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Basic Biology of Fungi
Garry T. Cole
General Concepts
Yeasts and Molds
These fungi grow as saprophytes, parasites, or both by using specific
proteolytic, glycolytic, or lipolytic enzymes to extracellularly break down
substrates and to absorb the products of digestion through the fungal cell
envelope.
Cell Wall
The fungal cell wall gives shape and form, protects against mechanical
injury, prevents osmotic lysis, and provides passive protection against the
ingress of potentially harmful macromolecules.
Filamentous Fungi and Filamentous Bacteria
Fungi are different from the Actinomycetes, a group of prokaryotic
filamentous bacteria having peptidoglycans in their cell walls and an absence of
nuclear membranes and organelles, but the two groups of microorganisms are
usually considered together in texts.
Hyphal and Yeast Morphogenesis
Hyphal extension growth occurs apically by a sophisticated organization of
tip-growth-related organelles and cytoskeletal elements. Hyphal wall and yeast
cell wall polysaccharide synthetases are active at sites where growth is
occurring and inactive when no growth is occurring. Morphogenesis is a balance
between wall synthesis and wall lysis.
Sexual Reproduction
Sexual reproduction occurs by the fusion of two haploid nuclei (karyogamy),
followed by meiotic division of the diploid nucleus. The union of two hyphal
protoplasts (plasmogamy) may be followed immediately by karyogamy, or it may be
separated in time.
Asexual Reproduction
Asexual reproduction occurs via division of nuclei by mitosis. With the
absence of meiosis, other mechanisms associated with the nuclear cycle result in
recombination of hereditary properties and genetic variation.
INTRODUCTION
Macroscopic fungi such as morels, mushrooms, puffballs, and the cultivated
agarics available in grocery stores represent only a small fraction of the
diversity in the kingdom Fungi. The molds, for example, are a large group of
microscopic fungi that include many of the economically important plant
parasites, allergenic species, and opportunistic pathogens of humans and other
animals. They are characterized by filamentous, vegetative cells called hyphae.
A mass of hyphae forms the thallus (vegetative body) of the fungus, composed of
mycelium. The more phylogenetically primitive molds (e.g., water molds, bread
molds, and other sporangialsaclikeforms) produce cenocytic filaments
(multinucleate cells without cross-walls), while the more advanced forms produce
hyphae with cross-walls (septa) that subdivide the filament into uninucleate and
multinucleate compartments. The septum, however, still provides for cytoplasmic
communication, including intercellular migration of nuclei. Many fungi occur not
as hyphae but as unicellular forms called yeasts, which reproduce vegetatively
by budding. Some of the opportunistic fungal pathogens of humans are dimorphic,
growing as a mycelium in nature and as a vegetatively reproducing yeast in the
body. Candida is an example of such a dimorphic fungus (Fig. 73-1). It can
undergo rapid transformation from the yeast to the hyphal phase in vivo, which
partly contributes to its success in invading host tissue.
FIGURE 73-1 Dimorphism in C albicans. DYC, Daughter yeast cell;
GT, germ tube; H, hypha; Ph, pseudohypha; YMC, yeast mother cell. (X8,980) (From
Cole, GT, Kendrick B: Biology of Conidial Fungi. Vol. 1. Academic Press, San
Diego, 1981, with permission.)
The true fungi obtain their carbon compounds from nonliving organic
substrates (saprophytes) or living organic material (parasites) by absorption of
nutrients through their cell wall. Small molecules (e.g., simple sugars and
amino acids) accumulate in a watery film surrounding the hyphae or yeast and
simply diffuse through the cell wall. Macromolecules and insoluble polymers
(e.g., proteins, glycogen, starch, and cellulose), on the other hand, must
undergo preliminary digestion before they can be absorbed by the fungal cell.
This process involves release of specific proteolytic, glycolytic, or lipolytic
enzymes from the hypha or yeast, extracellular breakdown of the substrate(s),
and diffusion of the products of digestion through the fungal cell envelope
(Fig. 73-2). Fungal pathogens rely on these digestive enzymes to penetrate
natural host barriers.
FIGURE 73-2 (A) Extracellular digestion and absorptive nutrition in fungi.
(B) Invasive hyphae of C albicans in stratified epithelial tissue of
mouse stomach. (X4,250).
Cell Walls
Not all species of fungi have cell walls, but in those that do, cell wall
synthesis is an important factor in determining the final morphology of fungal
elements. Thus, our knowledge of fungal morphogenesis has evolved in parallel
with our understanding of fungal cell wall biosynthesis. The fungal wall also
protects cells against mechanical injury and blocks the ingress of toxic
macromolecules. This filtering effect may be especially important in protecting
fungal pathogens against certain fungicidal products of the host. The fungal
cell wall is also essential to prevent osmotic lysis. Even a small lesion in the
cell wall can result in extrusion of cytoplasm as a result of the internal (turgor)
pressure of the protoplast. The composition of fungal cell walls is relatively
simple and includes substances not typically found in animal and plant hosts
(e.g., chitin). On this basis, it may be possible to identify pathogen-specific
molecular targets from investigations of the biosynthesis of fungal wall
components. Such targets may prove pivotal for the successful development of
antifungal drugs that are not toxic to mammalian cells.
Filamentous Fungi and Filamentous Bacteria
Fungi, like bacteria, are ecologically important as decomposers as well as
parasites of plants and animals. Both groups of microbes often inhabit the same
ecosystem and thus compete for the same food supply. Associated with this
competition is the production by both the fungi and bacteria of secondary
products that function as microbial growth inhibitors or toxins. These compounds
constitute a rich library of antimicrobial agents, many of which have been
developed as pharmacologic antibiotics (e.g., penicillin from Penicillium
chrysogenum, nystatin from Streptomyces noursei, amphotericin B from S
niveus).
The superficial morphologic similarities between actinomycetes (filamentous
bacteria) and molds suggest that the two groups have undergone parallel
evolution. Despite the production of branching filaments and mold-like spores,
the actinomycetes are clearly prokaryotes, whereas fungi are eukaryotes.
Moreover, the sexual reproduction of bacteria, which typically occurs by
transverse binary fission, should not be confused with asexual processes of
budding and fragmentation associated with mitotic nuclear division in fungi.
Most of the molds that produce septate vegetative hyphae reproduce exclusively
by asexual means, giving rise to airborne propagules called conidia. On the
other hand, elaborate mechanisms of sexual reproduction are also demonstrated by
members of the Eumycota. Four distinct kinds of meiospores (products of
karyogamy-meiosis-cytokinesis) are recognized: oospores (Oomycetes), zygospores
(Zygomycetes), ascospores (Ascomycetes), and basidiospores (Basidiomycetes).
A summary of these and other diagnostic features of the fungi is presented in
Table 73-1.
Hyphal and Yeast Morphogenesis
Hyphal growth occurs by extension at the tips. This polarization is at least
partially determined by directional movement and accumulation of vesicles that
carry wall precursors and wall synthetases to the site of exocytosis at the
apical dome of the hypha (Fig. 73-3). Despite the apparent simplicity of hyphal
morphogenesis, ultrastructural investigations have shown a sophisticated
organization of tip-growth-related organelles and cytoskeletal elements. There
is evidence that intussusception and polymerization of chitin microfibrils occur
at the apical dome of the hypha and that the biosynthesis of this major cell
wall product is controlled by the activity of membrane-bound chitin synthetase.
The zymogen form of chitin synthetase has been detected in microvesicles called
chitosomes, which appear to transport this enzyme to the hyphal tip. The
chitosomes may arise from Golgi-like bodies or by a process of self-assembly of
subunits freely within the cytoplasm or within larger vesicular bodies.
Activation of chitin synthetase occurs upon fusion of the chitosome with the
plasmalemma and may be due to the interaction of a membrane-bound protease and
the zymogen. Chitin microfibrillogenesis is initiated at these sites of fusion.
FIGURE 73-3 Polarized growth of hypha.
Evidence has also been presented, primarily from studies of the yeast Saccharomyces
cerevisiae, that biosynthesis of skeletal polysaccharides is catalyzed by
polysaccharide synthetases (e.g., chitin synthetase and b1-3-glucan synthetase),
which are uniformly distributed within the plasmalemma. These wall-synthesizing,
cell - membrane-bound enzymes occur in either zymogen or active forms. The model
of yeast morphogenesis (Fig. 73-4) suggests that the synthetase is active at
sites where the wall is growing and inactive where it is quiescent. One
possibility is that microvesicles transport activating factors (e.g., proteases,
ATP, and GTP) to the plasmalemma at specific sites of wall biosynthesis (zones
of bud emergence and of septal formation). These two concepts of regulation of
wall biosynthesis in fungal hyphae and yeasts have been supported by
considerable bodies of evidence, and it is likely that both are correct.
FIGURE 73-4 Stages (A to F) of bud emergence and yeast cell cycle.
Extension growth of hyphal tips and yeast buds logically requires a balance
between processes of insertion of newly synthesized polymeric material and
modification of the existing microfibrillar matrix to accommodate expansion and
further intussusception of wall polymers. In other words, a balance between wall
synthesis and wall lysis, or plasticization, is essential for maintaining the
orderly processes of hyphal tip elongation and bud emergence. The presence of
lytic enzymes in the fungal wall has been reported, including, b1-3-glucanase,
N-acetyl-b-D-glucosaminadase, and chitinase. Localization of such activity may
be mediated by macrovesicles. These organelles, like microvesicles, are probably
derived from Golg-like bodies and are directed to the hyphal tip or yeast bud
and fuse with the plasmalemma, thereby delivering their contents to the site of
wall synthesis.
Reproduction
Sexual reproduction in the fungi typically involves fusion of two haploid
nuclei (karyogamy), followed by meiotic division of the resulting diploid
nucleus (Fig. 73-5A). In some cases, sexual spores are produced only by fusion
of two nuclei of different mating types, which necessitates prior conjugation of
different thalli. This condition of sexual reproduction is known as
heterothallism, and the nuclear fusion is referred to as heterokaryosis.
Normally plasmogamy (union of two hyphal protoplasts which brings the nuclei
close together in the same cell) is followed almost immediately by karyogamy. In
certain members of the Basidiomycotina, however, these two processes are
separated in time and space, with plasmogamy resulting in a pair of nuclei (dikaryon)
contained within a single cell. Karyogamy may be delayed until considerably
later in the life history of the fungus. Meanwhile, growth and cell division of
the binucleate cell occur. The development of a dikaryotic mycelium results from
simultaneous division of the two closely associated nuclei and separation of the
sister nuclei into two daughter cells (Fig. 73-5B). An alternative mechanism of
sexual reproduction in the fungi is homothallism, in which a nucleus within the
same thallus can fuse with another nucleus of that thallus (i.e., homokaryosis).
An understanding of these nuclear cycles is fundamental to investigations of
fungal genetics.
FIGURE 73-5 (A) Life cycle of S cerevisiae. (B) Basidiospore
formation by Filobasidiella neoformans, sexual state of Cryptococcus
neoformans. (1 and 2) Dikaryon formation. (3) Nuclear fusion (Karyogamy). (4
and 5) Meiosis. (6) Basidiospore formation. (7) Mitosis and basidiospore
proliferation.
As mentioned above some fungi are classified as strictly asexually
reproducing forms. These include the large group of asexual (imperfect) yeasts
(e.g., Candida species) and conidial fungi (e.g., Coccidioides immitis).
Most members of this group have permanently lost their ability to produce
meiospores. A few undergo rare sexual reproduction, and perhaps for some species
we have yet to discover their sexual (perfect) stage. The most common methods of
asexual reproduction, in addition to simple budding in yeasts, are blastic
development of conidia from specialized hyphae (conidiogenous cells),
fragmentation of hyphae into conidia, and conversion of hyphal elements into
conidia or chlamydospores (thick-walled resting spores) (Fig. 73-6).
FIGURE 73-6 Methods of asexual reproduction in the conidial fungi. (A)
Terminal blastic conidium. (B) Repetitive blastic conidium formation from
specialized conidiogenous cell (phialide). (C) Conidium formation by hyphal
fragmentation. (D) Conidium formation by conversion of apical segment of hypha
into single, asexual propagule. (E) Conversion of hyphal element into an
intercalary chlamydospore.
Despite the absence of meiosis during the life cycle of these imperfect
fungi, recombination of hereditary properties and genetic variation still occur
by a mechanism called parasexuality. The major events of this process (Fig.
73-7) include the production of diploid nuclei in a heterokaryotic, haploid
mycelium that results from plasmogamy and karyogamy; multiplication of the
diploid along with haploid nuclei in the heterokaryotic mycelium; sorting out of
a diploid homokaryon; segregation and recombination by crossing over at mitosis;
and haploidization of the diploid nuclei. Sexual and parasexual cycles are not
mutually exclusive. Some fungi that reproduce sexually also exhibit
parasexuality.
FIGURE 73-7 The parasexual cycle (genetic recombination without meiosis).
Stages of the parasexual cycle are numbered as follows (1) Hyphal conjugation (plasmogamy).
(2) Heterokaryosis. (3) Nuclear fusion (karyogamy). (4) Mitotic recombination
and nondisjunction. (5) Haploidization and nuclear segregation leading to
homokaryosis.
An extensive foundation of knowledge on the basic biology of fungi is at
hand, including fungi that cause superficial, deep-seated, and systemic
infections of humans and other animals. Much less is known, however, of the
intricacies of interactions between these largely opportunistic pathogens and
their hosts. Many areas of research in medical mycology are still in their
infancy and offer formidable challenges and potential rewards. The current
application of methods of recombinant DNA technology to problems of fungus-host
interactions, especially the identification of pathogenicity genes, holds
promise for significant contributions to our knowledge of medically important
fungi.
Non-Specific Cellular Defense
- How the body attacks Candida
REFERENCES
Bulawa CE: Genetics and molecular biology of chitin synthesis in fungi. Ann
Rev Microbiol 47:505, 1993
Cole GT, Hoch HC (eds): The Fungal Spore and Disease Initiation in Plants and
Animals. Plenum, New York, 1991
Gow NAR, Gadd GM (eds): The Growing Fungus. Chapman and Hall, London, 1995
Kwon-Chung KJ, Bennett JE: Medical Mycology. Lea and Febiger, Philadelphia,
1992
Latgé JP, Boucias D (eds): Fungal Cell Wall and Immune Response. Springer-Verlag,
New York, 1991
Odds FC: Candida and Candidosis, 2nd edition. Baillière Tindall,
Philadelphia, 1988
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