CH 7-Fungal Classification and Replication

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CH 7-Fungal Classification and Replication

• Medical Micro
• S.S. Magrogan

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The Fungi

• ubiquitous and diverse group of organisms
• degrade organic matter.
• heterotrophic existence as saprobes (organisms
  that live on dead or decaying matter),
• symbionts (organisms that live together and in
  which the association is of mutual advantage),
• commensals (organisms living in a close
  relationship in which one benefits from the
  relationship and the other neither benefits nor
  is harmed),
• or as parasites (organisms that live on or
  within a host, from which they derive benefits
  without making any useful contribution in return
  in the case of pathogens the relationship is
  harmful to the host).

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More on Fungi

• Can be major causes of human disease
• Among immunocompromised
• Among hospitalized with serious underlying
  diseases.
• Fungi are opportunistic pathogens with high
  morbidity and mortality.
• Overall incidence of specific invasive mycoses
  continues to increase with time (Table 7-1)
• The list of opportunistic fungal pathogens
  likewise increases each year.
• In short, there are no nonpathogenic fungi!

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Fungal Infections

• This increase in fungal infections can be
  attributed to
• the ever-growing number of immunocompromised
  patients,
• including transplant patients, individuals with
  acquired immune deficiency syndrome (AIDS),
• patients with cancer and who are undergoing
  chemotherapy,
• individuals who are hospitalized with other
  serious underlying conditions and who undergo a
  variety of invasive procedures.

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Basic Anatomy of a Fungal Cell
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Disease Causing Fungi

• Table 7-1.
• Candida species
• Cryptococcus neoformans
• Coccidioides immitis
• Aspergillus species
• Histoplasma capsulatum
• Agents of Zygomycosis
• Agents of Hyalohyphomycosis
• Agents of Phaeohyphomycosis
• Sporothrix schenckii
• Malassezia furfur

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Fungi Life Style

• aerobic respiration
• some are facultatively anaerobic (fermentative)
• others are strict anaerobes.
• fungi are heterotrophic and biochemically
  versatile,
• producing both primary (e.g., citric acid,
  ethanol, glycerol) and secondary (e.g.,
  antibiotics penicillin, amanitens, aflatoxins)
  metabolites.
• Relative to the bacteria, fungi are slow growing
  with cell-doubling times in terms of hours rather
  than minutes.

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Classifying Fungi

• Five major classes of fungi of medical importance
  is shown in Table 7-3
• Only approximately 200 are known to cause human
  disease, (Deuteromycetes) although this number
  appears to be increasing.
• Reproduce by the formation of spores, which may
  be sexual (involving meiosis, preceded by fusion
  of the protoplasm and nuclei of two compatible
  mating types) or asexual (involving mitosis
  only).
• Classes Zygomycetes, Ascomycetes,
  Archiascomycetes, and Basidiomycetes
• produce both sexual and asexual spores -Table 7-4
• The form of the fungus producing sexual spores is
  termed the teleomorph,
• the form producing asexual spores is termed the
  anamorph.
• The fact that the teleomorph and anamorph of the
  same fungus have different names (e.g.,
  Ajellomyces capsulatum teleomorph and
  Histoplasma capsulatum anamorph) is a source of
  confusion for nonmycologists.

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More Classification

• morphology and
• mode of spore production
• ultrastructural features,
• biochemical and molecular characteristics,
• Fungi may be unicellular or multicellular.
• The simplest grouping yeasts or moulds.

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Yeast vs Mold

• Yeast
• reproduces by budding or by fission
• "mother" cell pinches off a portion of itself to
  produce "daughter" cell.
• The daughter cells may elongate to form
  sausagelike pseudohyphae.
• Yeasts are usually unicellular and produce round,
  pasty, or mucoid colonies on agar.

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Molds

• Moulds
• multicellular organisms
• Have threadlike tubular structures called hyphae
  that elongate at their tips by a process known as
  apical extension.
• Hyphae are either coenocytic (hollow and
  multinucleate) or septate (divided by partitions
  or cross-walls) (see Figure 7-2).
• The hyphae form together to produce a matlike
  structure called a mycelium.
• The colonies formed by moulds are often described
  as filamentous, hairy, or woolly.
• On or beneath the surface hyphae, termed
  vegetative hyphae,
• hyphae that project above the surface of the
  medium, so-called aerial hyphae.
• The aerial hyphae may produce specialized
  structures known as conidia (asexual reproductive
  elements
• The conidia are easily airborne and serve to
  disseminate the fungus.
• The size, shape, and certain developmental
  features of conidia are used as a means of
  identifying fungi to genus and species. Many
  fungi of medical importance are termed dimorphic
  because of the fact that they may exist in both a
  yeast form and a mould form.

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Asexual Spores

• Asexual spores consist of two general types
• sporangiospores
• asexual spores produced in a containing structure
  or sporangium
• Sporangiospores are characteristic of genera
  belonging to the class Zygomycetes,
• Rhizopus
• Mucor spp.
• conidia.
• Conidia are asexual spores that are borne naked
  on specialized structures,
• Aspergillus spp,
• Penicillium spp.,
• dermatophytes.

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Four Other Groups of Fungi

• Ascomycetes
• Basidiomycetes
• Zygomycetes
• Archiascomycetes

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ZYGOMYCETES

• Moulds
• broad, sparsely septate, coenocytic hyphae.
• Produce sexual zygospores following the fusion of
  two compatible mating types.
• The asexual spores of the order Mucorales are
  contained within a sporangium (sporangiospores).
• The sporangia are borne at the tips of stalklike
  sporangiophores that terminate in a bulbous
  swelling called the columella
• The presence of rootlike structures, called
  rhizoids, is helpful in identifying specific
  genera within the Mucorales.
• Most Zygomycetes encountered clinically belong to
  the order Mucorales.
• The other order, the Entomophthorales, are less
  common and include the genera, Basidiobolus and
  Conidiobolus. T
• These organisms cause tropical subcutaneous
  zygomycosis.
• The asexual spores are borne singly on short
  sporophores and are forcibly ejected when mature.

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ASCOMYCETES

• include both yeasts (Saccharomycetes) and
  moulds.
• The hyphae are septate,
• asexual spores are produced from conidiogenous
  cells borne on conidiophores.
• The sexual spore of the Ascomycetes is the
  ascospore, characterized by its production within
  a sac or ascus.

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ARCHIASCOMYCETES

• new class that was recently described to include
  an organism, Pneumocystis carinii, that had
  formerly been considered a protozoan.
• The reclassification of Pneumocystis was based on
  molecular evidence that it was most closely
  related to the ascomycete Schizosaccharomyces
  pombe.
• Further molecular studies resulted in the naming
  of human-derived strains as Pneumocystis
  jiroveci.
• The organism exists in a vegetative, trophic form
  that reproduces asexually by binary fission.
• Fusion of compatible mating types results in a
  spherical cyst or spore case, which on maturity
  contains eight spores.

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BASIDIOMYCETES

• rarely encountered clinically
• The only human pathogen is Filobasidiella
  neoformans,
• the sexual form of Cryptococcus neoformans.
• The sexual spore of the Basidiomycetes is the
  basidiospore,
• characterized by the extension from a club-shaped
  structure, the basidium.

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DEUTEROMYCETES

• includes both yeasts and moulds that share a
  common lack of a sexual phase
• Many of the fungi pathogenic to humans are
  included in this class
• septate hyphae
• produce conidia from conidiophores and
  conidiogenous cells.
• The yeasts reproduce by budding, and the moulds
  produce conidia by either a blastic (budding)
  process or a thallic process, in which hyphal
  segments fragment into individual cells or
  arthroconidia.
• Identification of individual genus and species
  is based in part on microscopic evaluation of the
  mode of development of the conidium from the
  conidiogenous cell.

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Disease Causing Fungi

• The largest group of fungi causing infections in
  humans, the Deuteromycetes Table 7-4
• does not produce known sexual spores.
• Its important to know sexual nature of fungi
  because
• Irrespective of the ability of a given fungus to
  produce sexual spores, in clinical situations it
  is common to refer to the organisms by their
  asexual designations.
• The anamorphic (asexual) state is isolated from
  clinical specimens
• Sexual or teleomorphic phase occurs only under
  very specialized conditions in the laboratory.

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HUMAN MYCOSES

• Classification of Human Mycoses
• In addition to the formal taxonomic
  classification of fungi, fungal infections may be
  classified according to the tissues infected, as
  well as by specific characteristics of organism
  groups.
• These classifications include the superficial,
  cutaneous, and subcutaneous mycoses the endemic
  mycoses and the opportunistic mycoses

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SUPERFICIAL MYCOSES

• Infections that are limited to the very
  superficial surfaces of the skin and hair.
• Nondestructive and of cosmetic importance only.
• The clinical infection pityriasis versicolor is
  characterized by discoloration or depigmentation
  and scaling of the skin. Tinea nigra refers to
  brown- or black-pigmented, macular patches
  localized primarily to the palms. The clinical
  entities of black and white piedra involve the
  hair and are characterized by nodules composed of
  hyphae that encompass the hair shaft. The fungi
  associated with these superficial infections
  include Malassezia furfur, Phaeoannelomyces
  (Exophiala) werneckii, Piedraia hortae, and
  Trichosporon spp.

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CUTANEOUS MYCOSES

• infections of the keratinized layer of skin,
  hair, and nails.
• These infections may elicit a host response and
  become symptomatic.
• Signs and symptoms include itching, scaling,
  broken hairs, ringlike patches of the skin, and
  thickened, discolored nails.
• The Dermatophytes are fungi classified in the
  genera Trichophyton, Epidermophyton, and
  Microsporum.
• Infections of the skin involving these organisms
  are called dermatophytoses.
• Tinea unguium refers to infections of the toes
  involving these agents.
• Onychomycoses includes infections of the nails
  caused by the dermatophytes, as well as
  nondermatophytic fungi such as Candida spp. and
  Aspergillus spp.

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SUBCUTANEOUS MYCOSES

• Subcutaneous mycoses involve the deeper layers of
  the skin, including the cornea, muscle, and
  connective tissue, and are caused by a broad
  spectrum of taxonomically diverse fungi.
• The fungi gain access to the deeper tissues,
  usually by traumatic inoculation, and remain
  localized, causing abscess formation, nonhealing
  ulcers, and draining sinus tracts.
• The host immune system recognizes the fungi,
  resulting in variable tissue destruction and,
  often, epitheliomatous hyperplasia.
• Infections may be caused by hyaline moulds such
  as Acremonium spp. and Fusarium spp. and by
  pigmented or dematiaceous fungi such as
  Alternaria spp., Cladosporium spp., and Exophiala
  spp. (Phaeohyphomycoses, Chromoblastomycoses).
• Subcutaneous mycoses tend to remain localized and
  rarely disseminate systemically.

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ENDEMIC MYCOSES

• Endemic mycoses are fungal infections caused by
  the classic dimorphic fungal pathogens
  Histoplasma capsulatum, Blastomyces dermatitidis,
  Coccidioides immitis, and Paracoccidioides
  brasiliensis.
• These fungi exhibit thermal dimorphism (i.e.,
  exist as yeast at 37C and mould at 25C) and are
  generally confined to specific geographic regions
  where they occupy specific environmental or
  ecologic niches.
• The endemic mycoses are often referred to as
  systemic mycoses, because these organisms are
  true pathogens and can cause infection in healthy
  individuals.
• Recently the dimorphic fungus Penicillium
  marneffei was added to the list of agents causing
  endemic mycoses.
• All of these agents produce a primary infection
  in the lung, with subsequent dissemination to
  other organs and tissues.

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OPPORTUNISTIC MYCOSES

• The opportunistic mycoses are infections caused
  by fungi that are normally found as human
  commensals or in the environment.
• With the exception of Cryptococcus neoformans,
  these organisms exhibit inherently low or limited
  virulence and cause infection in individuals who
  are debilitated, immunosuppressed, or who carry
  implanted prosthetic devices or vascular
  catheters.
• Virtually any fungus can serve as an
  opportunistic pathogen, and the list of those
  identified as such becomes longer each year.
• The most common opportunistic fungal pathogens
  are the yeasts Candida spp. and Cryptococcus
  neoformans, the mould Aspergillus spp., and
  Pneumocystis jiroveci.
• Because of its inherent virulence, Cryptococcus
  neoformans is often considered a "systemic"
  pathogen.
• Although this fungus may cause infection in
  immunologically normal individuals, it clearly is
  seen more often as an opportunistic pathogen in
  the immunocompromised population.