1. Yeasts reproduce asexually
by a process called budding (see Fig. 1 and Fig. 5). A bud is formed on the outer surface of the parent cell as
the nucleus divides. One nucleus migrates into the elongating bud. Cell
wall material forms between the bud and the parent cell and the bud breaks
away.
2. A few yeasts, such as Candida
albicans, also produce clusters of asexual reproductive
spores called
blastoconidia (blastospores) (def) and thick-walled survival spores called chlamydoconidia (chlamydospores)
(def) ; see
Fig. 4.
3. Yeasts can also reproduce
sexually by means of sexual spores called ascospores which
result from the fusion of the nuclei from two cells followed by meiosis
(def).
Sexual reproduction is much less common than asexual reproduction but
does allow for genetic recombination (def).
1. Candida albicans
Candida
albicans is found as normal flora on the mucous membranes
and in the gastrointestinal tract but is usually held in
check by:
a) normal flora bacteria;
and
b) normal body defenses.
Candida can cause a variety
of opportunistic infections in people who are debilitated, immunosuppressed,
or have received prolonged antibacterial therapy. Women who are diabetic,
pregnant, taking oral contraceptives, or having menopause are also more
prone to vaginitis. These conditions alter the sugar concentration and
pH of the vagina making it more favorable for the growth of Candida.
People who are immunosuppressed frequently develop thrush, vaginitis,
and sometimes disseminated infections.
Any Candida infection
is called candidiasis. Candida most commonly causes vaginitis
(def)
(see Fig. 6 and Fig. 6A); thrush (def) (an infection of the mouth) (see Fig. 7 and Fig. 7A); balantitis (def) (an infection of the foreskin and head of the penis) (see Fig. 8); onchomycosis (def) (an infection of the nails) (see Fig. 9); and dermatitis (def) (diaper
rash and other infections of moist skin).
Less commonly, Candida can infect the lungs, blood, heart, and
meninges (def),
especially in the compromised or immunosuppressed host. Candida now
causes about 10% of all cases of septicemia (def).
Candidiasis of the esophagus, trachea, bronchi, or lungs, in conjunction
with a positive HIV antibody test, is one of the indicator diseases for
AIDS.
Candida is said to be dimorphic, that is it has two different growth forms. It can grow as an oval, budding yeast,
but under certain culture conditions, the budding yeast may elongate and remain attached producing filament-like structures called pseudohyphae (see Fig. 10). C. albicans may also produce true hyphae (see Fig. 3) similar to molds.
In this case long, branching filaments lacking complete septa form. The pseudohyphae and hyphae help the yeast to invade deeper tissues after it colonizes the epithelium.
Asexual spores called blastoconidia are reproductive units produced by budding in yeasts. Under certain growth conditions, thick-walled
survival spores called chlamydoconidia (def) may also form at the tips or as a part of the hyphae (see
Fig. 5)
The most common Candida species
causing human infections is C. albicans, causing 50-60% of all Candida infections. Candida glabrata is second, causing 15-20% of Candida infections; Candida parapsilosis is third, responsible for 10-20%. More recently, Candida auris has been causing relatively rare but severe infections in patients that have been hospitalized for long periods of time, sometimes entering the bloodstream causing invasive infections throuout the body. C. auris is often resistant to common antifungal drugs and kills as many as 1 out of 3 patients with invasive infections.
2. Cryptococcus neoformans
A lesser known but often more
serious pathogenic yeast is Cryptococcus neoformans.
Like many fungi, this yeast can also reproduce sexually and the name
given to the sexual form of the yeast is Filobasidiella neoformans.
It appears as an oval yeast 5-6 µm in diameter,
forms buds with a thin neck, and is surrounded by a thick capsule (see Fig. 11).
It does not produce pseudohyphae and chlamydospores. The capsule
enables the yeast to resist phagocytic engulfment. The yeast is dimorphic. In its sexual form, as well as in its asexual form under certain conditions, it can produce a hyphal form.
Cryptococcus infections
are usually mild or subclinical but, when symptomatic, usually begin
in the lungs after inhalation of the yeast in dried bird feces.
It is typically associated with with pigeon and chicken droppings and
soil contaminated with these droppings. Cryptococcus, found in
soil, actively grows in the bird feces but does not grow in the bird itself.
Usually the infection does not proceed beyond this pulmonary stage. In
the immunosuppressed host, however, it may spread through the blood to
the meninges and other body areas, often causing cryptococcal meningoencephalitis.
Any disease by this yeast is usually called cryptococcosis.
Dissemination of the pulmonary
infection can result in a very severe and often fatal cryptococcal
meningoencephalitis.
Cutaneous and visceral infections are also found. Although exposure to
the organism is probably common, large outbreaks are rare, indicating
that an immunosuppressed host is usually required for the development
of severe disease. Extrapulmonary cryptococcosis, in conjunction with
a positive HIV antibody test, is another indicator disease for AIDS.
3. Pneumocystis jiroveci
Pneumocystis jiroveci (formerly
called Pneumocystis carinii) (see Fig.
7
and Fig. 8) is thought to be transmitted from
person to person by the respiratory route and is almost always asymptomatic.
However, in persons with highly depressed immune responses, such as people
with leukemias or infected with the Human Immunodeficiency Virus (HIV),
P. jiroveci can cause a severe pneumonia called PCP
(Pneumocystis pneumonia).
P. jiroveci can be found in 3 distinct morphologic stages:
In biopsies from lung tissue or
in tracheobronchial aspirates, both a trophic form about 1-4 µm
in diameter with a distinct nucleus and a cyst form between 5-8 µm
in diameter with 6-8 intracystic bodies (ascospores) can be
seen.
4. Malassezia globosa
Malassezia globosa is a (see Fig. 13) dimorphic yeast that is the most frequent cause of a superficial skin infection called tinea versicolor that commonly appears as a hypopigmentation of the infected skin. M. globosa is also the most common cause of dandruff and seborrheic dermatitis. The yeast is naturally found on the skin.
