Pathogenicity and Other Microbes

Fungi

A fungus is a member of a large group of eukaryotic organisms that includes microorganisms that exhibit pathogenicity.

Learning Objectives

Give examples of pathogenic fungi

Key Takeaways

Key Points

  • There are various examples of pathogenic fungi including but not limited too: Candida species, Aspergillosis, Cryptococcus, Histoplasma, Pneumocystis and Stachybotrys.
  • Many fungal species produce bioactive compounds called mycotoxins, such as alkaloids and polyketides that are toxic to animals including humans, contributing to pathogenecity and disease.
  • The study of pathogenic fungi is referred to as a medical mycology.

Key Terms

  • symbiont: An organism that lives in a symbiotic relationship; a symbiote.
  • mycotoxin: Any substance produced by a mold or fungus that is injurious to vertebrates upon ingestion, inhalation, or skin contact.
  • opportunist: when an organism takes advantage of any opportunity to advance its own situation.

A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds. These organisms are classified as kingdom Fungi, separate from plants, animals, and bacteria. Fungi have a worldwide distribution and cangrow in a wide range of habitats, including extreme environments such as deserts or areas with high salt concentrations or ionizing radiation, as well as in deep sea sediments. Most fungi are inconspicuous because of the small size of their structures and their cryptic lifestyles in soil, on dead matter, and as symbionts of plants, animals, or other fungi. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange. Many fungal species produce bioactive compounds called mycotoxins, such as alkaloids and polyketides that are toxic to animals including humans, contributing to pathogenecity and disease.

The study of pathogenic fungi is referred to as a medical mycology. There are various examples of pathogenic fungi including but not limited too: Candida species, Aspergillosis, Cryptococcus, Histoplasma, Pneumocystis and Stachybotrys.

Candida species are commonly known to cause opportunist infections in immunocompromised hosts. The immunocompromised hosts that commonly become infected with Candida include transplant patients, cancer patients and AIDS sufferers. Candida infections are difficult to treat and can result in systemic infections leading to death.

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Candida: A Candida infection seen from a pap test specimen.

One of the most commons fungal pathogenic species includes Aspergillus strains, specifically Aspergillus fumigatus and Aspergillus flavus. Aspergillus can cause disease via production of mycotoxins, induction of allergic responses and through localized or systemic infections. Aspergillus flavus specifically produces aflatoxin which is both a toxin and carcinogen whereas Aspergillus fumigatus causes allergic disease. Symptoms of diseases caused by Aspergillus can include fever, cough, chest pain or breathlessness.

Cryptococcus neoformans causes severe forms of meningitis and meningo-encephalitis in patients with HIV infection and AIDS. Cryptococcus species live in the soil and do not cause disease in humans thus, Cryptococcus neoformans is the major pathogen in both human and animals.

Histoplasma capsulatum results in the formation of histoplasmosis in humans, dogs and cats. This specific fungus is endemic in certain areas of the United States and infection is due to inhaling contaminated air.

Pneumocystis jirovecii results in the formation of pneumonia in individuals with weakened immune systems including premature children, the elderly and AIDS patients.

Stachybotrys chartarum, also referred to as black mold, causes respiratory damage and severe headaches. This type of black mold frequently occurs in households that are chronically damp.

Protozoa

Protozoa are a diverse group of unicellular eukaryotic organisms, many of which can cause disease.

Learning Objectives

Compare and contrast the proliferative and dormant stages in pathogenic protozoa and the diseases protazoa cause

Key Takeaways

Key Points

  • Examples of human diseases caused by protozoa are: malaria, amoebiasis, giardiasis, toxoplasmosis, cryptosporidiosis, trichomoniasis, Chagas disease, leishmaniasis, and dysentery.
  • The life stages of these protozoa play a major role in their ability to function as pathogens and infect various hosts.
  • Protozoa were regarded as the partner-group of protists to protophyta, which have plant-like behavior (e.g., photosynthesis). In general, protozoa are referred to as animal-like protists because they are capable of movement, or motile.
  • Some protozoa are human parasites, causing diseases.

Key Terms

  • trophozoite: A protozoan in the feeding stage of its life cycle.
  • protozoa: Protozoa are a diverse group of unicellular eukaryotic organisms, many of which are motile. Originally, protozoa had been defined as unicellular protists with animal-like behavior, e.g., movement. Protozoa were regarded as the partner group of protists to protophyta, which have plant-like behavior, e.g., photosynthesis.
  • dormant cyst: A resting or dormant stage of a microorganism
  • cyst: a pouch or sac without opening, usually membranous and containing morbid matter, which develops in one of the natural cavities or in the substance of an organ

Protozoa (or protozoans) are a diverse group of unicellular eukaryotic organisms, many of which are motile. Originally, protozoa had been defined as unicellular protists with animal-like behavior (e.g., movement). Protozoa were regarded as the partner-group of protists to protophyta, which have plant-like behavior (e.g., photosynthesis). In general, protozoa are referred to as animal-like protists because they are capable of movement, or motile. While there is no exact definition for the term protozoa, it often refers to a unicellular heterotrophic protist, such as the amoebas and ciliates.

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Leishmania donovani: Leishmania donovani, (a species of protozoa) in a bone marrow cell.

Protozoa can display pathogenicity and are the cause of various diseases. The life stages of these protozoa play a major role in their ability to function as pathogens and infect various hosts. Some protozoa have life stages alternating between proliferative stages (e.g., trophozoites ) and dormant cysts. As cysts, protozoa can survive harsh conditions, such as exposure to extreme temperatures or harmful chemicals, or long periods without access to nutrients, water, or oxygen for a period of time. The ability of protozoa to thrive under extreme environments contributes to their ability to evade immune system responses, drug therapies and survive for prolonged periods of time before infection. Being a cyst enables parasitic species to survive outside of a host, and allows their transmission from one host to another. When protozoa are in the form of trophozoites they actively feed. The conversion of a trophozoite to cyst form is known as encystation, while the process of transforming back into a trophozoite is known as excystation.

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Malaria Life Cycle: Example of a life cycle promoting pathogenicity of a protozoa, specifically the malaria parasite.

Protozoa such as the malaria parasites (Plasmodium spp. ), trypanosomes, and leishmania are also important as parasites and symbionts of multicellular animals. Examples of human diseases caused by protozoa are: malaria, amoebiasis, giardiasis, toxoplasmosis, cryptosporidiosis, trichomoniasis, Chagas disease, leishmaniasis, and dysentery. The life cycle of protozoan are successful based on successful transmission between hosts and host and environment. Infection and disease by protozoan parasites are often times associated with developing countries with poor hygiene and sanitation conditions that may promote transmission of these protozoa.

Helminths

Parasitic worms, often referred to as helminths, are a division of eukaryotic parasites.

Learning Objectives

List the four groups of parasitic worms (helminths), routes of transmission and risk factors

Key Takeaways

Key Points

  • Helminths are worm-like organisms that live and feed off of living hosts, receiving nourishment and protection while disrupting the nutrient absorption of their hosts, which causes weakness and disease.
  • Helminths that live inside the digestive tract are called intestinal parasites.
  • Helminths often find their way into a host through contaminated food or water, soil, mosquito bites, and sexual acts.
  • Response to worm infection in humans is a Th2 response in the majority of cases.

Key Terms

  • helminth: A parasitic roundworm or flatworm.
  • lymphatic system: In mammals, including humans, a network of lymph vessels and lymph nodes that transport fluid, fats, proteins, and lymphocytes to the bloodstream as lymph, and remove microorganisms and other debris from tissues.

Parasitic worms, often referred to as helminths, are a division of eukaryotic parasites. They are worm-like organisms that live and feed off of living hosts, receiving nourishment and protection while disrupting the nutrient absorption of their hosts, which causes weakness and disease. Those that live inside the digestive tract are called intestinal parasites. They can live inside humans as well as other animals.

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Hookworms: The hookworms attached to the intestinal mucosa.

Parasitic worms belong to four groups:

  • Monogeneans
  • Cestodes (tapeworms)
  • Nematodes (roundworms)
  • Trematodes (flukes)

Helminths often find their way into a host through contaminated food or water, soil, mosquito bites, and even sexual acts. Poorly washed vegetables eaten raw may contain eggs of nematodes such as Ascaris, Enterobius, Thichuris, and or cestodes such as Taenia, Hymenolepis, and Echinococcus. Plants may also be contaminated with fluke metacercaria, such as Fasciola. Schistosomes and nematodes such as hookworms (Ancylostoma an Necator) and Strongyloides can penetrate the skin. Finally, Wuchereria, Onchocerca, and Dracunculus are transmitted by mosquitoes and flies.

Populations in the developing world are at particular risk for infestation with parasitic worms. Risk factors include the following:

  • Inadequate water treatment
  • Use of contaminated water for drinking, cooking, washing food, and irrigation
  • Undercooked food of animal origin
  • Walking barefoot

Simple measures—such as use of shoes, soaking vegetables with 1.5% bleach, adequate cooking of foods (not microwaving), and sleeping under mosquito-proof nets—can have a strong impact on prevention.

Response to worm infection in humans is a Th2 response in the majority of cases. Inflammation of the gut may also occur, resulting in cyst-like structures forming around the egg deposits throughout the body. The host’s lymphatic system is also increasingly taxed the longer helminths propagate, as they excrete toxins after feeding. These toxins are released into the intestines and absorbed by the host’s bloodstream, making the host susceptible to more common diseases such as seasonal viruses and bacterial infections.

Parasitic worms have been used as a medical treatment for various diseases, particularly those involving an overactive immune response. As humans have evolved with parasitic worms, proponents argue that they are needed for a healthy immune system. Scientists are looking to see if there is a connection between the prevention and control of parasitic worms and the increase in allergies such as hay-fever in developed countries. Parasitic worms may be able to damp down the immune system of their host, making it easier for them to live in the intestine without coming under attack. This may be one mechanism for their proposed medicinal effect.

Algae

Algae can act as pathogens like any other microbe.

Learning Objectives

Discuss the various types of pathogenic algae

Key Takeaways

Key Points

  • While algal blooms can lead to negative consequences, the effect of an algal bloom are often indirect, the alga is not directly infecting a host.
  • Cephaleuros are a genus of parasitic alga which infect plants, causing red rust, which affects many commercial crops that humans consume.
  • Prototheca are a type of green alga that lack chlorophyll, that can infect mammals including humans causing the disease protothecosis.
  • To some degree the distribution of algae is subject to floristic discontinuities caused by geographical features, such as Antarctica, long distances of ocean, or general land masses.

Key Terms

  • thalloid: Of a plant, alga, or fungus lacking complex organization, especially lacking distinct stems, roots, or leaves.
  • alga: any of many aquatic photosynthetic organisms, whose size ranges from a single cell to giant kelps and whose form is very diverse
  • basionym: An earlier valid scientific name of a species that has since been renamed and from which the new name is partially derived.
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Cephaleuros virescens: Infestation of the algal leaf spot (Cephaleuros virescens) on ther southern magnolia (Magnolia grandiflora); Green-orange algal spots or “green scruf” on leaf surface. The grayish-white and darker “crusts” are lichens of the genus Strigula resulting from fungal colonization of the alga.

Algae, are not normally considered common pathogens. Algal blooms are often associated with negative impacts on humans and the surrounding environment in which they occur. A harmful algal bloom (HAB) is an algal bloom that causes negative impacts to other organisms via production of natural toxins, mechanical damage to other organisms, or by other means. HABs are often associated with large-scale marine mortality events and have been associated with various types of shellfish poisonings. However, the damage to other organisms is not due to the algae infecting a host but rather indirectly excreting a toxin, or in some cases blocking out light or competing for resources.

However notable examples of algae acting as pathogens are known. For example Cephaleuros which is a genus of parasitic thalloid alga comprising approximately 14 species. Its common name is red rust. Chrooderma is its basionym. Specimens can reach around 10 mm in size. Dichotomous branches are formed. The alga is parasitic on some important economic plants of the tropics and subtropics such as tea, coffee, mango and guava causing damage limited to the area of algal growth on leaves (algal leaf spot), or killing new shoots, or disfiguring fruit. Members of the genera may also grow with a fungus to form a lichen that does not damage the plants.

Examples of algae acting as a mammalian pathogen are known as well, notably the disease Protothecosis. Protothecosis is a disease found in dogs, cats, cattle, and humans caused by a type of green alga known as Prototheca that lacks chlorophyll. It and its close relative Helicosporidium are unusual in that they are actually green algae that have become parasites.The two most common species are Prototheca wickerhamii and Prototheca zopfii. Both are known to cause disease in dogs, while most human cases are caused by P. wickerhami. Prototheca is found worldwide in sewage and soil. Infection is rare despite high exposure, and can be related to a defective immune system. In dogs, females and Collies are most commonly affected. The first human case was identified in 1964 in Sierra Leone.