin-layer chromatography (HPTLC) and bioautography were tested as screening techniques in the detection and analysis of bioactive FQEMs.

Environmental metabolites of fluoroquinolones: synthesis, fractionation and toxicological assessment of some biologically active metabolites of ciprofloxacin

Biowastes produced by humans and animals are routinely disposed of on land, and concern is now growing that such practices provide a pathway for fluoroquinolone (FQs) antibacterial agents and their environmental metabolites (FQEMs) to contaminate the terrestrial environment. The focus of concern is that FQs and FQEMs may accumulate in amended soils to then adversely impact on the terrestrial environment. One postulated impact is the development of a selective environment in which FQ-resistant bacteria may grow. To find evidence in support of an accumulation of antibacterial-like activity, it was first necessary to establish whether any biologically active FQEMs could be synthesized by physicochemical factors that are normally present in the environment. However, many FQEMs are not commercially available to be used as standards in such studies. FQEMs were therefore synthesized using well-defined processes. They were subsequently analyzed using spectroscopy (UV-vis) and high performance liquid chromatography with mass spectral detection. The antibacterial-like activities of fractionated FQEMs were then assessed in novel bacterial growth inhibition bioassays, and results were compared to those obtained from instrumental analyses. Parent FQs were either exposed to sunlight or were synthesized using defined aerobic microbial (Mycobacterium gilvum or a mixed culture derived from an agricultural soil) fermentation processes. Mixtures of FQEMs derived from photo- and (intracellular) microbial processes were isolated by preparative chromatography and centrifugation techniques, respectively. Mixtures were subsequently fractionated using analytical high-performance thin layer chromatography (HPTLC), and excised analytes were tested in bioautography assays for their antibacterial-like activities. Two bacteria, Escherichia coli (E. coli) and Azospirillum brasilense (A. brasilense) were used as reporter organisms in testing FQ standards and any subtle differences between biologically active FQEMs of ciprofloxacin (CF). FQEMs produced in the photo-synthetic process had UV-vis profiles that were indistinguishable from the parent FQs, and yet mass spectral data revealed the presence of N-formylciprofloxacin (FCF). In contrast, the UV-vis profiles of FQEMs synthesized by M. gilvum and a mixed culture of microorganisms had UV-vis profiles that were similar to one another and markedly different to the parent fluoroquinolones. Mass spectral studies confirmed the presence of FCF and N-acetylciprofloxacin in both microbial ferments. In addition, a photo-FQEM (Cp 6), three M. gilvum FQEMs (Cm 5, Cm 8, and Cm 10) and a mixed culture FQEM (Cs 6) of CF and many other FQEMs of CF, norfloxacin (NF), and enrofloxacin (EF) were fractionated using HPTLC, although their identities have yet to be confirmed. Differences between bioautography results were obtained when E. coli or A. brasilense were used as reporter organisms. Parent FQs (CF and EF) and the FQEMs of CF (Cp 6, Cm 8, and Cs 6) displayed antibacterial-like activity when using E. coli as the reporter organism. In contrast, A. brasilense was insensitive to parent CF and sensitive to EF and all tested FQEMs of CF. Results are consistent with photo- and microbial processes modifying CF in different ways, with the latter changing the UV-vis chromophores. It can be inferred that a lack of detection of analytes (especially photo-FQEMs) when using UV-vis does not necessarily indicate an absence of analyte. Additionally, similarities between the UV-vis profiles of FQEMs extracted from the (monoculture) M. gilvum and the mixed culture microbial aerobic ferments are consistent with similar processes operating in both ferments. Results of HPTLC and bioautography studies revealed that mixtures of (photo- and microbial) FQEMs could be fractionated into individual components. Bioactive FQEMs of ciprofloxacin, as a representative FQ, can be synthesized by photo- and microbial processes, and their detection required the use of both instrumental and bioautography analytical techniques. It is likely that such FQEMs will also be present on agricultural land that has been repeatedly amended with FQ-contaminated biosolids. The use of instrumental analytical techniques alone and especially photometric detection techniques will underestimate antibacterial-like activities of FQEMs. Moreover, the extraction technique(s) and the selected toxicological endpoint(s) require careful consideration when assessing bioactivity. It is therefore recommended that instrumental analytical techniques and several bioautography assays be performed concurrently, and bioautography assays should use a variety of reporter organisms. Two types of bacterial growth bioassays are recommended in any assessment of antibacterial-like activity derived from CF (and possibly from other FQs). A standardized E. coli bioassay should be used as a general screening procedure to facilitate intra- and inter-laboratory exchange of data. Additionally, soil-specific (region-specific) growth inhibition bioassays should be undertaken using several species of endemic soil bacteria. It is likely that the two sets of data will be useful in future risk assessment processes.

Evaluation of the bioremoval of Cr(VI) and TOC in biofilters under continuous operation using response surface methodology

In the present study, the bioremoval of Cr(VI) and the removal of total organic carbon (TOC) were achieved with a system composed by an anaerobic filter and a submerged biofilter with intermittent aeration using a mixed culture of microorganisms originating from contaminated sludge. In the aforementioned biofilters, the concentrations of chromium, carbon, and nitrogen were optimized according to response surface methodology. The initial concentration of Cr(VI) was 137.35mgl(-1), and a bioremoval of 85.23% was attained. The optimal conditions for the removal of TOC were 4 to 8gl(-1) of sodium acetate,>0.8gl(-1) of ammonium chloride and 60 to 100mgl(-1) of Cr(VI). The results revealed that ammonium chloride had the strongest effect on the TOC removal, and 120mgl(-1) of Cr(VI) could be removed after 156h of operation. Moreover, 100% of the Cr(VI) and the total chromium content of the aerobic reactor output were removed, and TOC removals of 80 and 87% were attained after operating the anaerobic and aerobic reactors for 130 and 142h, respectively. The concentrations of cells in both reactors remained nearly constant over time. The residence time distribution was obtained to evaluate the flow through the bioreactors.

Biodegradation of beet molasses vinasse by a mixed culture of micro organisms: Effect of aeration conditions and pH control

The effect of aeration conditions and pH control on the progress and efficiency of beet molasses vinasse biodegradation was investigated during four batch processes at 38°C with the mixed microbial culture composed of Bifidobacterium, Lactobacillus, Lactococcus, Streptococcus, Bacillus, Rhodopseudomonas, and Saccharomyces. The four processes were carried out in a shake flask with no pH control, an aerobic bioreactor without mixing with no pH control, and a stirred-tank reactor (STR) with aeration with and without pH control, respectively. All experiments were started with an initial pH 8.0. The highest efficiency of biodegradation was achieved through the processes conducted in the STR, where betaine (an organic pollutant occurring in beet molasses in very large quantities) was completely degraded by the microorganisms. The process with no pH control carried out in the STR produced the highest reduction in the following pollution measures: organic matter expressed as chemical oxygen demand determined by the dichromatic method + theoretical COD of betaine (COD sum, 85.5%), total organic carbon (TOC, 78.8%) and five-day biological oxygen demand (BOD 5, 98.6%). The process conditions applied in the shake flask experiments, as well as those used in the aerobic bioreactor without mixing, failed to provide complete betaine assimilation. As a consequence, reduction in COD sum, TOC and BOD 5 was approximately half that obtained with STR.

Polyethylene Destruction by the Mixed Culture of Microorganisms

Polyethylene film destruction by mixed culture of microorganisms was studied. The initial culture was adapted for growth on the media with ion exchange resin. During cultivation of the microorganisms on the mineral media with a polyethylene as the only source of carbon and energy the increase of film roughness and decrease of amorphous fraction and crystal size was detected. Using atomic-force microscopy and scanning electron microscopy it was found the film reduce to the crystals of micron size and unilamellar crystal fragments. The film relief change and crystal size decrease was detected after contact with oil-oxidizing bacteria and after composting with plant residuals with redworm Eisenia foetida.

Use of Bacteria that Produce Linamarase in Cassava Processing for Gari

Assessment of the effectiveness of microorganisms involved in gari fermentation in the degradation of cyanogenic glucosides was studied. Alcaligenes faecalis, Lactobacillus plantarum, Bacillus subtilis and Leuconostoc cremoris were isolated from cassava waste water while standard analytical methods were used to monitor the fermentation for total cyanogens. The linamarase activity observed for the isolates ranged from 0.0416 imol/ml/nmol (Lactobacillus plantarum) to 0.2618 imol/ml/nmol (Leuconostoc cremoris). Bacillus subtilis did not express any activity for the enzyme. Values obtained for total cyanogenic glucosides differed significantly at 24 h and 48 h (p 0.05). At 24 h, the cyanohydrins did not differ among the fermenting organisms. However, the mixed culture had the least amounts from 48 h to 96 h. Free cyanide produced by each organism did not differ at 24 h and 48 h but visible differences were observed at 72 h and 96 h. B. subtilis had the highest amount at 96 h (38 mg HCN/ kg) while the mixed culture had the least amount (26 mg HCN/kg). There was no significant difference in the rate of degradation of the linamarin by the microorganisms. pH decreased with increase in time of fermentation with the mash fermented by the mixed culture having the lowest pH of 4.19 at the end of fermentation. Titrable acidity increased with increase in fermentation time with the highest value of 1.05% at 96 h of fermentation produced by the mixed culture of microorganisms. These results show that although these isolates produced linamarase, their activities did not significantly affect the breakdown of the cyanogens.

Experimental Study on Color Removal from Textile Industry Wastewater Using the Rotating Biological Contactor

Textile industry wastewater treatment is a cause for concern due to its intense color and high COD, which makes it difficult to treat it by conventional treatment options. Also, many of the dyes used in the industry fall under the category of hazardous wastes and are known to be toxic to microorganisms and aquatic life forms. An experimental study was conducted to observe the performance of Rotating Biological Contactor (RBC) system for color and COD removal from textile industry wastewaters. The RBC system could achieve up to 90±5% removal of color and 95±3% removal of COD in all four stages of RBC under optimum operating conditions. The first two stages of the RBC showed up to 75±2% of color and 85±2% of COD removal. The biofilm study concluded that a mixed culture of microorganisms (both bacteria as well as fungi) was responsible for the treatment of textile industry wastewater and the recovery of the RBC system could be very fast in case of any shock-loading occurred.

Effect of thermochemical pretreatment on sewage sludge and its impact on carboxylic acids production

This paper investigates the potential of converting sewage sludge into a useful product, namely carboxylic acids. To potentially enhance acid yields, the effect of pretreatment using 0.3 g lime/g dry biomass and water at 100 °C for 10–240 min was studied. The pretreated sludges were anaerobically fermented to mixed-acids using a mixed culture of microorganisms; methanogens were suppressed using iodoform. Batch fermentations were performed at 55 °C using ammonium bicarbonate buffer. The first batch experiments compared treated and untreated sludge as the only substrate. The second batch experiments used a mixture of sludge plus lime-treated bagasse (20:80 by weight). Analysis of liquor shows that the pretreatment were effective in solubilizing constituent compounds of sewage sludge. Nitrogen content and carboxylic acids increased with increasing pretreatment time. However, the soluble sugars peaked at 60 min, and then decreased with longer pretreatment time, showing that the solubilised sugars were undergoing intermolecular reactions, such as Maillard reactions. Fermentation experiments were a good indicator of the biodegradability of the pretreated sludges. Results clearly showed that lime-treating sludge, using even the minimum pretreatment time (10 min), negatively impacted acid production. The likely causes of this observation are attributed to the production of recalcitrant complexes and toxic compounds. Batch fermentation of untreated sludge yielded 0.34 g total acids/g VS fed, whereas sludge with 240-min lime pretreatment yielded only 0.20 g total acids/g VS fed. Co-fermentation of untreated sludge with pretreated bagasse gave a yield of 0.23 g total acids/g VS fed.

Detection and Identification of Microorganisms in Mixed Cultures by Nanoparticle-Induced NanoSPR Enhanced FTIR Spectroscopy and Chemometrics

Routine identification of pathogenic microorganisms predominantly based on nutritional and biochemical tests is a time-consuming process, but delay may lead to fatal consequences at times. In this work, nanoparticle-induced nanoSPR enhanced IR spectroscopy was used in conjunction with a background elimination data processing algorithm to directly identify microorganisms in mixed cultures. It was demonstrated that the microbial composition of mixtures of different E. coli strains could be accurately identified using our method, at a concentration level of 103 CFU mL -1 . The procedure was also applied to determine the presence or absence of pathogenic microorganisms in a simple but real food matrix (apple juice). Results indicated that microorganisms in a cocktail of up to eight different species suspended in an apple juice matrix could be accurately identified for its presence or absence.

Characterizations of Environmental Factors in Conjugated Linoleic Acid Production by Mixed Rumen Bacteria

Conjugated linoleic acid (CLA) production by rumen bacteria is closely related to biohydrogenation of linoleic acid (LA) and affected by various environmental factors. Ruminal biohydrogenation and isomerization were characterized in view of incubation conditions using a mixed culture of microorganisms obtained from surgically prepared cows. Free-floating bacteria (FFB) produced more CLA than particle-attached bacteria (PAB). Some major factors affecting the ruminal environment such as diet, concentrations of fat substrates, incubation time, pre-incubation, and the presence of glucose in the medium were found to be important determinants for the ruminal production of CLA and in a close relationship with biohydrogenation. The mixed bacterial culture, which was pre-exposed to LA, produced more CLA than an unexposed control in a medium containing 30% rumen fluid. The rate of conversion of fat substrate (LA) to hydrogenated products (trans-C18:1, C18:0) was negatively correlated with the initial LA concentration. Overall, the present study showed that CLA accumulation can be increased by modification of diet-induced environmental conditions, which affect changes in ruminal microflora.

Synergic degradation of phenanthrene by consortia of newly isolated bacterial strains

Three different bacteria capable of degrading phenanthrene were isolated from sludge of a pulp wastewater treatment plant and identified as Acinetobacter baumannii, Klebsiella oxytoca, and Stenotrophomonas maltophilia. Phenanthrene degradation efficiencies by different combinations (consortia) of these bacteria were investigated and their population dynamics during phenanthrene degradation were monitored using capillary electrophoresis-based single-strand conformation polymorphism (CE-SSCP). When a single microorganism was used, phenanthrene degradation efficiency was very low (48.0, 11.0, and 9.0% for A. baumannii, K. oxytoca, and S. maltophilia respectively, after 360 h cultivation). All consortia that included S. maltophilia degraded ∼80.0% of phenanthrene and reduced lag time to 48 h compared to the 168 h of pure A. baumannii culture. CE-SSCP analysis showed that S. maltophilia was the predominant species during phenanthrene degradation in the mixed culture. The results indicate that mixed cultures of microorganisms may effectively degrade target chemicals, even if the microorganisms show low degradation activity in pure culture.

Kinetics of growth and multi substrate degradation by an indigenous mixed microbial culture isolated from a wastewater treatment plant in Guwahati, India

An indigenous mixed culture of microorganisms, isolated from a sewage treatment plant, was investigated for its potential to simultaneously degrade phenol and m-cresol during its growth in batch shake flasks. 2(2) full factorial designs with the two substrates as the factors, at two different levels and two different initial concentration ranges, were employed to carry out the biodegradation experiments. For complete utilisation of phenol and m-cresol, the culture took a minimum duration of 21 hrs at their low concentration of 100 mg/L each, and a maximum duration of 187 hrs at high concentration of 600 mg/L each in the multisubstrate system. The biodegradation results also showed that the presence of phenol in low concentration range (100-300 mg/L did not inhibit m-cresol biodegradation; on the other hand, presence of m-cresol inhibited phenol biodegradation by the culture. Moreover, irrespective of the concentrations used, phenol was degraded preferentially and earlier than m-cresol. During the culture growth, a lag phase was observed above a combined concentration of 500 mg/L i.e., 200 mg/L m-cresol and 300 mg/L of phenol and above). Statistical analysis of the specific growth rate of the culture in the multisubstrate system was also performed in the form of ANOVA and Student 't' test, which gave good interpretation in terms of main and interaction effects of the substrates.

Feasibility of m-cresol degradation using an indigenous mixed microbial culture with glucose as co-substrate

An indigenous mixed culture of microorganisms, isolated from the soil of a gasoline filling station, was used in degrading m-cresol in presence of glucose as an alternative carbon source. Initial glucose concentration was kept at either 250 or 500 mg l−1, to initiate and support necessary culture growth, and that of m-cresol was varied between 50 and 400 mg l−1. A maximum total biomass yield value of 0.925 g g−1 was obtained at 100 mg l−1m-cresol and 500 mg l−1 glucose initial concentrations in the media. Variation in the experimentally observed specific growth rate revealed that m-cresol initial concentrations, above 100 mg l−1, inhibited the culture growth irrespective of the glucose concentrations used in the study. Complete degradation of m-cresol was observed within a time period of 18–26 h depending upon the initial concentrations of m-cresol and glucose in the media; on the other hand, glucose utilization was quick and preceded m-cresol degradation. A sum kinetics model was used to describe the variation in the culture specific growth rate, which gave a high coefficient of determination (R2) value >0.98. From the interaction parameter values obtained by solving this model, the inhibitory effect of glucose on m-cresol degradation by the culture was found to be more pronounced compared to the effect of m-cresol on glucose utilization. This study showed good potential of the indigenous mixed culture in degrading m-cresol when it is provided with a simple alternative carbon source, such as glucose, for supporting its growth.

Effect of bacterial interactions on the spoilage of cold-smoked salmon

Cold-smoked salmon is a lightly preserved fish product in which a mixed microbial flora develops during storage and where the interactive behaviour of micro-organisms may contribute to their growth and spoilage activity. The aim of this study was to assess the effect of the bacterial interactions between the main species contaminating the cold-smoked salmon on bacterial growth, chemical and sensory changes, and spoilage. First, Carnobacterium piscicola, Photobacterium phosphoreum, Lactobacillus sakei, Vibrio sp., Brochothrix thermosphacta and Serratia liquefaciens-like were inoculated as pure cultures on sterile cold-smoked salmon. All bacterial species grew well; Vibrio sp. was the fastest and L. sakei strains developed very rapidly as well with a high maximum cell density on cold-smoked salmon blocks (up to 10 9 cfu g − 1 after 10 days at 8 °C). Based on sensory analysis, Vibrio sp. was identified as non-spoilage bacteria, C. piscicola as very lightly and B. thermosphacta as lightly spoiling. L. sakei and S. liquefaciens-like were found to be the most spoiling bacteria. Secondly, C. piscicola and L. sakei, two species frequently occurring in the lactic flora of the product, were inoculated together and each of them in mixed cultures with respectively P. phosphoreum, Vibrio sp., B. thermosphacta, and S. liquefaciens-like. The growth of L. sakei was shown to strongly inhibit most of the co-inoculated strains i.e. P. phosphoreum, B. thermosphacta, S. liquefaciens-like and, to a lesser extent, Vibrio sp. The growth of C. piscicola seemed to be enhanced with B. thermosphacta and to develop earlier with P. phosphoreum and Vibrio sp. Conversely, S. liquefaciens-like and P. phosphoreum were weakly inhibited by C. piscicola. The main observation resulting from the sensory evaluation was the delay in the appearance of the spoilage characteristics in the mixed cultures with L. sakei, in particular L. sakei/ S. liquefaciens-like. On the other hand, the spoilage activity of the non-spoiler strains Vibrio sp. or the moderate spoilage strains B. thermosphacta and C. piscicola was increased when they were associated together. It is concluded that the spoilage behaviour of micro-organisms in mixed culture is significantly different from pure culture and explain the difficulty to find robust quality indices for this product.