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2.4: Eukaryotic Pathogens and Hosts

-uni- or multicellular organisms with cells containing membrane-bound nucleus and other membrane-bound organelles such as mitochondria .

(uni= 1 cell or multicellular=made of many cells;See figure 2.18 p 46 andpages 473-474 in Belken’s Biology; note photo of “malaria pathogen” on p 474 is incorrect)

Domain Eukarya: 4 kingdoms (based on “old” Whittaker 5 Kingdom taxonomy):

a. Kingdom Protista e.g. protozoa

                                                e.g. Plasmodium/malaria

b. Kingdom Fungi  e.g. yeasts and molds

                                                e.g. Coccidioides immitis/San Joaquin Valley Fever

Candida albicans/opportunistic infections , “thrush”, vaginal yeast infections

                                c. Kingdom Plantae: plants

                                d. Kingdom Animalia:

                                                -includes large, multicellular endo- and ecto-parasites

                                                -includes helminth/worm parasites eg filarial worms

                                                -includes arthropod vectors e.g. mosquitoes, ticks, fleas





















Fig 2: composite cells            a=prokaryotic cell  ( see fig  2.17 p  41 Belk’s Biology)

                  b=eukaryotic cell (see fig 2.18 and Table 2.1 p 42-46 in Biology)

Table 1: Classification of pathogens and hosts

Acellular microbes= not made of cells

Cellular organisms= made of cells

Prions: “misfolded” proteins causing “TSE’s” , Transmissible Spongiform Encephalopathies


Viruses: RNA or DNA genomes + protein coat +/- envelope

         -obligate intracellular parasites




    -Domain Bacteria

          -includes bacterial pathogens

  -Domain Archaea


   -Domain Eukarya

        Kingdom Protista

             e.g. protozoa

        Kingdom Fungi

             e.g. yeasts and molds

        Kingdom Plantae

        Kingdom Animalia

           e.g. ecto- and endo-parasites

                  vertebrates including humans




Table 2: Specific examples of agents of infectious diseases/pathogens


                Acellular agents                                                                   Cellular


prions                                     viruses                   prokaryotes                                          eukaryotes


spongiform encephalopathies                  HIV                          Bacteria:                                                  Protozoa:

“Mad Cow Disease”                 Ebola                        Mycobacterium tuberculosis/TB                              Plasmodium/malaria

Kuru                                         Hanta                       Staphylococcus                                        Cryptosporidium

Creutzfeldt-Jakobs Disease       Polio                        Bacillus anthracis/anthrax                       Giardia                   

Chronic Wasting Disease          Rabies                      E. coli                                                       Trypanosoma/sleeping sickness

                                                Hepatitis                  Salmonella

                                                Smallpox                  Chlamydia/STD                                        Fungi:

                                                                                Treponema pallidum/syphilis                   Candida/yeast






                                                                                                                                                mosquitoes, ticks, fleas




All microbes and hosts




























Fig 3: Cast of Characters Tree








Fig 4 Tree of Life         see fig 2.19 p 47 Belk’s Biology














Microbial hosts

Hosts are the organisms in which microbial pathogens replicate. Microbes can infect all cellular organisms therefore all cellular organism may act as microbial hosts.


  • Bacterial hosts and bacteriophage therapy: microbes as therapeutic agents:  Bacteria (unicellular prokaryotes) can be infected and killed by bacterial viruses called bacteriophage.  Bacteriophage therapy is the use of bacterial viruses to kill pathogenic bacteria. Bacteriophage therapy was first proposed during World War I and was actively pursued in the former Soviet Union. During World War II when the first antibiotic penicillin was introduced, bacteriophage therapy fell out of favor. With the increase in antibiotic resistant bacteria, there is renewed interest in developing bacteriophage as a way to treat antibiotic resistant bacteria and to treat food which may be contaminated with  pathogenic bacteria (e.g. phage are used to kill Listeria monocytogenes in contaminated foods)
  • Eukaryotic hosts: All eukaryotic organisms (“protists”, fungi, plants and animals) can be infected by microbial pathogens.
  • Plants as hosts for microbial pathogens; impact on human health:  Microbial diseases of plants cause immense losses of food worldwide. Lack of food, malnutrition and starvation greatly reduce animals/humans ability to resist infection by pathogen thus plant diseases indirectly increase infectious diseases in humans.


-Historically plant diseases have had huge impacts on human history. Two examples are the “water mold” Phytophthora infestans andthe fungus Claviceps purpurea

-Late blight of potatoes caused by Pytophthora infestans triggered failure of the Irish potato crop with subsequent starvation. As a result, many poor Irish emigrated to the US, Canada, Australia. Phytophthora infestans continues to cause potato disease and starvation in parts of the world.

- The fungus/mold Claviceps purpurea is a plant pathogenthat cause “ergot poisoning” in animals/humans. Claviceps grows on moist grain and produces a heat-stable toxin, ergotamine. Ergotamine’s chemical structure is similar to LSD and is a powerful hallucinogen. IfClaviceps-contaminated grain is made into flour and subsequently into bread, th ergotamine cause hallucinations, vasoconstriction (blood vessels narrow, decreases blood flow causes necrosis, death of tissues), smooth muscle contraction (triggers abortion) . Ergot poisoning is also called “St Anthony’s fire” possible because the lack of blood flow caused a painful burning sensation. The monks of the order of St. Anthony frequently cared for victims of ergot poisoning. Some historians believe the Salem witch trials were a product of ergot poisoning. Ergot poisoning also occurs in farm animals fed “moldy grain”


Animals as microbial hosts

  • Non-human animals as microbial hosts: Veterinary  infectious diseases (infectious diseases of non-human animals) have a huge impact on human health. 

1. Most human microbial pathogens probably evolved from veterinary pathogens as described in the “Short History of Infectious Diseases” unit.

2. Non-human animals as reservoirs of pathogens for humans. Many medical microbiologists believe most of the new “emerging” diseases will be new zoonoses , diseases which will be transferred to humans from non-human animal reservoirs

3. Infectious diseases of farm animals including food and work animals (example oxen used in plowing/harvesting) have contributed to lack of food and starvation.

-In areas of the world where tractors are not affordable, farmers rely on animals for plowing, harvesting and transporting crops to market and as a source of plant fertilizers in the form of manure. If their animals sicken or die, they cannot grow sufficient food.

-Malnutrition in many parts of the world is caused by lack of protein (kwashiorkor) which animal products (meat, poultry, milk products, eggs) could supply.

-Lack of food/protein decreases humans ability to resist infectious diseases thus veterinary disease indirectly increase human infectious diseases

  • Arthropods as vectors of pathogens: arthropods including insects and arachnids can transfer pathogens from an infected host to a new human host. Biological arthropod vectors become infected with the pathogen which replicates and/or matures within the arthropod host.
  • Humans as microbial hosts: Humans are eukaryotes as are all animals. As discussed below, it is important to be able to classify a pathogen infecting a human so that the proper antimicrobial drug may be selected. Most anti microbial drugs are very selective for a specific group of pathogens thus if the pathogen is incorrectly classified, the incorrect drug will be given with no benefit.  Understanding that humans are eukaryotic organisms also helps us understand why drugs used to inhibit/kill eukaryotic pathogens often have more unpleasant/dangerous side-effects than drugs used to treat bacterial (prokaryotic) pathogens.