Eukaryotic Microorganisms

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Eukaryotic Microorganisms

WHY YOU NEED TO KNOW

HISTORY

The bacterial pathogen Yersinia pestis has been identified as the causative agent of the “Black Death,” or bubonic plague, which killed about a third of the known world’s population during several centuries of the Middle Ages. A viral pathogen (influenza virus A subtype H1N1) caused the Great Influenza Pandemic of 1918, which took an estimated 100 million lives over a period of about 9 months in 1918 to 1919! On occasion there are related current outbreaks of bird flu and bubonic plague. The threat of these pandemics is so horrendous that there is a tendency to underplay the significance of other groups of organisms.

However, fungi, algae, protozoans, and helminths have also proven to be significant players in the field of human pathogens. For example, a type of algae containing a neurotoxin is responsible for “red tide” fish kill outbreaks that periodically occur off the shores of the Pacific, New England, and Gulf Coast states. These algae release a cellular product, saxitoxin, which is toxic to fish and other marine life. When shellfish from contaminated waters is consumed by humans it can cause serious neuromuscular problems and/or death, if the condition is untreated. Other protozoans are human pathogens responsible for such diseases as amoebic dysentery, malaria, and toxoplasmosis, to name but a few. Interactions between helminths and humans also result in diseases such as trichinosis, tapeworm infections, elephantiasis, and schistosomiasis. Virtually all the diseases mentioned have been recorded throughout the history of human diseases.

Moreover, in cases such as malaria, once the complex life cycle of the organism (Plasmodium spp.) and its vector (Anopheles spp.) was uncovered and understood, great strides were made in eradicating the disease in developed countries, but it is far from eradicated in developing countries. Malaria remains one the most costly and prevalent unconquered human diseases at large in the world. One of the first recorded references to the disease can be found in early Chinese and Hindu writings. It is estimated that malaria—the name comes from the Medieval Italian mal aria, or bad air, because it was associated with the foul-smelling odors of the swamps near Rome—has been a pathogen of humans throughout all of recorded history. In the late fifth century bce, Hippocrates described aspects of the disease in his writings. A series of discoveries were made in the late 1800s by independent researchers and physicians that led to a complete picture of the life cycle of Plasmodium, the organism responsible for causing malaria. Sir Ronald Ross, a British army surgeon who, using birds as experimental models, established the major stages of the life cycle, in 1902 received the Nobel Prize for his work. Malaria accounted for much suffering and death among troops participating in the American Civil War, the Spanish–American War, World War II, the Korean War, and the Vietnam War. In an attempt to free the world of malaria, in 1955 the World Health Organization (WHO) began a worldwide program to eradicate the disease. Unfortunately, the program collapsed by 1976 because of the emergence of resistance to DDT (dichlorodiphenyltrichloroethane) by the mosquito vectors that play a critical part in the life cycle and the development of resistance to chloroquine, the drug used to fight Plasmodium, the causative protozoan agent.

IMPACT

Malaria is endemic in countries in a belt area roughly about the equator in tropical and subtropical climates. These equatorial countries include South and Central America, Africa, China, the Middle East, India, and Asia. This disease is responsible for about half a billion infections per year; 1 to 3 million deaths per year or, as you read this, about a death every half a minute! If the disease remains uncontrolled, this death rate is estimated to double in 20 years.

Malaria, being a disease associated with poverty, is also a hindrance to economic progress, particularly where it is endemic in developing countries. The disease causes loss of work days, school days, and loss of days/weeks for the caregivers who must be absent from their occupations, not to mention loss of investments from tourism. For example, affected areas such as sub-Saharan Africa, where about 90% of the fatalities occur, cost Africa $12 billion annually, which is about 40% of the total public health expenditure.

With the recognition of extraerythrocytic (latent liver) forms of the disease in the 1980s, new strategies have been viewed to institute better treatment and/or a potential cure or vaccine.

FUTURE

Protection from severe endemic diseases such as malaria is best achieved with vaccines. Some of the ideal characteristics of a vaccine include safety, simple application, life-long protection, low cost, and ease of distribution, to mention a few. Not all these features are obtainable and are generally approached to varying degrees. Prevention and treatment of malaria begin with eradication of the female Anopheles mosquito vector followed by the development of a vaccine and treatments that can attack the sporozoite, liver, merozoite, or sexual stages. All of this must be cost-effective as sub-Saharan Africa is also beset by HIV/AIDS and tuberculosis. Given the socioeconomic environment, additional factors of application of successful drug candidates come to play important roles. The Malaria Vaccine Initiative (MVI), sponsored by the Bill & Melinda Gates Foundation; Roll Back Malaria (RBM); and Medicines for Malaria Venture (MMV) of the WHO are examples of complementing programs currently in the works.

Introduction

Eukaryotic organisms differ from bacteria and archaea in many ways including cell size, internal structure, and genetic properties (see Chapter 3, Cell Structure and Function). The classification of eukaryotes has changed over the centuries starting in the late eighteenth century with Linnaeus, who classified all organisms as either plants or animals. At present, instead of the traditional classification schemes, many taxonomists favor classification schemes that are based on distinguishing characteristics at the molecular level. Modern taxonomy attempts to explain the genetic relationships of microbes and place them in groups based on similarities in their biochemical and metabolic characteristics, their nucleotide sequences, as well as their cellular ultrastructure as revealed by electron microscopy. Although universal support for a particular classification scheme has not been reached, many taxonomists favor a scheme similar to the one shown in Figure 8.1.

Many eukaryotic organisms are pathogens, and according to the World Health Organization (WHO, Geneva, Switzerland), parasitic diseases rank among the top 20 microbial causes of death in the world, especially in the developing countries. These pathogens are not only fungi, algae, and protozoans, but also include parasitic helminths. Although helminths are not microorganisms, they play an important role in infectious disease and for that reason are also addressed in this chapter.

Fungi

The study of fungi is called mycology, and although more than 100,000 species of fungi are known, only about 200 are pathogenic to humans and animals, and relatively few fungi are virulent enough to be considered primary pathogens (Table 8.1). Moreover, the incidence of fungal infections has been increasing, especially those acquired in hospitals (nosocomial), other institutional settings such as nursing homes, and in people with a compromised immune system.

TABLE 8.1

Primary and Opportunistic Fungal Pathogens

Organism Reservoir Transmission Clinical Manifestation
Primary Pathogens
Blastomyces dermatitidis Soil and organic debris; endemic area southeastern U.S., Ohio and Mississippi River Valleys Inhalation of conidia Primary pulmonary blastomycosis; chronic pulmonary blastomycosis; disseminated blastomycosis (cutaneous, bone, genitourinary tract, brain)
Coccidioides immitis Desert soil; southwestern U.S.; Mexico; certain regions of Central and South America Inhalation of conidia Initial pulmonary infection; chronic pulmonary coccidioidomycosis; disseminated coccidioidomycosis (meningitis, bone, joints, genitourinary tract, cutaneous, ophthalmic)
Histoplasma capsulatum Soil infested with bird/bat guano; eastern half of U.S.; most of Latin America; parts of Asia, Europe, and Middle East Inhalation of conidia Clinically asymptomatic pulmonary and “cryptic dissemination”; acute pulmonary histoplasmosis, mediastinitis, pericarditis; chronic pulmonary histoplasmosis
Paracoccidioides brasiliensis Soil and vegetation; Central and South America Inhalation of conidia Diverse clinical manifestations; chronic multifocal involvement (lungs, mouth, nose); juvenile progressive disease (lymph nodes, skin, and viscera)
Opportunistic Pathogens
Candida spp. Gastrointestinal mucosa, vaginal mucosa, skin, nails Gastrointestinal translocation, intravascular catheters Mucocutaneous candidiasis; oral/vaginal thrush; hematogenous dissemination; hepatosplenic candidiasis; endophthalmitis
Cryptococcus neoformans Soil infested with bird guano Inhalation of aerosolized yeast; percutaneous inoculation Primary cryptoccal pneumonia; meningitis (particularly in HIV-infected patients); hematogenous dissemination; genitourinary cryptococcosis; primary cutaneous cryptococcosis
Aspergillus spp. Soil, plants, water, pepper, air Inhalation of conidia; transfer to wounds via contaminated bandages and/or tape Allergic bronchopulmonary aspergillosis, sinusitis; aspergilloma; invasive aspergillosis (lung, brain, skin, gastrointestinal tract, heart)

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Fungi have important symbiotic relationships with other organisms and therefore can be beneficial to life in general. For example, they play a major role in the decomposition of dead plant material, and they are the primary decomposers of material such as cellulose, which cannot be digested by animals. The majority of plants are dependent on their symbiotic relationship with mycorrhizae (fungi that help the roots of plants to absorb minerals and water from the soil). Fungi provide food for humans in the form of mushrooms and truffles, and they are used in the manufacture of foods and beverages, including breads, alcoholic beverages, citric acid, soy sauce, and some cheeses. Fungi also produce pharmaceuticals such as the antibiotics penicillin and cephalosporin (see Chapter 22, Antimicrobial Drugs, for details). In addition, fungi are of use in the making of the immunosuppressive drug cyclosporine for organ transplant patients, and mevinic acids, which are used in the manufacture of cholesterol-reducing drugs. Fungi are also important research tools in genetics and biotechnology (see Chapter 25, Biotechnology).

Characteristics of Fungi

Fungi are eukaryotes but are quite different from plants and animals. Fungal colonies are vegetative structures because they are made up of cells involved in catabolism and growth. Whether microscopic or macroscopic, they generally grow as filamentous, multinucleated organisms that form threadlike filaments called hyphae, or as unicellular fungal organisms (Figure 8.2, A). Morphologically, fungi are often divided into three groups.

Most fungi that are identified as human pathogens are microscopic. Hyphae are long, branching filamentous cells of a fungus and they represent its main vegetative growth. A collective mass of the threadlike hyphae is called the mycelium (Figure 8.2, B). A mycelium can be small, forming colonies too small to see with the naked eye, or they can be extensive, such as Armillaria gallica, which was described as extending through 37 acres of forest soil and was estimated to weigh at least 9700 kg.

Depending on environmental conditions such as temperature, fungi can exhibit different morphological characteristics, a condition referred to as dimorphism. They may appear as yeasts, molds, or fleshy fungi. For instance, some fungi can grow as a mold at room temperature (about 25° C) and as yeast at 37° C. Fungi often adapt to an environment that is hostile to bacteria. They are free-living and heterotrophic organisms (i.e., they use organic compounds as their source of carbon). Although fungi, like bacteria, absorb nutrients rather than ingest them, they differ in several nutritional characteristics as well:

Fungi have a cell wall, typically composed of a strong, flexible, nitrogenous polysaccharide called chitin, and a plasma membrane that contains ergosterol. Fungi differ from plants because they lack chlorophyll and therefore do not use photosynthesis in their metabolic processes. Moreover, their ribosomal RNA is an 80S rRNA, which is the same as in humans and animals. For medical/treatment purposes, important aspects of fungal metabolism include the following:

Fungal pathogens can be classified on the basis of their growth forms—filamentous, yeast, or dimorphic—and on the type of infection they can cause. For example, in superficial mycoses, the fungus grows on the surface of the skin or hair. In cutaneous and subcutaneous mycoses, nails and deeper layers of the skin are involved, and in systemic (deep) mycoses, the fungus spreads to internal organs (Box 8.1). Patients with a compromised immune system are especially susceptible to fungal infections. Whereas superficial and cutaneous mycoses are generally mild, systemic mycoses are serious and difficult to treat.

Yeasts

Yeasts exist as single cells that reproduce by budding. However, some species may become multicellular through the formation of connected budding cells referred to as pseudohyphae, or true hyphae (Figure 8.3). They can be classified according to the presence or absence of capsules, the size and shape of the cells, the mechanism of daughter cell formation, formation of hyphae, presence of sexual spores, and other physiological/genetic data. Yeasts may require oxygen for their metabolic activities as obligate aerobes or may be able to continue their metabolic activities in the absence of oxygen as facultative anaerobes. Yeasts do not require light for their growth.

Molds

Molds are rapidly growing, asexually reproducing fungi that grow on a variety of substances. They are characterized by the development of hyphae (long filaments of cells joined together), which results in colony characteristics that can be used for identification purposes. Hyphae can grow to immense proportions (see Armillaria gallica, earlier this chapter). In general, the hyphae in molds contain cross-walls called septa, which divide the hyphae into distinct uninuclear, cell-like units called septate hyphae. Some classes of fungi have coenocytic hyphae, which do not contain septa and appear as long, continuous cells with many nuclei (Figure 8.4). In an appropriate environment, the hyphae grow to form a mycelium that is visible to the naked eye.

Dimorphic Fungi

In general, microscopic fungi can be divided into two basic morphological forms: yeast and molds (hyphae). Some fungi can exhibit both growth forms depending on the conditions in which growth occurs, especially temperature. These fungi are called dimorphic fungi. Their patterns of growth are as follows:

Several dimorphic fungi can cause systemic mycoses (see Table 8.1) that usually start by the inhalation of spores from the mold form and then germinate in the lungs, where the fungus grows as yeast. These fungi include, but are not limited to, Sporothrix schenckii (sporotrichosis), Histoplasma capsulatum (histoplasmosis), Blastomyces dermatitidis (blastomycosis), Paracoccidioides brasiliensis (paracoccidioidomycosis), and Coccidioides spp. (coccidioidomycosis).

Life Cycle of Fungi

Although all fungi have some means of asexual reproduction (mitosis), most fungi can also reproduce sexually. Asexual reproduction does not involve genetic recombination between two sexual types, whereas sexual reproduction does involve genetic recombination. Sexual reproduction, as expected, introduces variation in a population.

Asexual Reproduction

Yeast typically bud in a manner similar to the binary fission of prokaryotic organisms (see Chapter 3, Cell Structure and Function; and Chapter 6, Bacteria and Archaea). Filamentous fungi can reproduce asexually by fragmentation of their hyphae, and by spore formation either sexually or asexually. Asexual reproduction results in the formation of sporangia, which ultimately release spores into the environment. Spores, after entering the respiratory tract, are common causes of infection (see Chapter 11, Infections of the Respiratory System). These asexual spores are often categorized on the basis of the nature and manner of spore development:

• Sporangiospores form inside a sac, the sporangium, which is attached to a stalk, the sporangiophore, located on the tips or sides of hyphae. The spores are released when the sporangium ruptures.

• Conidia (conidiospores) are produced at the tips or sides of hyphae but are not enclosed by a saclike structure. Their development involves the pinching off of the tip of a fertile hypha or the segmentation of a preexisting vegetative hypha. They are the most common of the asexual spores and exist in different forms (Figure 8.5):

Sexual Reproduction

Sexual reproduction introduces genetic variation into a population. The sexual reproduction phase of fungi, unlike asexual spore formation, introduces variable characteristics into its population. Moreover, sexual reproduction in fungi occurs when nutrients are limited, and/or other conditions are unfavorable for growth. Therefore, by this mechanism, survival of the species through genetic changes is made possible.

Because fungal mycelia are morphologically indistinguishable, rather than using the terms female and male, fungal mating types are designated as “+” and “−.” Sexual reproduction of fungi can be summarized into four basic steps, with some differences occurring between groups.