Introduction
Molds are multinucleated, filamentous fungi composed of hyphae. A hypha is
a branching, tubular structure from 2–10 µm in diameter and is usually divided
into cell-like units by crosswalls called septa. The total mass of hyphae is
termed a mycelium. The portion of the mycelium that anchors the mold and
absorbs nutrients is called the vegetative mycelium; the portion that produces
asexual reproductive spores is termed the aerial mycelium.
Molds possess a rigid polysaccharide cell wall composed mostly of chitin
and, like all fungi, are eukaryotic. Molds reproduce primarily by means of
asexual reproductive spores such as conidiospores, sporangiospores, and
arthrospores. These spores are disseminated by air, water, animals, or objects.
Upon landing on a suitable environment, they germinate and produce new
hyphae. Molds may also reproduce by means of sexual spores such as ascospores
and zygospores, but this is not common. The form and manner in which the
spores are produced, along with the appearance of the hyphae and mycelium,
provide the main criteria for identifying and classifying molds.
Nonpathogenic Molds
To illustrate how morphological characteristics, such as the type and form of
asexual reproductive spores and the appearance of the mycelium, may be used
in identification, we will look at 3 common nonpathogenic molds.
The 2 most common types of asexual reproductive spores produced by
molds are conidiospores and sporangiospores. Conidiospores are borne externally
in chains on an aerial hypha called a conidiophore; sporangiospores are
produced within a sac or sporangium on an aerial hypha called a sporangiophore.
Penicillium and Aspergillus are examples of molds that produce conidiospores.
Penicillium is one of the most common household molds and is a frequent food
contaminant. The conidiospores of
Penicillium usually appear gray, green, or
blue and are produced in chains on finger-like projections called sterigmata.
Aspergillus is another common contaminant. Although usually nonpathogenic,
it may become opportunistic in the respiratory tract of a compromised host and,
in certain foods, can produce mycotoxins. The conidiospores of Aspergillus
appear brown to black and are produced in chains on the surface of a ball-like
structure called a vesicle.
- Scanning electron micrograph of the conidiospores of Penicillium.
- Scanning electron micrograph of the conidiospores of Aspergillus.
-
Rhizopus is an example of a mold that produces sporangiospores. Although
usually nonpathogenic, it sometimes causes opportunistic wound and
respiratory infections in the compromised host. The sporangiospores of Rhizopus appear brown or black and are found within sacs called
sporangia. Anchoring structures called rhizoids are also produced on the
vegetative hyphae.
Rhizopus can also reproduce sexually. During sexual reproduction, hyphal
tips of a (+) and (–) mating type join together and their nuclei fuse to form a
sexual spore called a zygospore. This gives rise to a new sporangium-producing
sporangiospore with DNA that is a recombination of the 2 parent strains’ DNA.
Nonpathogenic molds are commonly cultured on fungal-selective or enriched
media such as saboraud dextrose agar (SDA), corn meal agar, and potato
dextrose agar.
Dermatophytes
The
dermatophytes are a group of molds that cause superficial mycoses of the
hair, skin, and nails and utilize the protein keratin. Infections are commonly
referred to as ringworm or tinea infections and include tinea capitis (head), tinea
barbae (face and neck), tinea corporis (body), tinea cruris (groin), tinea unguium
(nails), and tinea pedis (athlete’s foot).
The 3 common dermatophytes are
Microsporum, Trichophyton, and Epidermophyton. These organisms grow well at 25°C. They may produce large leaf- or
club-shaped asexual spores called macroconidia, as well as small spherical
asexual spores called microconidia, both from vegetative hyphae.
Microsporum commonly infects the skin and hair,
Epidermophyton, the skin
and nails, and Trichophyton, the hair, skin, and nails. Dermatophytic infections
are acquired by contact with fungal spores from infected humans, animals, or
objects. On the skin, the dermatophytes cause reddening, itching, edema, and
necrosis of tissue as a result of fungal growth and a hypersensitivity of the host
to the fungus and its products. Frequently there is secondary bacterial or Candida
invasion of the traumatized tissue.
To diagnose dermatophytic infections, tissue scrapings can be digested
with 10% potassium hydroxide (which causes lysis of the human cells but not
the fungus) and examined microscopically for the presence of fungal hyphae
and spores. To establish the specific cause of the infection, fungi from the
affected tissue can be cultured on dermatophyte test medium (DTM) and
saboraud dextrose agar (SDA).
DTM has phenol red as a pH indicator with the medium yellow (acid)
prior to inoculation. As the
dermatophytes utilize the keratin in the medium,
they produce alkaline end products, which raise the pH, thus turning the
phenol red in the medium from yellow or acid to red or alkaline. On SDA, the
types of macroconidia and microconidia can be observed. Many dermatophyte
species produce yellow- to red-pigmented colonies on SDA, and the most common
species of
Microsporum fluoresce under ultraviolet light.
Dimorphic Fungi
Dimorphic fungi may exhibit 2 different growth forms. Outside the body they
grow as a mold, producing hyphae and asexual reproductive spores, but inside
the body they grow in a nonmycelial form. Dimorphic fungi may cause systemic
mycoses, which usually begin by inhaling spores from the mold form. After
germination in the lungs, the fungus grows in a nonmycelial form. The infection
usually remains localized in the lungs and characteristic lesions called
granuloma may be formed in order to wall-off and localize the organism. In rare
cases, usually in an immunosuppressed host, the organism may disseminate to
other areas of the body and be life-threatening. Examples of dimorphic fungi
include
Coccidioides immitis, Histoplasma capsulatum, and Blastomyces dermatitidis.
Coccidioides
immitis is a dimorphic fungus that causes coccidioidomycosis,
a disease endemic to the southwestern United States. The mold form of the
fungus grows in arid soil and produces thick-walled, barrel-shaped asexual
spores called
arthrospores by a fragmentation of its vegetative hyphae. After
inhalation, the arthrospores germinate and develop into endosporulating
spherules in the lungs. Coccidioidomycosis can be diagnosed by culture, by a
coccidioidin skin test, and by indirect serologic tests.
Histoplasma capsulatum is a dimorphic fungus that causes histoplasmosis, a
disease commonly found in the Great Lakes region and the Mississippi and
Ohio River valleys. The mold form of the fungus often grows in bird or bat
droppings, or soil contaminated with these droppings, and produces large
tuberculate macroconidia and small microconidia. After inhalation of these
spores and their germination in the lungs, the fungus grows as a budding,
encapsulated yeast. Histoplasmosis can be diagnosed by culture, by a histoplasmin
skin test, and by indirect serologic tests.
Symptomatic and disseminated histoplasmosis and coccidioidomycosis are
seen primarily in individuals who are immunosuppressed. Along with a positive
HIV antibody test, both are indicator diseases for the diagnosis of AIDS.
Blastomycosis, caused by
Blastomyces dermatitidis, produces a mycelium
with small
conidiospores and grows actively in bird droppings and contaminated
soil. When spores are inhaled or enter breaks in the skin, they germinate
and the fungus grows as a yeast with a characteristic thick cell wall. Blastomycosis
is common around the Great Lakes region and the Mississippi and
Ohio River valleys. It is diagnosed by culture and by biopsy examination.
Procedure
Nonpathogenic Molds
- Using a dissecting microscope, observe the SDA plate cultures of Penicillium,
Aspergillus, and Rhizopus. Note the colony appearance and color and the
type and form of the asexual spores produced.
- Observe the prepared slides of Penicillium, Aspergillus, and Rhizopus under high magnification. Note the type and form of the asexual spores
produced.
- Observe the prepared slide showing the zygospore of Rhizopus produced
during sexual reproduction.
Dermatophytes
- Observe the dermatophyte Microsporum growing on DTM. Note the red
color (from alkaline end products) characteristic of a dermatophyte.
- Microscopically observe the SDA culture of Microsporum. Note the macroconidia
and microconidia.
- Observe the photographs of dermatophytic infections.
Dimorphic Fungi
- Observe the prepared slide of Coccidioides immitis arthrospores.
- Observe the pictures showing the mold form and endosporulating spherule
form of Coccidioides immitis.
- Observe the pictures showing the mold form and yeast form of Histoplasma
capsulatum.
- Observe the photographs of systemic fungal infections.
Results
Nonpathogenic Molds
Make drawings of the molds as they appear microscopically under high
magnification and indicate the type of asexual spore they produce. Also note
their color and appearance on SDA.
Dermatophytes
- Describe the results of Microsporum growing on DTM:
- Original color of DTM =
- Color following growth of Microsporum =
- Reason for color change =
- Draw the macroconidia and microconidia seen on the SDA culture of
Microsporum.
Dimorphic Fungi
- Draw the arthrospores of Coccidioides immitis.
- Draw the mold form and endosporulating spherule form of Coccidioides
immitis.
- Draw the mold form and yeast form of Histoplasma capsulatum.
Performance Objectives
Discussion
- Define the following: hypha, mycelium, vegetative mycelium, and aerial
mycelium.
- Describe the principle way molds reproduce asexually.
- List the main criteria used in identifying molds.
Nonpathogenic Molds
- Describe conidiospores and sporangiospores and name a mold that
produces each of these.
- Recognize the following genera of molds when given an SDA plate culture
and a dissecting microscope and list the type of asexual spore seen:
(a) Penicillium
(b) Aspergillus
(c) Rhizopus
- Recognize the following genera of molds when observing a prepared slide
under high magnification and list the type of asexual spore seen:
(a) Penicillium
(b) Aspergillus
(c) Rhizopus
- Recognize Rhizopus zygospores.
Dermatophytes
- Define dermatophyte and list 3 common genera of dermatophytes.
- Name 4 dermatophytic infections and explain how they are contracted by
humans.
- Describe macroconidia and microconidia.
- Describe how the following may be used to identify dermatophytes:
potassium hydroxide
preparations of tissue scrapings,
DTM, and SDA.
- Recognize a mold as a dermatophyte and explain how you can tell when
given the following:
- flask of DTM showing alkaline products.
- an SDA culture (under a microscope) or picture showing macroconidia.
- Recognize macroconidia and microconidia.
Dimorphic Fungi
- Define dimorphic fungi and describe how they are usually contracted by
humans.
- Name 3 common dimorphic fungal infections found in the United States,
explain how they are transmitted to humans, and indicate where they are
found geographically.
- Describe the mold form and the nonmycelial form of the following:
(a) Coccidioides immitis
(b) Histoplasma capsulatum
(c) Blastomyces dermatitidis
- Recognize Coccidioides immitis and its arthrospores when given a prepared
slide and a microscope.
