Ogbu, H.I.
(2016)
Amino acids utilisation by Clostridium difficile strains.
PhD thesis, University of Nottingham.
Abstract
The carbon and energy metabolism of the human pathogen Clostridium difficile is poorly understood. Amino acid metabolism by the Stickland reactions has previously been described as a primary source of energy in a number of Clostridium species, especially when grown in a medium containing only amino acids. Deeper insights may be gained by metabolic analyses using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) platforms, but such experiments are best performed in well-defined growth media. A number of C. difficile strains have been successfully cultivated on defined media but these media provide an excess of nutrients, particularly in terms of amino acid provision, resulting in undesirable background growth in the absence of glucose. To overcome these challenges, three variants of a defined medium were developed that contain the essential nutrients that support the growth of this bacterium, in the presence of a carbon and energy source such as glucose together with an LC-MS/MS method that will simultaneously measure all twenty amino acids.
Since the focus in this study was amino acid utilisation, a comprehensive and most effective technique that will provide as much information as possible for understanding the metabolic requirements of eight Clostridium difficile strains (CD630∆erm, DH196, R20291, EK15, EK28, R12801, L26, O17 Serotype F) and two transposon mutants (CRG-2979, CRG-3887) was required. Due to high water solubility and the range of ionic characteristics of amino acids, an aqueous normal phase chromatographic method was considered to simultaneously separate a mixture of amino acids without derivatisation. Aqueous normal phase chromatographic method represents an important new technology with a capability of the silica-hydride-base stationary phases, offering a distinct advantage for practical application with a high degree of reproducibility and long-term stability of polar and non-polar compounds.
The developed methods were subsequently adapted to study amino acid utilisation in the presence or absence of a fermentable carbohydrate and/or selenium. OD determinations were performed by measuring absorbance at 600 nm (or OD600) using a Biomate 3 spectrophotometer. The concentration of individual amino acids remaining in the spent C. difficile culture media was measured after 24 h and/or 48 h using the developed LC-MS/MS method in order to determine which amino acids were being utilised by these organisms and in which order. This analysis should give further insight on the importance of amino acids for the survival of this bacterium in the gut, which may possibly lead to discovery of novel fermentable products and metabolic pathways and/or eventually aid in the successful control of the disease.
Data obtained for CD630∆erm, DH196, R20291, EK15, EK28, R12801, L26, O17 Serotype F strains showed that they all grew on the fully defined medium, with different growth profiles in terms of lag phase, growth rate, and maximum OD reached. The LC-MS/MS data generated suggest that cysteine, glutamine, isoleucine, leucine, serine, threonine and tyrosine are preferentially utilised, both in the presence and absence of glucose. Several other amino acids, including asparagine, glycine, phenylalanine, proline and valine were also utilised but to a lesser extent. Notable in this study was the lack of glutamate utilisation, except by strain L26, and the excretion of alanine after its initial uptake by most of the tested stains. The excretion of alanine may be due to the use of pyruvate as an amino acceptor during the degradation of preferentially fermented amino acids, whereas, glutamate is not a substrate for most C. difficile strains. Thus, LC-MS/MS profiling confirmed that these organisms derive most of their carbon and energy from the fermentation of a selected range of amino acids.
Given the importance of selenium-dependent Stickland reactions to the growth of this bacterium, further studies were undertaken to evaluate the metabolism of amino acids by two different C. difficile strains (630∆erm, R20291) and two transposon mutants CRG-2979 (defective in hadB, encoding one of the two subunits of hydroxyisocaproyl-CoA dehydratase required for reductive degradation of leucine) and CRG-3887 (defective in selA encoding selenocysteine synthase) in the presence or absence of glucose/selenium. LC-MS/MS data reveal that amino acid utilisation was affected by the presence of selenite, notably proline utilisation, which could be explained by the presence of the enzyme proline reductase and the lack of glycine consumption, known to be selenium-dependent. The non-utilisation of glycine could be explained by the presence of proline which represses the formation of the necessary enzyme systems required for glycine degradation. Data generated for CRG-2979 reveals that this mutant could only thrive in the presence of selenium when glucose is present, possibly due to the presence of proline replacing leucine as the major Stickland acceptor. The results of the transposon mutant CRG-3887, were much of a surprise too, because this mutant was predicted to be deficient in proline and glycine breakdown which is also reliant on selenoenzymes. This suggests the presence of selenium independent proline fermentation pathway although this hypothesis is not supported by the existing literature.
Experimental data provided further evidence about the ability of this bacterium to obtain its carbon and energy in the absence of a fermentable carbohydrate; by Stickland reactions and that the presence or absence of certain amino acids could repress the utilisation or biosynthesis of other amino acids.
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