"C. albicans has been studied for over 100 years, but it has never revealed
a sexual stage in its life cycle and has defied attempts to mate," explains
Dennis M. Dixon, chief of the Bacteriology and Mycology Branch of NIAID's
Division of Microbiology and Infectious Diseases. "This work is extremely
important because it begins to explain how the organism can adapt to changes
in its environment and cause disease."
C. albicans, a common cause of thrush, can infect the skin, mucous
membranes, and blood. In the latter case, the organism can invade multiple
organ systems where it causes death in 30 to 50 percent of infected
individuals. The fungus is particularly prevalent as a pathogen of the oral
cavity and the female genital tract and as an opportunistic infection that
strikes people with impaired immune systems. Existing anti-candida drugs
are often highly toxic, and drug-resistant infections are becoming more
Unlike baker's yeast, where the genetic systems and mating have been
extensively analyzed in the laboratory, C. albicans has proven difficult to
study until more recently. Baker's yeast can reproduce by mating, during
which two single-celled parent yeasts fuse to produce a single organism with
the combined genetic material of both parents. Because C. albicans has not
previously been found to mate, however, scientists have had difficulty
exchanging genetic information between different strains, thereby
complicating research efforts. Now Beatrice B. Magee, M.S., and Paul T.
(Pete) Magee, Ph.D., are the first to produce mating strains of C. albicans.
This discovery promises to accelerate research into the fungus and enable
researchers to more quickly understand its biology and identify new drug
targets. "There is no doubt that the identification of a sexual cycle will
facilitate ongoing drug discovery programs and motivate pharmaceutical
companies to begin new searches," says Dr. Magee.
Interest in C. albicans reproduction increased as scientists began to
unravel the organism's genetic blueprint, a process that is nearing
completion. Christina Hull and Alexander Johnson, Ph.D., researchers at the
University of California in San Francisco, analyzed this blueprint and
isolated potential genes that resembled those controlling mating in the
common baker's yeast. When the Magees removed one of these genes from a C.
albicans strain, they paired the organism with a mate that contained the
missing gene. Once the two strains met, they fused just like their baker's
yeast cousins. Hull and Johnson accomplished the same feat independently.
The finding has important implications beyond simplifying Candida research.
"Scientists have shown that in another disease-causing fungus, Cryptococcus,
one mating type is much more virulent than the other. If this is true for
C. albicans it opens up a new approach to understanding how this microbe
causes disease," explains Dr. Magee. The researchers expect their discovery
to accelerate studies on how the fungus adapts to different environments and
how it evades the body's defense mechanisms.
The Magee's studies also illustrate an important caveat of modern biomedical
research, now rife with announcements of newly deciphered genetic
blueprints. "Determining the sequence of the C. albicans genome was only
one step in the process," says Dr. Dixon. "The Magees have worked for years
to painstakingly analyze the biology and genetics of Candida. When the C.
albicans DNA sequence revealed a few hints about the organism's reproductive
processes, the Magees were poised to investigate these clues and take a
giant stride towards understanding an important human pathogen. Without
their strong history of basic research, it is unlikely that this discovery
would have been made."
NIAID is a component of the National Institutes of Health (NIH). NIAID
conducts and supports research to prevent, diagnose, and treat illness such
as HIV disease and other sexually transmitted diseases, tuberculosis,
malaria, asthma and allergies. NIH is an agency of the U.S. Department of
Health and Human Services.
Press releases, fact sheets and other NIAID-related materials are available
on the NIAID Web site at http://www.niaid.nih.gov/ .
The National Institute of Allergy and Infectious Diseases is a component of
the National Institutes of Health, U.S. Department of Health and Human
CM Hull, RM Raisner, and AD Johnson. Evidence for mating of the "asexual"
yeast Candida albicans in a mammalian host. Science 2000;289:307-309.