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PCAT Biological Processes: Eukaryotic pathogens

  • by Allison Chae
  • Oct 17, 2017
  • Biological Processes, PCAT Blog

Microbiology is a key component of the PCAT because pharmacists dispense many medications that treat infectious diseases. Often, when students hear the terms ‘microbiology’ and ‘infectious diseases,’ they immediately think of bacteria and viruses – and with good reason. Bacteria and viruses are everywhere in our environment, and pathogenic ones cause hundreds of diseases, ranging from strep throat to Ebola. However, some important pathogens are actually eukaryotic! Such pathogens are not affected by antibiotics, and it is a major challenge for drug designers to find ways of targeting eukaryotic pathogens without damaging the host body as well. For this reason, you should be aware of some of the main eukaryotic pathogens for the PCAT.


Fungi are heterotrophic eukaryotes that are defined by the presence of a substance known as chitin in their cell walls. Mushrooms—which can be edible, poisonous, or psychedelic—are probably the best-known example of fungi, but fungi play a surprisingly broad range of roles in the environment and in industrial processes. Some of them can also cause disease by colonizing the human body. Athlete’s foot (or tinea pedis), which involves a scaly, itchy rash, is one of the most familiar examples of a condition caused by a fungal infection. Ringworm is another; despite what its name might suggest, ringworm doesn’t involve worms – instead, it’s a highly contagious fungal infection that manifests as circular rashes on the skin. Both athlete’s foot and ringworm are nonspecific conditions that can be caused by diverse types of fungi. In contrast, candidiasis is a blanket term for infections caused by yeast (an example of fungi) in the Candida genus. Oral candidiasis is known as thrush, and the commonly encountered term ‘yeast infection’ generally refers to vaginal candidiasis. Candidiasis of other parts of the body is more common in immunocompromised individuals.

As we mentioned, targeting eukaryotic pathogens is challenging because drugs are needed that target pathogens without hurting the human body too badly. Many antifungal medications take advantage of the fact that fungal cell membranes contain ergosterol instead of cholesterol, and therefore target ergosterol or its precursors.


Some protozoa can also cause disease in humans. Protozoa are a vaguely-defined category of single-celled eukaryotic organisms that are neither plants nor fungi, and often exhibit characteristics such as motility. Malaria is probably the best-known example of a protozoan disease; it is caused by Plasmodium parasites whose life cycle alternates between mosquitoes and humans. Another well-known example is toxoplasmosis, which is caused by a protozoan known as Toxoplasma gondii that can be spread via poorly-cooked food and the feces of infected cats. This, in combination with the fact that toxoplasmosis can affect developing fetuses, is why pregnant women are told to avoid contact with cat litter.

Antiprotozoan medications are challenging to develop because protozoa are very diverse, making it difficult to find medications that target protozoa without injuring host cells. For that reason, many antiprotozoan medications have highly unpleasant side effects. Interestingly, the mechanism of chloroquine, the first-line medication for malaria, has not been fully established.


Helminths are our final major class of eukaryotic pathogens. Put simply, helminths are worms—that is, multicellular eukaryotic prokaryotes that have a linear, worm-like shape. A surprising variety of helminths exist; they can be categorized as tapeworms, flukes, and roundworms based on characteristics like shape, the presence of an internal body cavity, and the organs they use to attach to their hosts. They reproduce by laying eggs, and their life cycle can depend on the interactions of multiple hosts in specific environmental niches. Helminths can cause long-lasting infections, with examples including tapeworm infections, schistosomiasis, and trichinosis. Antihelminthics, sometimes contracted to anthelminthics, are drugs that target helminths. They have many different mechanisms of action and can be specific to certain subgroups of helminths.

For the PCAT, you should have a general sense of these classes of pathogens and ideally know an example or two of relevant diseases. You don’t have to know much about the medications used to treat them—there will be time for that in pharmacy school!—but you absolutely should understand for the PCAT that antibiotics cannot be successfully used to treat fungal, protozoan, or helminthic infections (the same way antibiotics cannot treat viral infections).

We hope this review was helpful! If you’re still looking for more practice, Next Step offers various PCAT resources (including some that are free!).

If you’re just getting started with the PCAT, we recommend beginning with a diagnostic. Our Free PCAT Bundle includes a full-length diagnostic, our first full-length practice PCAT, a demo of our course, and a sampling of our review videos. You can also check out our free pre-pharmacy webinars for extra help on the PCAT. 

Our online PCAT course, and private, one-on-one PCAT tutoring offer the resources and personalized attention students need to succeed on the PCAT exam. We wish you the best of luck in your journey towards pharmacy school!

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