Analysis of the development of arsenical resistance in trypanosomes in vitro

Laboratory Selection of Trypanosomatid Pathogens for Drug Resistance

The selection of parasites for drug resistance in the laboratory is an approach frequently used to investigate the mode of drug action, estimate the risk of emergence of drug resistance, or develop molecular markers for drug resistance. Here, we focused on the How rather than the Why of laboratory selection, discussing different experimental set-ups based on research examples with Trypanosoma brucei, Trypanosoma cruzi, and Leishmania spp. The trypanosomatids are particularly well-suited to illustrate different strategies of selecting for drug resistance, since it was with African trypanosomes that Paul Ehrlich performed such an experiment for the first time, more than a century ago. While breakthroughs in reverse genetics and genome editing have greatly facilitated the identification and validation of candidate resistance mutations in the trypanosomatids, the forward selection of drug-resistant mutants still relies on standard in vivo models and in vitro culture systems. Critical questions are: is selection for drug resistance performed in vivo or in vitro? With the mammalian or with the insect stages of the parasites? Under steady pressure or by sudden shock? Is a mutagen used? While there is no bona fide best approach, we think that a methodical consideration of these questions provides a helpful framework for selection of parasites for drug resistance in the laboratory.

Eimeria tenella: experimental studies on the development of resistance to amprolium, clopidol and methyl benzoquate

The development of resistance by the Houghton strain of Eimeria tenella to the anticoccidial drugs amprolium, clopidol and methyl benzoquate has been studied. Resistance to amprolium and clopidol developed more readily in experiments where a large number of coccidia were exposed to the drug, either by increasing the number of oocysts in the inoculum or by increasing the number of birds in the group. When 45 birds were given 2.0 X 10(6) oocysts, resistance to amprolium and clopidol appeared after 6 and 7 passages respectively. In previous experiments, under similar conditions, resistance to robenidine developed after 6 passages, suggesting little difference between these three drugs. Resistance to amprolium and clopidol arose gradually as the concentration of drug was increased, but resistance to methyl benzoquate appeared in a single step from sensitivity to high-level resistance. Both amprolium and clopidol-resistant lines showed an 8-fold reduction in drug sensitivity. Attempts to measure the degree of resistance by calculation of the ED50 were unsuccessful.

The resistance to human plasma of Trypanosoma brucei, T. rhodesiense and T. gambiense: III. Clones of two plasma-resistant strains

Tests for resistance to human plasma were made on six clones of a stabilate of Trypanosoma rhodesiense (LUMP 10) which was calculated to contain about 3,000 resistant trypanosomes per million. Two of the clones were not resistant and four were only subresistant. Tests were also made on 12 lines (clones) of a stabilate of polymorphic trypanosomes isolated from tsetse flies. One of them, ETAT 10, had infected a laboratory worker and was found to be fully resistant to human plasma; the other lines showed only low or moderate resistance. Resistance of a strain to human plasma often depends upon a small minority of resistant trypanosomes. Strains of polymorphic trypanosomes may be classified as fully resistant, moderately resistant, subresistant, or sensitive to human plasma, if they contain respectively, all, some (e.g. one per hundred), very few (e.g. one per million) or no individuals which are resistant.

The resistance to human plasma of Trypanosoma brucei, T. rhodesiense and T. gambiense. I. Analysis of the composition of trypanosome strains

The sensitivity of strains of polymorphic trypanosomes to human plasma was investigated in mice. By measuring the prepatent period an approximate estimate could be made of how many trypanosomes resisted each of a graded series of doses of plasma and survived to produce infection. In this way the composition of typical strains could be analysed. Three types of strains could be recognized: (i) Strains in which all the individuals composing them were sensitive to full doses of plasma, the response to plasma probably having a "normal" distribution. These are termed "sensitive" strains. (ii) Strains composed mostly of sensitive individuals but containing a small sub-population of resistant individuals (perhaps one in a million). These are termed "subresistant" strains. Most of the strains which have been isolated from animals by previous workers and found to be resistant to human serum and/or infective for volunteers are of this type. (iii) Strains composed mostly of resistant individuals. These are "highly resistant" strains and are the type isolated from man. It is postulated that there are one (or two) genes responsible for resistance to human plasma and that the response of a strain in plasma-sensitivity tests depends upon whether the strain contains no trypanosomes with the R gene (sensitive strain), a few trypanosomes with the R gene (subresistant strain) or a jamority with the R gene (resistant strain). This model of a subresistant strain can be reproduced artificially by mixing a few resistant trypanosomes with a large number of sensitive ones. Passage of three resistant strains through mice for six weeks diminished their plasma resistance slightly to moderately, presumably through overgrowth of sensitive individuals. Passage trrough goats for six weeks diminished plasma resistance markedly but did not convert the strains into "sensitive" ones. Repeated exposure of a subresistant strain to human plasma in mice gradually increases the number of resistant trypanosomes present and so the resistance of a strain as a whole is enhanced.