Completed research projects

Formulation of high cell density medium for aerobic cultivation of Escherichia coli DH5α

Obtaining high cell yield of uniform characteristics is a key requirement for many areas of applied microbiology research where there is a need to perform large scale characterization and performance tests of the biomass under different scenarios. However, biomass yield from many commercially available growth media remained poor; thereby, resulting in batch-to-batch variation in cell characteristics. Using Escherichia coli DH5α as a model organism, this research seeks to develop a liquid growth medium capable of high cell density aerobic cultivation of the bacterium. Results revealed the formulated medium was capable of increasing the optical density of the culture broth by more than 3 times compared to LB Lennox medium without the generation of growth inhibitory compounds during cultivation. Obtained large population of cells could also be maintained for a longer period compared to commercially available medium such as Tryptic Soy Broth. Additionally, a high capacity phosphate buffer system helped modulate fluctuations in pH during cell cultivation which improved biomass formation. Collectively, a formulated medium capable of high cell density cultivation of E. coli DH5α was developed.

 

Representative publications

Wenfa Ng and Yen-Peng Ting, “Formulation of a semi-defined medium for high cell density cultivation of Escherichia coli in shake flasks”, PeerJ Preprints, Link to preprint

Wenfa Ng, “High cell density cultivation of Escherichia coli DH5α in shake flasks with a new formulated medium”, PeerJ Preprints, Link to preprint

 

Effect of wash buffers on zeta potential analysis of bacterial cells

Bacterial cells in contact with a solution such as a growth medium would inevitably adsorb ions and metabolites from the solution. Such nonspecific adsorption of ions and metabolites could thus confer additional electric charges to the cell surface and masks the real surface charge. Thus, various wash buffers of different ionic strength have been used in repeated steps of centrifugation and washing for removing nonspecifically adsorbed ions and metabolites from bacterial cell surface. However, their relative efficacy remained poorly understood. Thus, the goal of this project was in using a systematic study to understand the relative efficacies of different wash buffers in removing nonspecifically adsorbed ions and metabolites from the cell surface. Experiment results from the model system of Escherichia coli DH5α grown in two different growth media that differed in salt content revealed that high ionic strength of wash buffers was correlated with greater removal of nonspecifically adsorbed ions and metabolites. More importantly, a threshold exists beyond which high ionic strength wash buffers could remove ions intrinsic to the cell envelope and thereby alter cell surface characteristics. Depending on the cell wall structure and types and amounts of ions and metabolites in the growth medium, differing amounts of ions and metabolites would nonspecifically adsorb to the cell surface. Thus, the threshold ionic strength for removing nonspecifically adsorbed ions and metabolites is likely bacterial species and growth medium specific. In the case of E. coli DH5α grown in LB Lennox + 2g/L glucose and a formulated medium, the threshold ionic strength was 0.15M (e.g., 9 g/L sodium chloride wash buffer). High ionic strength wash buffers such as 0.6M sodium chloride and 0.1M sodium citrate would result in removal of ions intrinsic to the cell envelope that manifested as changes to the point of zero charge (pHzpc) of the cells. More importantly, the research also revealed that citrate ions could adsorb onto the cell surface and thus wash buffers with high concentration of citrate should not be used in preparing cell samples for zeta potential analysis. Finally, using wash buffer of 0.15M ionic strength (i.e., 9 g/L sodium chloride), the research possibly revealed the real surface charge of E. coli DH5α grown in the two growth media.

 

Representative publications

Wenfa Ng and Yen-Peng Ting, “Bacterial surface charge in “layers”: revealed by wash buffers of different ionic strength”, PeerJ Preprints, Link to preprint

Wenfa Ng and Yen-Peng Ting, “Zeta potential of bacterial cells: Effect of wash buffers”, PeerJ Preprints, Link to preprint

 

Colourless agar for enhanced colour contrast between agar and microbial colonies

Solid medium used for the cultivation of microorganisms are typically beige in colour. Given that many microbial colonies are also similar in colour, there is a lack of colour contrast necessary for easy and accurate identification of small microbial colonies on agar plates. Thus, the objective of this research is the development of a colourless agar which when coupled with coloured paper of suitable hue would help increase colour contrast between microbial colonies and agar background. However, reaction between glucose and ammonium compounds would generate coloured compounds during autoclave heat sterilization. By separately sterilizing glucose and ammonium compounds, this project developed a simple protocol for the preparation of colourless agar that remained colourless even with the addition of yeast extraction at 1 g/L. Growth experiments revealed that the colourless agar could cultivate common bacteria such as Escherichia coli, Bacillus subtilis, Pseudomonas protegens, and Pseudomonas aeruginosa to relatively high cell densities. In addition, the colourless agar could also support cell concentration common in viable cell counting of microbes, which opens up possibility in using the colourless agar together with automated image analysis for colony identification and counting. In assessing the microbial flora of deionized and tap water, the colourless agar was also able to support a large variety of different microorganisms. Specifically, colonies of different morphologies, size and pigmentation could be observed on the colourless agar where types of microbes cultivated positively correlated with yeast extract concentration. Overall, a method was developed for the preparation of colourless agar that lends a useful tool for automated image analysis of microbial colonies where accurate identification of colonies occurred through enhanced colour contrast between colonies and agar background.

 

Representative publications

Wenfa Ng, “Colourless agar for enhancing colour contrast between microbial colonies and agar”, PeerJ Preprints, Link to preprint

Wenfa Ng, “Microbes in deionized and tap water: Implications for maintenance of laboratory water production system”, PeerJ Preprints, Link to preprint

 

Ammonium interference in adsorption of copper by formaldehyde crosslinked seaweed from low concentration solutions

Ammonium ions co-exist with heavy metal ions in many environmental water matrixes and could possibly influence the removal of the heavy metal ions by various water treatment techniques. One such possibility is adsorption on seaweed such as Sargassum sp. which has been shown to be useful for adsorbing a variety of heavy metals such as copper, lead, cadmium and chromium. Given that ammonium ions exist as positively charged NH4+ ions at circumneutral pH of around 5-6, they could possibly interfere with the adsorption of cationic heavy metals on seaweed. Thus, the objective of this project is to understand the possible interference effect of ammonium ion on the adsorption of copper from low concentration solutions (4-20 ppm Cu2+) by formaldehyde crosslinked Sargassum sp. (modified seaweed). Modification of seaweed through formaldehyde crosslinking helped reduced organic leaching but the residual leaching remained at 4 ppm total organic carbon. Batch kinetic and equilibrium experiment results revealed that ammonium ion interfered with copper adsorption in a concentration-dependent manner. Specifically, beyond a threshold concentration of 50 ppm NH4+-N, ammonium ion reduced the uptake capacity of modified seaweed as well as increased the final equilibrium copper concentration. Overall, ammonium ion interfered with the uptake of copper by modified seaweed through possibly competing for the same functional groups on the seaweed surface. In addition, leaching of organics from the modified seaweed meant that the biosorbent could not be used in drinking water treatment applications.

 

Representative publication

Wenfa Ng, “Ammonium interference reduced copper uptake by formaldehyde-crosslinked Sargassum seaweed”, PeerJ Preprints, Link to preprint

 

 

 

 

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