THE IMPACT OF COMPETITION ON THE EMERGENCE OF DRUG RESISTANCE IN PLASMODIUM CHABAUDI INFECTIONS

Open Access
Author:
Garrity, Bridget
Area of Honors:
Biology
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Andrew Fraser Read, Thesis Supervisor
  • Michael Axtell, Honors Advisor
Keywords:
  • Malaria
  • antimicrobial resistance
  • competition
  • competitive suppression
  • competitive release
  • resistance emergence
Abstract:
Antimicrobial resistance poses a potential major health threat as the rate of resistant infections increases dramatically across the globe. Malaria infections tend to be mixed, meaning they contain multiple different strains of the parasite in the same infection. Because of this, it is also thought to be common for malaria infections to contain both drug resistant and susceptible parasites. It is therefore likely that competition between resistant and susceptible parasites plays a role in parasite density dynamics, including the emergence of resistant parasites to levels capable of causing disease and being transmitted. In this study we use an in-vivo model of malaria to evaluate the role of parasite competition in the presence and absence of drug on the emergence of resistance. To do so, we first developed isogenic drug-resistant lines of the malaria parasite Plasmodium chabaudi, AS-GFP and AS-mCherry, which differ only in their fluorescent markers. We then determined their growth rates in the absence of drugs, and confirmed the development of mutations which confer resistance of these two selected lines to ensure that drug selection had been successful. We then used these lines in competition experiments in which mice were coinfected with atovaquone susceptible and resistant parasites in the presence or absence of drug treatment to evaluate the role of competition on drug resistance emergence. Previous in-vivo competition experiments have been done using strains that are not isogenic, meaning they differ in other factors in addition to resistance. Therefore any conclusions drawn from those experiments could not be directly attributed to competition between resistant and susceptible parasites. The advantage of this experiment is that the parasites competing are isogenic and differ only in their resistance to atovaquone, allowing any conclusions drawn to be directly attributed to the effect of competition between atovaquone resistant and susceptible parasites. Mixed infections parasite densities prior to drug treatment were found to be decreased compared to single infections, demonstrating that the presence of the two competing lines resulted in the suppression of overall parasite densities. The mixed infection that was drug treated with atovaquone had increased resistant parasite densities following drug treatment compared to untreated mixed infections, demonstrating that the removal of competing parasites by drug treatment allowed for the increase in abundance of resistant parasites, otherwise known as competitive release. Therefore, in-vivo competition does play a role in resistance emergence in mixed infections and should be considered when treating infections known to contain resistant microbes.