From population ecology to metabolism: growth of Trypanosoma evansi, and implications of glucose depletion, in a live host

• The potential top-down effects of population growth on metabolism have received little attention.
• Trypanosoma evansi is a promising model for studying such effects.
• We assess population growth, glycolytic flux, and enzyme expression in T. evansi.
• We discuss the results in the context of a metabolic map based on previous knowledge.
• We predict density dependent metabolic changes as growing parasites deplete host nutrients.

Little attention has been paid to the potential top-down effects of population ecology on metabolism. Because it is capable of a fast growth that dramatically modifies its blood environment, the parasitic protozoan, Trypanosoma evansi, is a promising model for the study of density-dependent metabolic processes. We assess the in vivo growth rate, doubling time, biomass yield, glycolytic flux, and enzyme expression of T. evansi. Then, we explore the metabolic changes likely occurring during its growth. At low T. evansi densities, most host glucose is used to produce energy, which results in the release of pyruvate. Part of glucose is used for NADPH production through the pentose phosphate pathway. At high T. evansi densities, fructose, mannose, and glycerol become additional energy sources. Part of host glucose is used for biosynthesis and for NADPH production through alternative metabolic pathways, which results in the release of succinate, alanine, and acetate. The ability of organisms to adjust to resource changes is crucial to their survival. Irrespective if the triggering mechanism is direct (nutrient limitation) or indirect (a pheromone-like factor), nutrient availability is necessarily the main evolutionary factor responsible for the density-dependent metabolic properties of trypanosome populations.

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