Growth, CO2 consumption and H2 production of Anabaena variabilis ATCC 29413-U under different irradiances and CO2 concentrations

Aims:  The objective of this study is to develop kinetic models based on batch experiments describing the growth, CO₂ consumption, and H₂ production of Anabaena variabilis ATCC 29413-U^TM as functions of irradiance and CO₂ concentration.
Methods and Results:  A parametric experimental study is performed for irradiances from 1120 to 16100 lux and for initial CO₂ mole fractions from 0·03 to 0·20 in argon at pH 7·0 ± 0·4 with nitrate in the medium. Kinetic models are successfully developed based on the Monod model and on a novel scaling analysis employing the CO₂ consumption half-time as the time scale.
Conclusions:  Monod models predict the growth, CO₂ consumption and O₂ production within 30%. Moreover, the CO₂ consumption half-time is an appropriate time scale for analysing all experimental data. In addition, the optimum initial CO₂ mole fraction is 0·05 for maximum growth and CO₂ consumption rates. Finally, the saturation irradiance is determined to be 5170 lux for CO₂ consumption and growth whereas, the maximum H₂ production rate occurs around 10 000 lux.
Significance and Impact of the Study:  The study presents kinetic models predicting the growth, CO₂ consumption and H₂ production of A. variabilis . The experimental and scaling analysis methods can be generalized to other micro-organisms.     

Models for mineralization kinetics with the variables of substrate concentration and population density.

The rates of mineralization of [14C]benzoate by an induced population of Pseudomonas sp. were measured at initial substrate concentrations ranging from 10 ng/ml to 100 micrograms/ml. Plots of the radioactivity remaining in the culture were fit by nonlinear regression to six kinetic models derived from the Monod equation. These models incorporate only the variables of substrate concentration and cell density. Plots of the mineralization kinetics in cultures containing low, intermediate, and high initial substrate concentrations were well fit by first-order, integrated Monod, and logarithmic kinetics, respectively. Parameters such as maximum specific growth rate, half-saturation constant, and initial population density divided by yield agreed between cultures to within a factor of 3.4. Benzoate mineralization by microorganisms in acclimated sewage was shown to fit logistic (sigmoidal), Monod, and logarithmic kinetics when the compound was added at initial concentrations of 0.1, 1.0, and 10 micrograms/ml, respectively. The mineralization of 10 micrograms of benzoate per ml in sewage also followed logarithmic kinetics in the absence of protozoa. It is concluded that much of the diversity in shapes of mineralization curves is a result of the interactions of substrate concentration and population density. Nonlinear regression with models incorporating these variables is a valuable means for analysis of microbial mineralization kinetics.     

Modelling the growth of Weissella cibaria as a function of fermentation conditions

Aims:  To investigate the effect of pH, water activity ( a _w) and temperature on the growth of Weissella cibaria DBPZ1006, a lactic acid bacterium isolated from sourdoughs.
Methods and Results:  The kinetics of growth of W. cibaria DBPZ1006 was investigated during batch fermentations as a function of pH (4·0–8·0), a _w (0·935–0·994) and temperature (10–45°C) in a rich medium. The growth curve parameters (lag time, growth rate and asymptote) were estimated using the dynamic model of Baranyi and Roberts (1994. A dynamic approach to predicting bacterial growth in food. Int J Food Microbiol 23, 277–294). The effect of pH, a _w and temperature on maximum specific growth rate (μ_max) were estimated by fitting a cardinal model. μ_max under optimal conditions (pH = 6·6, a _w = 0·994, T  = 36·3°C) was estimated to be 0·93 h⁻¹. Minimum and maximum estimated pH and temperature for growth were 3·6 and 8·15, and 9·0°C and 47·8°C, respectively, while minimum a _w was 0·918 (equivalent to 12·2% w/v NaCl).
Conclusions:  Weissella cibaria DBPZ1006 is a fast-growing heterofermentative strain, which could be used in a mixed starter culture for making bread.
Significance and Impact of the Study:  This is the first study reporting the modelling of the growth of W. cibaria , a species that is increasingly being used as a starter in sourdough and vegetable fermentations.     

Kinetic Constants for Aerobic Growth of Microbial Populations Selected with Various Single Compounds and with Municipal Wastes as Substrates

The general applicability of the Monod relationship between the logarithmic growth rate constant and substrate concentration was studied for heterogeneous populations metabolizing a variety of substrates including concentrated municipal sewage. It was found that growth could be described by the Monod equation, mu = mu(m)/k(s) + s. The kinetic "constants" for heterogeneous populations growing on concentrated sewage were comparable to those found with glucose as substrate.     

Survival kinetics of faecal bacterial indicators in spent broiler litter composting

Aims:  The objective of this study was to deduce and analyse equations that best describe the behaviour of faecal bacterial indicators and two decomposition parameters during broiler litter composting. Mathematical models were fitted and the order of rate equations were identified.
Methods and Results:  The Levenburg-Marquardt algorithm was used to fit nonlinear mathematical models to total coliforms (TC), faecal coliforms (FC), Escherichia coli (EC), faecal enteroccocci (FE), organic-C and volatile solids reduction, VS_Red, by the least squares procedure. The rate equations showed that TC, FC and EC reductions were expressed by second-order decay kinetics. FE reduction followed first-order decay. Temperature dependency of decomposition rate was effectively verified by applying empirically derived rate equations.
Conclusions:  The governing mathematical models critically compare the inactivation kinetics of faecal indicators. TC, FC and EC were rapidly destroyed while FE was more resistant. Temperature elevation, organic-C and VS_Red dynamics provide an accurate understanding of composting-induced decomposition of the broiler litter.
Significance and Impact of the Study:  The conservative performance of FE with respect to the other indicators has been established. Hence, FE presents better opportunities to encompass the totality of the composting process in terms of attainment of hygiene efficacy compared with EC.     

Culture conditions affect the chemical composition of the exopolysaccharide synthesized by the fungus Aureobasidium pullulans

Aims:  To identify if culture conditions affect the chemical composition of exopolysaccharide (EPS) produced by Aureobasidium pullulans .
Methods and Results:  In batch airlift and continuously stirred tank (CSTR) reactors the EPS produced with low (0·13 g l⁻¹ N) initial NaNO₃ or (NH₄)₂SO₄ levels contained pullulan, with maltotriose as its major component, similar to that synthesized in the airlift reactor with high (0·78 g l⁻¹ N) initial NaNO₃ levels. EPS produced by CSTR grown cultures with high (NH₄)₂SO₄ levels contained little pullulan, possibly because of a population shift from unicells to mycelium. This chemical difference may explain why total EPS yields did not fall as they did with cultures grown under identical conditions with high NaNO₃ levels, where the pullulan component of the EPS disappeared. EPS synthesized in N-limiting chemostat cultures of A. pullulans changed little with growth rate or N source, being predominantly pullulan consisting of maltotriose units.
Conclusions:  While the EPS chemical composition changed little under N-limiting conditions, high initial medium N levels determined maltotriose content and/or pullulan content possibly by dictating culture morphology.
Significance and Impact of the Study:  These results emphasize the requirement of all studies to determine EPS chemical composition when examining the influence of culture conditions on EPS yields.     

Resistance to chlorine of freshwater bacterial strains

The disinfectant properties of chlorine have been known for centuries but in the last few years water chlorination has attracted some criticism due to its secondary effects and the increased resistance of bacterial strains to chlorine inactivation. In this paper the kinetics of inactivation by chlorine of different Gram-positive and Gram-negative bacterial strains isolated from chlorinated water is studied. The Gram-positive strains were more resistant to chlorine and the behaviour of some of them in the presence of chloramphenicol suggests either the synthesis of unique proteins or aggregation of the bacteria as mechanisms of resistance to inactivation. The concept of K _i, the inactivation rate constant, by comparison with K _s in Michaelis-Menten enzyme kinetics (considering enzymic saturation), or with K _s in Monod growth kinetics (considering limiting rates of transport and metabolism of substrates), may be an interesting parameter to define microbial resistance to disinfectants and toxics.     

Coexistence of nitrifiers, denitrifiers and Anammox bacteria in a sequencing batch biofilm reactor as revealed by PCR-DGGE

Aims:  The bacterial diversity in a sequencing batch biofilm reactor (SBBR) treating landfill leachate was studied to explain the mechanism of nitrogen removal.
Methods and Results:  The total microbial DNA was extracted from samples collected from landfill leachate and biofilm of the reactor with the removal efficiencies of NH₄⁺-N higher than 97% and that of chemical oxygen demand (determined by K₂Cr₂O₇, COD_Cr) higher than 86%. Denaturing gradient gel electrophoresis (DGGE) fingerprints based on total community 16S rRNA genes were analyzed with statistical methods, and excised DNA bands were sequenced. The results of phylogenetic analyses revealed high diversity within the SBBR biofilm community, and DGGE banding patterns showed that the community structure in the biofilm remained stable during the running period.
Conclusions:  A coexistence of nitrifiers, including ammonia-oxidizing bacteria and nitrite-oxidizing bacteria, denitrifiers, including aerobic or anaerobic denitrifying bacteria and Anammox bacteria were detected, which might be the real matter of high removal efficiencies of NH₄⁺-N and COD_Cr in the reactor.
Significance and Impact of the Study:  The findings in this study indicated that PCR-DGGE analysis could be used for microbial community detection as prior method, and the SBBR technique could provide preferable growing environment for bacteria with N removal function.     

Variation of broth composition by addition of broiler litter composting substrate extracts: influence on faecal bacterial growth

Aim:  This study investigates differences in bacterial growth response in broth amended with compost-substrate extracts periodically bypassed during broiler litter composting.
Methods and Results:  Compost samples, suspended in diluent were mixed with double strength broth into which ampicillin selective (0·3 g l⁻¹) Escherichia coli and E. faecalis were separately seeded. Growth was measured by viable cell count. The Levenberg–Marquardt algorithm was applied to obtain a four-parameter sigmoidal function that best described the diminishing height transitions of the curves for extracts of increasing composting age. The time course of the growth rate followed a unimodal bell-shaped curve. The Microfit^© application was run to generate information of direct microbiological interest: increasing λ and decreasing μ_max for both bacteria with time.
Conclusion:  More than the curve-fitting process, the Unified model option of the Microfit^© application has confirmed the significant differences ( P  <   0·05) in the growth curve behaviour with more stabilized substrate extracts. The study demonstrates further scopes for characterization of the sanitization potential and indirectly, the impact of indigenous microbial competitive exclusion effects on enteric bacteria.
Significance and Impact of the Study:  A different outlook to understanding faecal bacterial growth dynamics in compost has been presented, using predictive microbiology concepts. Further structured studies are needed to fine-tune the generality of the findings for model development.     

Kinetics of microbial growth on pentachlorophenol.

Batch and fed-batch experiments were conducted to examine the kinetics of pentachlorophenol utilization by an enrichment culture of pentachlorophenol-degrading bacteria. The Haldane modification of the Monod equation was found to describe the relationship between the specific growth rate and substrate concentration. Analysis of the kinetic parameters indicated that the maximum specific growth rate and yield coefficients are low, with values of 0.074 h-1 and 0.136 g/g, respectively. The Monod constant (Ks) was estimated to be 60 micrograms/liter, indicating a high affinity of the microorganisms for the substrate. However, high concentrations (KI = 1,375 micrograms/liter) were shown to be inhibitory for metabolism and growth. These kinetic parameters can be used to define the optimal conditions for the removal of pentachlorophenol in biological treatment systems.     

Kinetics of microbial growth on pentachlorophenol

Batch and fed-batch experiments were conducted to examine the kinetics of pentachlorophenol utilization by an enrichment culture of pentachlorophenol-degrading bacteria. The Haldane modification of the Monod equation was found to describe the relationship between the specific growth rate and substrate concentration. Analysis of the kinetic parameters indicated that the maximum specific growth rate and yield coefficients are low, with values of 0.074 h-1 and 0.136 g/g, respectively. The Monod constant (Ks) was estimated to be 60 micrograms/liter, indicating a high affinity of the microorganisms for the substrate. However, high concentrations (KI = 1,375 micrograms/liter) were shown to be inhibitory for metabolism and growth. These kinetic parameters can be used to define the optimal conditions for the removal of pentachlorophenol in biological treatment systems.     

A Thermodynamic Interpretation of the Monod Equation

The Monod equation for microbial growth is purely empirical, and the theoretical basis of this model stays unclear. Similar to any chemical reactions, overall microbial growth process is dependent upon the changes in free energy. This study showed that the Monod equation could be interpreted in a thermodynamic sense very well. It was probably for the first time demonstrated that the Monod constant (K _s ) was inversely related to the equilibrium constant of the overall microbial growth process. Received: 2 August 2002 / Accepted: 23 September 2002     

Batch kinetics of microbial polysaccharide biosynthesis

A modified form of logistic equation has been proposed to quantity the batch kinetics of microbial growth during the biosynthesis of extra- and intracellular polymers. Based on the experimental data developed in this study, the proposed model appeared to provide adequate growth and fermentation kinetics of Aureobasidium pullulans. The model was also applicable for representing the reported data on pullulan, xanthan, and poly-beta-hydroxybutyricacid. In comparison to the logistic and Monod kinetics, this model fitted the data better and more accurately described the overall fermentation, both concentrations and fermentation time.     

Kinetic behavior of heterogeneous populations in completely mixed reactors

The kinetic behavior of heterogeneous microbial populations was studied in a continuous flow completely mixed reactor operated at various dilution rates. Glucose was used as the growth-limiting nutrient. The physiological growth parameters for cells harvested from continuous flow reactors were determined using batch experiments. It, was found that the growth parameters, maximum growth rate (μ_m), saturation constant (k_s), and cell yield (Y) vary for each dilution rate, and cannot be considered as precise constants in depicting the kinetic behavior of heterogeneous populations. In addition, it was found that the yield coefficients obtained from batch experiments were always lower than those obtained from continuous flow experiments. Levels of substrate and biological solids calculated for different dilution rates using growth constants from batch experiments did not agree with the experimental values observed in steady-state experiments. However, when the yield values from, the continuous flow experiments were used in conjunction with batch values for μ_m and k_s the theoretical and experimental dilute-out curves agreed fairly closely (within the range needed for engineering prediction) until the culture began to wash out of the unit. In general, the data substantiated the use of the single phase relationship between growth rate and substrate concentration described by the Monod equation, μ = μ_mS/(k_s + s).     

A mechanism of microbial cell growth

Presently empirical expressions, especially the Monod equation, are used to quantitatively relate microbial growth rate to limiting substrate concentration in the solution. In this paper microbial growth is postulated to occur by a mechanism involving a mass transfer or assimilation process. The assimilation process is assumed to be substrate mass transfer limited and hence proportional to the limiting substrate concentration. The ingestion is assumed independent of limiting substrate concentration and only dependent upon internal reaction rates. The quantitative relationship between limiting substrate and microbial growth rate resulting from this mechanism is developed. Under certain limiting conditions this expression is shown to reduce to the Monod equation and under other conditions it reduces to the Lotka-Volterra relationship. This mechanism is applied to batch and continuous cultures and the results obtained are compared quantitatively with experiment.     

Effects of feeding and induction strategy on the production of BmR1 antigen in recombinant E. coli

Aim:  To investigate the effects of feeding and induction strategies on the production of Bm R1 recombinant antigen.
Methods and Results:  Fed-batch fermentation was studied with respect to the specific growth rate and mode of induction to assess the growth potential of the bacteria in a bioreactor and to produce high yield of Bm R1 recombinant antigen. Cells were grown at a controlled specific growth rate (μ_set) during pre-induction, followed by constant feeding postinduction. The highest biomass (24·3 g l⁻¹) was obtained during fed-batch process operated at μ_set of 0·15 h⁻¹, whereby lower μ_set (0·075 h⁻¹) gave the highest protein production (9·82 mg l⁻¹). The yield of Bm R1 was increased by 1·2-fold upon induction with 1 mmol l⁻¹ IPTG (isopropyl-β- d -thiogalactoside) compared to using 5 mmol l⁻¹ and showed a further 3·5-fold increase when the culture was induced twice at the late log phase.
Conclusions:  Combination of feeding at a lower μ_set and twice induction with 1 mmol l⁻¹ IPTG yielded the best result of all variables tested, promising an improved method for Bm R1 production .
Significance and Impact of the Study:  This method can be used to increase the production scale of the Bm R1 recombinant antigen to meet the increasing demand for Brugia Rapid^™, a commercial diagnostic test for detection of brugian filariasis.     

The survival of Salmonella enterica serovar Typhimurium DT104 and total viable counts on beef surfaces at different relative humidities and temperatures

Aims:  The aim of this study was to investigate changes in Salmonella and total viable count (TVC) survival on beef carcass surfaces stored for 72 h under different combinations of relative humidity (i.e. RH 75% or 96%) and temperature (5°C or 10°C).
Methods and Results:  The influence of low water activity ( a _w) and temperature on the survival and growth of Salmonella enterica serovar Typhimurium DT104 and the aerobic mesophilic flora on meat pieces from different sites on beef carcasses was investigated, under controlled conditions (75% or 96% RH; 5 or 10°C) in an environmental cabinet. Salmonella counts declined during storage at low a _w (75% RH) conditions at 5°C or 10°C. Salmonella counts increased during storage at high a _w (96% RH) at 10°C only. At 5°C, TVCs increased during storage at high a _w, but not at low a _w. TVCs increased on all samples from carcasses stored at high or low a _w at 10°C, except those samples taken from areas of surface fat.
Conclusions:  This suggests that substrate composition dictates growth rates under low a _w conditions. The results are discussed in terms of the possible protective effects of substrate osmolyte accumulation in bacterial survival and/or growth.
Significance and Impact of the Study:  The data obtained in this study provides useful insights on the influence of a _w and temperature on pathogen survival on meat surfaces at chill temperature.     

Diffusion-limited binding explains binary dose response for local arterial and tumour drug delivery

Background:  Local drug delivery has transformed medicine, yet it remains unclear how drug efficacy depends on physicochemical properties and delivery kinetics. Most therapies seek to prolong release, yet recent studies demonstrate sustained clinical benefit following local bolus endovascular delivery.
Objectives:  The purpose of the current study was to examine interplay between drug dose, diffusion and binding in determining tissue penetration and effect.
Methods:  We introduce a quantitative framework that balances dose, saturable binding and diffusion, and measured the specific binding parameters of drugs to target tissues.
Results:  Model reduction techniques augmented by numerical simulations revealed that impact of saturable binding on drug transport and retention is determined by the magnitude of a binding potential, B_p , ratio of binding capacity to product of equilibrium dissociation constant and accessible tissue volume fraction. At low B_p (< 1), drugs are predominantly free and transport scales linearly with concentration. At high B_p (> 40), drug transport exhibits threshold dependence on applied surface concentration.
Conclusions:  In this paradigm, drugs and antibodies with large B_p penetrate faster and deeper into tissues when presented at high concentrations. Threshold dependence of tissue transport on applied surface concentration of paclitaxel and rapamycin may explain threshold dose dependence of in vivo biological efficacy of these drugs.     

Mycelial growth and ochratoxin A production by Aspergillus section Nigri on simulated grape medium in modified atmospheres

Aims:  To evaluate the impact of modified atmosphere packaging on in vitro growth of Aspergillus carbonarius and Aspergillus niger , and possible effects on ochratoxin A (OTA) biosynthesis.
Methods and Results:  Ochratoxigenic isolates belonging to the species A. carbonarius and A. niger were grown on a synthetic grapejuice medium (SNM) and packaged in combinations of controlled O₂ (1% and 5%) and CO₂ levels (0% and 15%), and in air as a control. Colony diameters were recorded every 3 days up to 21 days, and OTA was analysed after 7, 14 and 21 days. The greatest reductions in mycelial growth rate were observed at 1% O₂ followed by 1% O₂/15% CO₂, whereas 5% O₂ stimulated the growth of all isolates. OTA production by A. carbonarius and A. niger isolates was minimized at 1% O₂/15% CO₂ and 1% O₂, respectively, after 7 days of incubation. Maximal OTA accumulation after 7 days was observed for all isolates in the control pack and at 5% O₂.
Conclusions:  Of the atmospheres tested, only 1% O₂ combined with 15% CO₂ consistently reduced fungal growth and OTA synthesis by A. carbonarius and A. niger .
Significance and Impact of the Study:  Storage under modified atmospheres is unlikely to be suitable as the sole method for OTA minimization and grape preservation; other inhibitory factors are necessary.