COMPARATIVE PHYSIOLOGY OF 2 PROTOZOAN PARASITES, LEISHMANIA-DONOVANI AND TRYPANOSOMA-BRUCEI, GROWN IN CHEMOSTATS

Cultures of the insect stage of the protozoan parasites Leishmania donovani and Trypanosoma brucei were grown in chemostats with glucose as the growth rate-limiting substrate. L. donovani has a maximum specific growth rate (mu(max)) of 1.96 day-1 and a K(s) for glucose of 0.1 mM; the mu(max) of T. brucei is 1.06 day-1 and the K(s) is 0.06 mM. At each steady state (specific growth rate, mu, equals D, the dilution rate), the following parameters were measured: external glucose concentration (Glc(out)), cell density, dry weight, protein, internal glucose concentration (Glc(in)), cellular ATP level, and hexokinase activity. L. donovani shows a relationship between mu and yield that allows an estimation of the maintenance requirement (m(s)) and the yield per mole of ATP (Y(ATP)). Both the m(s) and the Y(ATP) are on the higher margin of the range found for prokaryotes grown on glucose in a complex medium. L. donovani maintains the Glc(in) at a constant level of about 50 mM as long as it is not energy depleted. T. brucei has a decreasing yield with increasing mu, suggesting that it oxidizes its substrate to a lesser extent at higher growth rates. Glucose is not concentrated internally but is taken up by facilitated diffusion, while phosphorylation by hexokinase is probably the rate-limiting step for glucose metabolism. The K(s) is constant as long as glucose is the rate-limiting substrate. The results of this study demonstrate that L. donovani and T. brucei have widely different metabolic strategies for dealing with varying external conditions, which reflect the conditions they are likely to encounter in their respective insect hosts.