Yeast, infectious WORLD OF MICROBIOLOGY AND IMMUNOLOGY612•
confirmed by the analysis of a yeast cell mutant called cdc28.
The cdc28 mutant is blocked at start and so does not enter S
phase where the synthesis of DNA occurs.
Analysis of this and other cdc mutations has found a myr-
iad of functions associated with the genetic mutations. For
example, in yeast cells defective in a gene dubbed cdc2, the pro-
tein coded for by the cdc2 gene does not modify various pro-
teins. The absence of these modifications causes defects in the
aggregation of the chromosomal material prior to mitosis, the
change in the supporting structures of the cell that are necessary
for cell division, and the ability of the cell to change shape.
Studies of such cdc mutants has shown that virtually all
eukaryotic cells contain a similar control mechanism that gov-
erns the ability of a cell to initiate mitosis. This central control
point is affected by the activities of other proteins in the cell.
A great deal of research effort is devoted to understanding this
master control, because scientists presume that knowledge of
its operation could help thwart the development of cancers
related to a defect in the master control.See also Cell cycle (eukaryotic), genetic regulation of;
Genetic regulation of eukaryotic cells; Molecular biology and
molecular geneticsYYeast, infectiousEAST, INFECTIOUS
Yeastare single-cell fungiwith ovoid or spherical shapes,
which are grouped according to the cell division process into
budding yeast (e.g., the species and strains of Saccharomyces
cerevisiaeand Blastomyces dermatitidis), or fission yeast
(e.g., Schizosaccharomyces) species.
Yeast species are present in virtually all natural envi-
ronments such as fresh and marine water, soil, plants, animals,
and in houses, hospitals, schools, etc. Some species are sym-
biotic, while others are parasitic. Parasitic species may be
pathogenic (i.e., cause disease) either because of the toxins
they release in the host organism or due to the direct destruc-
tion of living tissues such as skin, internal mucosa of the
mouth, lungs, gastrointestinal, genital and urinary tracts of
animals, along with plant flowers, fruits, seeds, and leaves.
They are also involved in the deterioration and contamination
of stored grains and processed foods.
Yeast and other fungal infections may be superficial
(skin, hair, nails); subcutaneous (dermis and surrounding
structures); systemic (affecting several internal organs, blood,
and internal epithelia); or opportunistic (infecting neutropenic
patients, such as cancer patients, transplant patients, and other
immunocompromised patients). Opportunistic infections
acquired by patients inside hospitals, or due to medical proce-
dures such as catheters are termed nosocomial infections, and
they are a major concern in public health, because they
increase both mortality and the period of hospitalization. An
epidemiological study, with data collected between 1997 and
2001 in 72 different hospitals in the United States, showed that
7–8% of the nosocomial blood-stream infections were due to
a Candidaspecies of yeast, especially Candida albicans.
About 80% of Candidainfections are nosocomial in theUnited States, and approximately 50% of them are acquired in
intensive care units. A national epidemiologyof mycoses sur-
vey in the early 1990s showed that in neonatal ICUs C. albi-
canswas the cause of about 75% of infections and Candida
parapsilosisaccounted for the remaining 25%. Candida albi-
cansfrequently infects infants during birth, due to its presence
in the mother’s vaginal mucosa, whereas C. parapsilosiswas
found in the hands of healthcare professionals of the neonatal
ICUs. In surgical ICUs, C. albicanswas implicated in 50% of
infections while Candida glabrataresponded for another 25%
of the cases. The most frequently community-acquired yeast
infections are the superficial mycoses, and among other path-
ogenic fungi, Candida albicansis the cause of mouth thrush,
and vaginitis. Gastrointestinal yeast infections are also trans-
mitted by contaminated saliva and foods.
Although immunocompetent individuals may host
Candidaspecies and remain asymptomatic for many years, the
eventual occurrence of a debilitating condition may trigger a
systemic candidiasis. Systemic candidiasis is a chronic infec-
tion that usually starts in the gastrointestinal tract and gradu-
ally spreads to other organs and tissues, and the Candida
species commonly involved is C. albicans.They release about
79 different toxins in the hosts’ organism, and the lesions they
cause in the intestinal membranes compromise nutrient
absorption by reducing it to about 50% of the normal capacity.
C. albicansintestinal colonization and lesions expose internal
tissues and capillary vessels to contamination by bacteria
present in fecal material. The elderly, cancer patients, and
infants are especially susceptible to Candidainfections, as are
AIDSpatients. In the long run, systemic candidiasis may lead
to a variety of symptoms, such as chronic fatigue, allergies,
cystitis, endometriosis, diarrhea, colitis, respiratory disorders,
dry mouth, halitosis (bad breath), emotional disorders, etc.
The indiscriminate prescription and intake of antibiotics
usually kills bacteria that are essential for normal digestion
and favors the opportunistic spread of Candidaspecies on the
walls of the digestive tract, which can be worsened when asso-
ciated with a diet rich in sugars and carbohydrates. Once yeast
species colonize the intestinal walls, treatment becomes diffi-
cult and is usually followed by recurrence. Another challengeLight micrograph of Candida albicans.womi_Y 5/7/03 9:15 AM Page 612