Pathogenicity

Growth and reproduction are fundamental aspects of the fungal life cycle, and they are important for pathogenic invasion within animals and plants. In general, the majority of species of fungi are saprophytes or parasites, but not pathogens. A parasite is an organism that acquires nutrition from a living host, but the term parasite does not imply a beneficial or harmful interaction. In contrast, a pathogen acquires nutrients at the expense of the host, and this interaction results from physiological changes in the host and subsequent appearance of disease symptoms. At least 8,000 and 100 species of fungi are known pathogens of plants and humans, respectively.13,26

Each of the four phyla in the Kingdom Fungi has pathogenic species, but the Ascomycota contains the largest number of pathogens. Within the Ascomy-cota, most pathogens of plants and animals are restricted to a few taxonomic classes.27 In general, plant pathogens are represented in different classes than animal pathogens. A large percentage of plant pathogens are members of the classes Taphrinomycetes, Sordariomycetes, Leotiomycetes, and Doth-ideomycetes, whereas animal pathogens belong mostly to the Eurotiomycetes and Chaetothyriomycetes.27 In addition, the Basidiomycota, Chytridiomycota, and Zygomycota also contain important pathogens of animals and plants.28,29 Most plant pathogenic fungi can infect and colonize healthy hosts. In contrast, the majority of fungi that cause diseases of humans infect hosts who are debilitated or compromised, and these fungi are known as opportunistic pathogens. However, two exceptions are Coccidioides immitis and Histoplasma capsulatum,30 which infect apparently healthy individuals.

Ascocarp wall

Ascocarp wall

Naked Asci

Perithecium Apothecium

FIGURE 6.4 Fruiting structures (ascocarps) produced by fungi in the phylum Ascomycota with asci and ascospores; (A) Naked asci, (B) Cleistothecium, (C) Perithecium, and (D) Apothecium. Fig. 11.11 from ref. (2). Printed with permission from John Wiley and Sons.

Perithecium Apothecium

FIGURE 6.4 Fruiting structures (ascocarps) produced by fungi in the phylum Ascomycota with asci and ascospores; (A) Naked asci, (B) Cleistothecium, (C) Perithecium, and (D) Apothecium. Fig. 11.11 from ref. (2). Printed with permission from John Wiley and Sons.

Over the last 20 years the steady increase in the number of human infections caused by opportunistic fungi has presented a greater concern to the medical community. This increase in human disease is associated with opportunistic fungal infections in compromised individuals who have AIDS or have

FIGURE 6.5 Different types of basidia produced by fungi in the phylum Basidiomycota. Note basidiospore on tip of sterigma on basidium on the far left. The three basidia to the far right are typical of those produced from teliospores of rust and smut fungi. Modification of Fig. 11 ref. (1). Printed with permission from CAB International.

FIGURE 6.5 Different types of basidia produced by fungi in the phylum Basidiomycota. Note basidiospore on tip of sterigma on basidium on the far left. The three basidia to the far right are typical of those produced from teliospores of rust and smut fungi. Modification of Fig. 11 ref. (1). Printed with permission from CAB International.

FIGURE 6.6 Zygospore and oospore produced by organisms in phyla Zygomycota and Oomycota, respectively (not drawn to scale). Modification of Fig. 10 from ref. (19), and Fig. 8.18 from ref. (2). Printed with permission from John Wiley and Sons.

undergone medical procedures that require immunosuppression, e.g., bone marrow and organ transplants and chemotherapy. Examples of opportunistic pathogens are Cryptococcus neoformans, Aspergillus fumigatus, species of Candida, Aspergillus, Fusarium, Penicillium, and several species within the Zygomycota. In the last few years the number of invasive infections by species of Aspergillus has risen steadily, replacing species of Candida as the mostly commonly encountered fungal pathogen in some institutions.31 Further, species of Fusarium are now recognized as second to species of Aspergillus as a major invasive mold.31 Also, species of Candida other than C. albicans are becoming increasingly more prevalent, possibly due to the extensive use of antifungal drugs toxic to C. albicans.31

The interrelationship between a host and a pathogen is complex and involves specific accommodations by both partners in the interaction. It is not possible to have a disease without a susceptible host, a pathogen, and a favorable environment. Fungi have adapted a number of strategies to infect plants and humans. The most comprehensive studies that involve the interaction of a host and fungal parasite have been conducted on plants. Fungal pathogens are heterotrophs capable of obtaining nutrients from both dead and living sources and can establish three types of trophic (e.g., feeding) relationships with plants: biotrophic, hemi-biotrophic, and necrotrophic. However, these relationships likely represent a continuum of interactions that occur been the host and fungal pathogen. In a biotrophic (obligate parasite) relationship, the fungus can live and multiply only on or within another living organism. Examples of obligate parasites (e.g., biotrophs) include fungi that cause mildew and rust diseases of plants. Interestingly, biotrophic relationships of fungi with animals and humans have not been documented. In a hemi-biotrophic relationship, the fungus is dependent on a living cell early in the infection process but then obtains nutrients from dead plant cells as a necrotroph. Examples of this include the smut and leaf curl diseases of plants. Many hemi-biotrophs have a yeast-like phase early in their life cycle and then convert to mycelial growth habit to cause infection of plants. In general, necrotrophy is the most common type of relationship between plants and fungi. In this type of relationship, the fungus obtains nutrients from dead cells often killed in advance by toxins or enzymes. Examples of necrotrophs include Sclerotinia sclerotiorum, a pathogen of more than 408 species of plants, Bipolaris maydis, the causal agent of Southern corn leaf blight, and species of Cercospora, pathogens of hundreds of plants.

Because host-pathogen interactions are complex, it is often difficult to identify specific factors required for pathogenicity. Consequently, only a few fungal traits have been shown conclusively to be involved in pathogenicity on either plants or humans. Traits identified as being required for pathogenicity include the ability of the fungus to produce adhesive compounds, melanins, toxins, and enzymes. Further, parasitic growth is associated with the tran scription of a suite of genes, the expression of which is coordinated by elaborate signaling networks involved in sensing environmental stimuli and transmitting the molecular communication between the pathogen and the host.32-38

Whether a human or plant pathogen, the ability of the fungus to adhere to the host is of paramount important for pathogenicity. For example, plant pathogens secrete mucilage that allows spores to adhere to plant surfaces.39 Likewise, the ability to adhere to host tissue and avoid expulsion during the early phase of infection has also been shown to be important for fungal pathogens of humans.30,37 Another factor important in the early phase of infection by both human and plant pathogens is melanin. Melanin is a component of the cell wall of most pathogenic fungi, and in animal pathogens it protects the invading fungus from reactive oxygen species produced by the host.30 Such a role for melanin in fungal interactions with plants has not been shown, but a similar role is likely because reactive oxygen species are often produced as a defense mechanism to microbial infection. A demonstrated role for melanin in plant pathogenic fungi is to provide structural integrity of specialized infection structures known as appressoria. These structures accumulate glycerol, which creates osmotic pressure required for direct penetration of plant tissue by the fungus.40,41

Many pathogenic fungi are thigmotrophic and able to recognize the surface of human and plant cells. Fungi can sense the topography of host cells and respond by altering their morphology, growth rates, and adhesion to cell sur-faces.30,42 Such recognition is known to be important for the bean rust pathogen Uromyces appendiculatus41 that infects through stomata of the leaf. Bean leaf stomata are slightly raised (0.5 mm) above the leaf surface, and the fungus recognizes the difference in leaf surface topography to locate stomata and initiate infection. Many human pathogens are also thigmotrophic, and this trait may be important in human tissue invasion.30

Toxins produced by some plant pathogenic fungi are pathogenicity factors and are required for disease development. Other toxins have no known role in pathogenicity. Most of these compounds show toxicity to several species of plants and are known as nonselective toxins.43 Cercosporin, a photosensitizing toxin produced by species of Cercospora,44 is an example of a nonselective toxin. Another class of toxins, known as host-selective toxins, is toxic to a specific plant genotype within a species, and they often determine the host range of the fungus. Over 20 species of fungi are known to produce host-selective toxins.45,46

Fungal toxins have not been shown to be pathogenicity factors for human fungal pathogens. This is interesting, as members of the Eurotiomycetes, a class of fungi with recognized pathogens of humans, are known to produce a number of toxins. These include gliotoxin, a known immunosuppressant and inducer of animal programmed cell death, produced by Aspergillus fumigatus,30

and aflatoxin, a carcinogen and immunosuppressant, produced by A. flavus.14 Also, most species of Aspergillus that are pathogens of humans are known to produce a number of toxic compounds that are harmful if ingested (myco-toxins), but none has been shown to be involved in pathogenicity. Similarly, no role has been shown for any of the numerous ribotoxins (e.g., alpha-sarcin, restrictocin, and mitogillin) produced by filamentous fungi.30

Filamentous fungi are known to secrete a wide array of extracellular enzymes that are important for degradation of complex molecules during saprophytic growth. However, it has been difficult to demonstrate a requirement for these degradative enzymes in the pathogenicity of plants or animals.47,48 Gene families often encode degradative enzymes, and creating mutant strains lacking all enzymatic activity is difficult. Further, some enzymes are induced only in host tissue or produced only during a certain phase of growth.30

Regardless of the interrelationship, the interaction of a fungus with its host involves molecular communication between the two, and many genes shown to regulate pathogenicity are components of signaling pathways. Fungi are known to have a number of signaling pathways including MAP kinases, G-proteins, and phosphokinases that have been shown to be required for pathogenicity.32,38 Research in this field may potentially lead to disease management strategies based on the inhibition of important metabolic and regulatory pathways in fungi.

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