Abstract
Microorganisms must maintain equilibrium and intracellular availability of ions for growth and persistence in a range of environments, either in the face of scarce resources or excesses, whereby they must buffer, exclude or export these ions preventing accumulation to harmful levels. Ion homeostasis is essential for energy generation, metabolism, regulation of internal pH and osmotic homeostasis. Mycobacteria are part of the native microflora found in soils and aquatic environments and harbour an extensive repertoire of uptake and efflux and secretion pathways to modulate ions and metabolites including, sodium, potassium, calcium, magnesium, iron, nickel, manganese, zinc, copper and cobalt. There remains a paucity of information regarding ion homeostasis networks and pathways in mycobacteria. The overarching aim of this thesis was to characterise ion homeostasis and energetics in the saprophyte Mycobacterium smegmatis, under normoxia and hypoxia, under ion chelation and deficiency, with heavy-metal exposure and in response to anti-tubercular compounds with putative disruptive effects on ion energetics, including bedaquiline and amiloride derivatives.
Inductively coupled plasma mass spectroscopy (ICP-MS) analysis of wild-type M. smegmatis and energy-driven ion transport mutants revealed the molecular inventory of cations in M. smegmatis under normoxia and hypoxia. The intracellular cation profile of M. smegmatis was comprised primarily of sodium (40-85%), potassium (10-55%) and magnesium (5-7%), while trace elements consisting of calcium, iron, nickel, manganese, zinc, copper and cobalt were less than 1.5% of the profile. ICP-MS analysis also demonstrated changes in ion homeostasis from disruptions made using chelation agents to reduce the availability of manganese, iron, cobalt and zinc. Cultures exposed to 2 mM nickel and manganese resulted in intracellular accumulation of these cations and significant depletion of intracellular cobalt, iron, magnesium and potassium concentrations. Disruption of potassium homeostasis in the ΔtrkA mutant of M. smegmatis resulted in the reduction of (~10-50%) intracellular potassium concentrations and elevated (~10-30%) intracellular sodium concentrations, coupled with reductions in viability and persistence under hypoxia.
This study has demonstrated the molecular inventory of intracellular ions in M. smegmatis under several key laboratory conditions and has characterised essential components of ion homeostasis and more specifically, potassium homeostasis and energetics in mycobacteria. The role of several energy-driven transport systems and the effects of anti- tubercular compounds on cellular and ion homeostasis in M. smegmatis and Mycobacterium tuberculosis were also studied.