- Open Access
Selective suppression of bacterial contaminants by process conditions during lignocellulose based yeast fermentations
© Albers et al; licensee BioMed Central Ltd. 2011
Received: 7 October 2011
Accepted: 20 December 2011
Published: 20 December 2011
Contamination of bacteria in large-scale yeast fermentations is a serious problem and a threat to the development of successful biofuel production plants. Huge research efforts have been spent in order to solve this problem, but additional ways must still be found to keep bacterial contaminants from thriving in these environments. The aim of this project was to develop process conditions that would inhibit bacterial growth while giving yeast a competitive advantage.
Lactic acid bacteria are usually considered to be the most common contaminants in industrial yeast fermentations. Our observations support this view but also suggest that acetic acid bacteria, although not so numerous, could be a much more problematic obstacle to overcome. Acetic acid bacteria showed a capacity to drastically reduce the viability of yeast. In addition, they consumed the previously formed ethanol. Lactic acid bacteria did not show this detrimental effect on yeast viability. It was possible to combat both types of bacteria by a combined addition of NaCl and ethanol to the wood hydrolysate medium used. As a result of NaCl + ethanol additions the amount of viable bacteria decreased and yeast viability was enhanced concomitantly with an increase in ethanol concentration. The successful result obtained via addition of NaCl and ethanol was also confirmed in a real industrial ethanol production plant with its natural inherent yeast/bacterial community.
It is possible to reduce the number of bacteria and offer a selective advantage to yeast by a combined addition of NaCl and ethanol when cultivated in lignocellulosic medium such as wood hydrolysate. However, for optimal results, the concentrations of NaCl + ethanol must be adjusted to suit the challenges offered by each hydrolysate.
Contamination by bacteria in industrial scale yeast fermentations is a huge problem with serious economic consequences. Such operations are not carried out under aseptic conditions and Lactobacilli, which are usually considered to be the most frequent contaminants, thrive under the very same conditions as the yeast Saccharomyces cerevisiae [1–3]. In some conditions and for certain products the bacteria can provide added value in the form of flavor, taste, and so on, but the levels must be maintained within certain limits . In other processes, such as production of biofuels like ethanol, bacterial contamination causes reductions in yield and/or productivity with a deteriorating economy of the process as a consequence. Despite massive amounts of time and effort spent on these matters, bacterial contamination is still a serious problem and a threat to the successful development of commercial bio-based fuel production. Traditional methods for keeping bacterial contaminants at a tolerable level include introduction of very low pH, for example, between 2 and 3 , and more modern techniques rely on the ancient knowledge that hops can provide not only a favorable taste of various beverages but also protection against bacterial decomposition of the product [6, 7]. However, these methods do not always function as expected, and there is still a need to find additional ways of preventing bacteria from flourishing in these environments. Another option sometimes considered is the use of antibiotics. However, this is questionable from an economic point of view but even more important is the increasing awareness and fear of the ever-increasing spread of bacterial resistance due to massive misuse of these compounds.
This investigation was undertaken in order to develop process conditions that would present a selective advantage to the yeast while suppressing growth and product formation of bacteria. In addition, several of the conditions were selected to be relevant for so-called high gravity or high solids fermentation as this would offer high product concentration and an improved economy of the ethanol production process . In order to make relevant comparisons of the selective effect on yeast and bacteria between different conditions, we isolated numerous bacterial isolates from the industrial ethanol production plant in Örnsköldsvik, Sweden from which the yeast strain used was originally isolated. The process conditions selected were enhanced levels of sugar, sodium chloride and ethanol as well as low pH. Initially these conditions were tested one by one using pure cultures of yeast and bacteria. Later on, cocultures of yeast and bacteria were studied in competition experiments and combinations of stress factors were also included. The results showed that a combination of NaCl + ethanol additions to the wood hydrolysate could suppress growth of bacteria while yeast viability and ethanol production was favored.
Results and Discussion
In order to identify what bacterial species that should be included in the study sampling of the microbial community at an industrial ethanol production plant, Domsjö Fabriker AB in Örnsköldsvik, Sweden, were performed. The most abundant species of Lactobacillus seemed to be Lactobacillus buchneri and Lactobacillus plantarum, that is, most of the isolates obtained belonged to these two species. A full report concerning identified bacterial species and their respective growth behavior, stress tolerance and so on, will be reported in a separate publication. The species of Lactobacillus and Acetobacter that are included in this investigation were all obtained from this plant with the exception of Lactobacillus fermentum that was obtained from the American Type Culture Collection (ATCC; http://www.lgcstandards-atcc.org/). This latter strain was used as a reference as it had previously been investigated in a similar type of study .
Viability of bacterial contaminants with or without cocultivation with yeast
The effect of nutrient supplementation (yeast extract) on viability of bacteria
Similarly, no effect on yeast viability and multiplication could be detected as a result of yeast extract additions to the hydrolysate (Figure 3).
Growth and product formation during cocultivation of yeast and Lactobacillus or Acetobacter
Coculture experiments revealed large effects on growth and viability of yeast and bacteria when coexisting in the lignocellulosic medium. How did this affect metabolism and major catabolic products such as ethanol, acetate and lactic acid?
A completely different picture emerged when S. cerevisiae was mixed with Acetobacter in cocultures. In this case there was production of ethanol during the initial 24 h but this was followed by declining ethanol concentrations concomitant with an increase in acetate concentrations (Figure 4B), similar to what is normally observed during growth of Acetobacter . Obviously, the Acetobacter not only has a very negative effect on the viability of the yeast (Figure 2) but also consumes a substantial part of the previously formed ethanol.
The selective effect on yeast and bacteria from addition of NaCl, sugar, ethanol, and low pH
Identification of optimum combinations of NaCl + ethanol selectively inhibiting bacteria
Enhancing yeast viability and productivity by a combination of NaCl and ethanol additions
Verification of laboratory results in an industrial ethanol production plant
Acetic acid bacteria can potentially be a much more serious threat than lactic acid bacteria as contaminants of industrial scale yeast fermentations. Acetic acid bacteria showed a capacity to drastically reduce the viability of yeast cells as well as consuming the previously formed ethanol. Lactic acid bacteria did not show any of these characteristics. A combined addition of NaCl and ethanol during cultivation in wood hydrolysate could be used to reduce the number of bacteria and to selectively support the viability of yeast cells and thereby increase the concentration of ethanol.
A strategy to implement this in an industrial setting could be to add ethanol by recycling of process streams and to start the process using only a fraction of the total volume. This will potentially offer a kick-start for yeast in comparison to bacteria and preclude the necessity of adding large total amounts of NaCl and EtOH.
An industrial strain of S. cerevisiae was used (CCUG53310, Culture Collection University of Göteborg, Göteborg, Sweden) . This strain was originally isolated from an industrial ethanol production plant, Domsjö Fabriker AB, Örnsköldsvik, Sweden. Lactic acid bacteria were obtained from a culture collection, Lactobacillus fermentum (ATCC14931) or isolated from Domsjö Fabriker AB, L. buchneri, L. plantarum. Acetic acid bacteria, Acetobacter tropicalis, A. syzygii, were isolated from the same industrial plant. The isolated bacteria were species determined by a combination of API test (bioMerieux, France) and 16S RNA gene sequencing.
Inoculum cultures were grown in YPD medium (10 g/l yeast extract (Sigma-Aldrich St. Louis, USA), 20 g/l peptone (Nordic Biolabs, Taby, Sweden), 20 g/l glucose (Merck, Darmstadt, Germany)) for yeast and MRS medium (Oxoid, Hampshire, England) for bacteria for 1 to 1.5 days at 30°C in falcon tubes or shake flasks depending on culture volume.
The cultivations were performed in a lignocellulosic hydrolysate of spruce chips pretreated with dilute acid, a composition determined previously . The pH was adjusted to 5.0 with ammonia (Merck, Darmstadt, Germany) and the hydrolysate medium was filter sterilized before usage.
For cocultivations on a small scale in 50 ml falcon tubes, the cells in the inocula were harvested after 1 day by centrifugation, resuspended in sterile water and the optical density at 610 nm (OD610) was measured. The hydrolysate (6 or 10 ml) was inoculated with cells (yeast and/or bacteria) and water in a total of 45 μl/ml hydrolysate to give an initial OD610 of 0.05 for yeast and at 0.09 for bacteria. The lid was closed and the tubes were incubated at 30°C in a rotary shaker and monitored for up to 4 days. Samples for medium analyses were centrifuged (2 min, at minimum 14,000 g) and stored at -20°C before analysis.
Cocultivations on a large bench scale were performed with 1 l of medium using 3 l bioreactors (Belach Bioteknik AB, Stockholm, Sweden) operated at 30°C with a stirring rate of 300 rpm and no gas inlet. The initial pH was set to 5.0 with ammonia (Merck, Darmstadt, Germany) during the preparation of medium and the decrease during cultivations was always less than 0.5 pH units.
Determination of bacteria and yeast viability at an industrial production plant
The start inoculum was a mixture of microorganisms harvested from the Domsjö Fabriker industrial ethanol production plant located in Örnsköldsvik, Sweden. This mixture (sludge) contained the complete microbiological community existing in an industrial ethanol fermentation plant: mainly yeast, lactic acid bacteria and acetic acid bacteria.
This microbiological community was cultivated for 32 h at 30°C in spent sulfite liquor supplemented with 10.2 ml 25% ammonium (Merck, Darmstadt, Germany) and 171 mg/l KH2PO4 (Merck, Darmstadt, Germany) with and without addition of NaCl (Merck, Darmstadt, Germany) (25 g/l) + ethanol (VWR, Leuven, Belgium) (12.5 g/l). The pH was adjusted to 5.0 by addition of 5 M NaOH (Merck, Darmstadt, Germany) prior to fermentation. The cultivations were performed in 300 ml Erlenmeyer flasks with a total volume of 200 ml.
Measurements of the cell viability were performed by colony forming unit (CFU) count.
Metabolites in the medium (ethanol, acetic acid, lactic acid) were analyzed using commercial enzymatic kits assays (R-Biopharm GmbH, Darmstadt, Germany) with adapted volumes in microtiter plates. Absorbance was measured with a Fluostar Galaxy plate reader (BMG Labtechnologies, Offenburg, Germany).
CFU determinations were performed on agar plates with YPD for yeast (when bacteria was present in large numbers, 20 μl of 50 g/l ampicillin (AppliChem, Darmstadt, Germany) was added to each plate) and MRS for bacteria with 0.1 g/l cycloheximide (Merck, Darmstadt, Germany)to suppress growth of yeast. Each dilution was spread on two or three plates. The plates were incubated at 30°C for 2 days for yeast and for 3 days for bacteria to establish distinct colonies before counting.
Multiple regression analysis of the specific growth rate as a function of pH, temperature, and concentrations of NaCl, glucose, ethanol and lactic acid was performed using the software Modde 9.0 (Umetrics AB, Umea, Sweden).
The project was funded by the Swedish energy agency, project no. 30188-1 and 30188-2, the Kempe foundation and the county administrative board of Vasternorrland, which is gratefully acknowledged.
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