Mycology

Mycology
Chapter 36
Microbiology for Surgical Technologists

Mycology is the study of fungi
Fungal diseases are called mycoses (singular, mycosis)
Fungi include yeasts, mushrooms, and mold
The species of fungi found in nature are saprophytes
What does that mean?
They are important in nature and in the food industry
Most fungal infections are superficial and are not life threatening
Unless the individual is immunocomprimised
The primary reservoirs of fungi are the soil and animals
Humans are reservoirs for certain opportunistic yeasts
They are contracted by either inhaling spores or through a wound on the skin

Classification of Mycoses
Superficial - affect outermost layer of the skin
Cutaneous – affect keratin in the skin, hair, and nails; causes inflammation in the skin
Subcutaneous – affect soft tissue, muscles, and bones directly under the skin
Systemic – affect deep tissues and organs of the body

Characteristics of Fungi
Fungi are eukaryotic plant organisms
The cell membrane is composed of lipids called sterols
What are lipids?
They do not contain chlorophyll
Fungi are either unicellular, such as yeasts, or multicellular, such as molds

Molds
Molds are made up of long filaments called hyphae
The hyphae are either aseptate or septate
Septate hyphae are divided into sections or separate cells by walls called septa (singular: septum)
Aseptate do not have a septum and cytoplasm flows from cell to cell
Hyphae grow by branching, a mass of hyphae called mycelium

Yeasts
Yeasts have round cells
Reproduce by budding, in which a protuberance called a blastoconidium is created
Some yeasts produce buds that grow longer and are similar to hyphae of molds called pseudohyphae (false hyphae)
Some fungi are dimorphic
They can live either as mold or yeast depending on growth conditions
An example is Candida albicans
When they grow in tissue at body temp, they are unicellular yeast
When grown at room temp or taken from the soil they are mold, with hyphae and spores

Reproduction
Fungi either reproduce sexually or asexually by producing spores
A true spore is formed by either asexual cleavage or sexual meiosis
Cleavage, the cell produces one spore
with complete DNA
Meiosis, the sex cell divides
and each cell receives half
of the DNA

Asexual Reproduction
There are two types of asexual spores
Sporangiospore (true spore)
Conidiospore (not a true spore)
The sporangiospore are only produced by the class of fungi called Zygomycetes
Conidiospore are produced by most of the fungi
They are not true spores since they are not formed by cleavage
They are produced:
Directly from hyphae
Directly from yeast
By specialized cells called conidiophores
Two types of conidiospore are produced from fungi
Macroconidium – multicellular
Microconidium - unicellular
Chlamydospores are only produced by the yeast Candida albicans
Chlamydospores are asexual spores
They are round, thick walled structures formed at the end, sides, or within the strand of a hyphae or pseudohyphae

Sexual Reproduction
Most fungi asexually reproduce, but sexual spores can be produced
This requires the mating of two different strains of the fungus
Three types of sexual spores
Ascospores
Basidiospores
Zygospores
Treatment of Fungal Infections
Treatment of systemic fungal diseases is difficult
Fungal cells are similar to human cells; therefore, the drugs that affect fungi also affect human cells causing toxicity
Newer classes of anti-fungal medications have been developed due to the large number of AIDS pts that contract fungal disease

Opportunistic Fungi
The majority of fungi are opportunistic pathogens
Risk factors or development of a fungal infection:
Immune deficiencies
Immunosuppressive drug therapy (organ transplant)
Steroid drug therapy
Types of cancer
Antibiotics
Diabetes

Opportunistic Molds
Aspergillosis
Zygomycosis

Aspergillosis
Caused by the genus Aspergillus
Occur most commonly in the respiratory tract through the inhalation of airborne conidiospores
Cause bronchitis that turns into a necrotic pneumonia
The necrosis is due to the fact that Aspergillus has an affinity for blood vessels and will obstruct the blood flow killing the tissue they feed
Once the organism enters the blood stream it will spread to other organs of the body and become systemic
The conidiospore can infect the nose and nasal passages causing a sinonasal infection
In few instances it can destroy the nasal bones and the bone at the base of the cranium, allowing access to the brain
Some pts have cavities in the lung from aspergillosis, or healed cavities from TB
Aspergillus spores will establish itself in the cavity and develop into hyphae that grow into a large mass; this is called a aspergilloma or fungus ball
Aspergilloma

Zygomycosis
Caused by Zygomycetes
Found in soil, on fruits, on plants and is common bread mold
Form aseptate hyphae
Like Aspergillus, Zygomycetes have an affinity for invading the vascular system
A severe disease is rhinocerebral zygomycosis
It is a rapidly progressive infection that causes extensive tissue damage and has a high mortality rate
Death occurs within a few days
Widespread destruction of the bones and tissue of the face occurs associated with the loss of one or both eyes

Characteristics of Yeast
All species of yeast are now considered opportunistic pathogens that can cause life threatening infections
This is due to the increase of immunocomprimised pts
Yeasts are unicellular and reproduce asexually by the process of blastoconidia formation (budding) or fission
Yeast infections are primarily transmitted by direct contact and aerosols

Candida
Are a species that are part of the normal human flora
The disease that it causes is candidiasis
Located in the mucosal surfaces of the GI tract
Rectal region, oral cavity, and vagina

Candida albicans
Causes several types of opportunistic infections in humans
Most are in connection with the use of antibiotics while treating another infection
In most cases when the antibiotic is discontinued the normal flora numbers will return
The most common are vaginal candidiasis and oral candidiasis (trench mouth)

Cryptococcus neoformans
Causes an infection called cryptococcosis (or European blastomycosis)
The cells vary in size, so it is referred to as a pleomorphic yeast
It has a polysaccharide capsule that surrounds the yeast cell
No other yeasts produce a capsule
The capsule is the key to its pathogenicity
The primary site of entry is the respiratory tract
The infection can be seen as a solitary node on x-ray and looks like a carcinoma
It can spread to the CNS and is the leading cause of fungal meningitis
STs will occasionally assist on pulmonary cases to excise the nodules

The End